Finding all intervals that match predicate in vector












8












$begingroup$


I have a function find_all_intervals_below that iterates through a vector and finds all the index intervals of at least a given length where each element within the interval is below a given threshold.



struct Interval {
int start;
int end;
};

std::vector<Interval>
find_all_intervals_below(const std::vector<int> &v, const int &threshold,
const int &min_length) {
auto start_position { 0 };
auto end_position { 0 };
std::vector<Interval> intervals;
bool found_start { false };

for (auto current_pos = 0; current_pos < v.size(); ++current_pos) {
if (v[current_pos] <= threshold and not found_start) {
start_position = current_pos;
end_position = 0;
found_start = true;
} else if (found_start and v[current_pos] > threshold and end_position == 0) {
end_position = current_pos;
if (end_position - start_position >= min_length) {
Interval interval;
interval.start = start_position;
interval.end = end_position;
intervals.push_back(interval);
}
start_position = 0;
found_start = false;
}
}
if (found_start and end_position == 0 and v.size() - start_position >= min_length) {
end_position = v.size();
Interval interval;
interval.start = start_position;
interval.end = end_position;
intervals.push_back(interval);
}
return intervals;
}


This function works perfectly fine, I would just like to get some input from others as I imagine there is likely much more succinct ways of doing this. search_n from STL looks like it might be a solution but I couldn't figure out how to use it for my case.



EDIT: I need the solution to be C++11 compatible, unfortunately.



Test example



const auto min_len { 3 };
const auto threshold { 3 };
const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

const auto actual { find_all_intervals_below(v, threshold, min_len) };
const std::vector<Interval> expected { Interval(1, 4) };

assert(actual == expected);









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    8












    $begingroup$


    I have a function find_all_intervals_below that iterates through a vector and finds all the index intervals of at least a given length where each element within the interval is below a given threshold.



    struct Interval {
    int start;
    int end;
    };

    std::vector<Interval>
    find_all_intervals_below(const std::vector<int> &v, const int &threshold,
    const int &min_length) {
    auto start_position { 0 };
    auto end_position { 0 };
    std::vector<Interval> intervals;
    bool found_start { false };

    for (auto current_pos = 0; current_pos < v.size(); ++current_pos) {
    if (v[current_pos] <= threshold and not found_start) {
    start_position = current_pos;
    end_position = 0;
    found_start = true;
    } else if (found_start and v[current_pos] > threshold and end_position == 0) {
    end_position = current_pos;
    if (end_position - start_position >= min_length) {
    Interval interval;
    interval.start = start_position;
    interval.end = end_position;
    intervals.push_back(interval);
    }
    start_position = 0;
    found_start = false;
    }
    }
    if (found_start and end_position == 0 and v.size() - start_position >= min_length) {
    end_position = v.size();
    Interval interval;
    interval.start = start_position;
    interval.end = end_position;
    intervals.push_back(interval);
    }
    return intervals;
    }


    This function works perfectly fine, I would just like to get some input from others as I imagine there is likely much more succinct ways of doing this. search_n from STL looks like it might be a solution but I couldn't figure out how to use it for my case.



    EDIT: I need the solution to be C++11 compatible, unfortunately.



    Test example



    const auto min_len { 3 };
    const auto threshold { 3 };
    const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

    const auto actual { find_all_intervals_below(v, threshold, min_len) };
    const std::vector<Interval> expected { Interval(1, 4) };

    assert(actual == expected);









    share|improve this question









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    $endgroup$















      8












      8








      8


      1



      $begingroup$


      I have a function find_all_intervals_below that iterates through a vector and finds all the index intervals of at least a given length where each element within the interval is below a given threshold.



      struct Interval {
      int start;
      int end;
      };

      std::vector<Interval>
      find_all_intervals_below(const std::vector<int> &v, const int &threshold,
      const int &min_length) {
      auto start_position { 0 };
      auto end_position { 0 };
      std::vector<Interval> intervals;
      bool found_start { false };

      for (auto current_pos = 0; current_pos < v.size(); ++current_pos) {
      if (v[current_pos] <= threshold and not found_start) {
      start_position = current_pos;
      end_position = 0;
      found_start = true;
      } else if (found_start and v[current_pos] > threshold and end_position == 0) {
      end_position = current_pos;
      if (end_position - start_position >= min_length) {
      Interval interval;
      interval.start = start_position;
      interval.end = end_position;
      intervals.push_back(interval);
      }
      start_position = 0;
      found_start = false;
      }
      }
      if (found_start and end_position == 0 and v.size() - start_position >= min_length) {
      end_position = v.size();
      Interval interval;
      interval.start = start_position;
      interval.end = end_position;
      intervals.push_back(interval);
      }
      return intervals;
      }


      This function works perfectly fine, I would just like to get some input from others as I imagine there is likely much more succinct ways of doing this. search_n from STL looks like it might be a solution but I couldn't figure out how to use it for my case.



      EDIT: I need the solution to be C++11 compatible, unfortunately.



      Test example



      const auto min_len { 3 };
      const auto threshold { 3 };
      const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

      const auto actual { find_all_intervals_below(v, threshold, min_len) };
      const std::vector<Interval> expected { Interval(1, 4) };

      assert(actual == expected);









      share|improve this question









      New contributor




      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.







      $endgroup$




      I have a function find_all_intervals_below that iterates through a vector and finds all the index intervals of at least a given length where each element within the interval is below a given threshold.



      struct Interval {
      int start;
      int end;
      };

      std::vector<Interval>
      find_all_intervals_below(const std::vector<int> &v, const int &threshold,
      const int &min_length) {
      auto start_position { 0 };
      auto end_position { 0 };
      std::vector<Interval> intervals;
      bool found_start { false };

      for (auto current_pos = 0; current_pos < v.size(); ++current_pos) {
      if (v[current_pos] <= threshold and not found_start) {
      start_position = current_pos;
      end_position = 0;
      found_start = true;
      } else if (found_start and v[current_pos] > threshold and end_position == 0) {
      end_position = current_pos;
      if (end_position - start_position >= min_length) {
      Interval interval;
      interval.start = start_position;
      interval.end = end_position;
      intervals.push_back(interval);
      }
      start_position = 0;
      found_start = false;
      }
      }
      if (found_start and end_position == 0 and v.size() - start_position >= min_length) {
      end_position = v.size();
      Interval interval;
      interval.start = start_position;
      interval.end = end_position;
      intervals.push_back(interval);
      }
      return intervals;
      }


      This function works perfectly fine, I would just like to get some input from others as I imagine there is likely much more succinct ways of doing this. search_n from STL looks like it might be a solution but I couldn't figure out how to use it for my case.



      EDIT: I need the solution to be C++11 compatible, unfortunately.



      Test example



      const auto min_len { 3 };
      const auto threshold { 3 };
      const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

      const auto actual { find_all_intervals_below(v, threshold, min_len) };
      const std::vector<Interval> expected { Interval(1, 4) };

      assert(actual == expected);






      c++ c++11 interval






      share|improve this question









      New contributor




      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.











      share|improve this question









      New contributor




      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      share|improve this question




      share|improve this question








      edited 13 hours ago







      Michael Hall













      New contributor




      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.









      asked 16 hours ago









      Michael HallMichael Hall

      1435




      1435




      New contributor




      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.





      New contributor





      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      Michael Hall is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






















          2 Answers
          2






          active

          oldest

          votes


















          7












          $begingroup$

          Code Review





          1. This piece



                    Interval interval;
            interval.start = start_position;
            interval.end = end_position;
            intervals.push_back(interval);


            can be transformed into



            intervals.emplace_back(start_position, end_position);


          2. Don't accept small objects by reference for read-only purposes. Although it usually doesn't hurt, in most implementations reference (which is implemented as pointer) will take up more space (compiler will probably inline the function or just pass by value though).



          3. Algorithm. When there is a state which is represented by combination of flags and some metadata, flags usually go out of hand quickly. I would instead implement something like this:



            1. Set previous, current to start of the input
            2. previous = current
            3. current = first index of element that is higher than threshold
            4. if current - previous >= minlength, add to result
            5. increment current
            6. Go to 2


            One could also create it the other way around, e.g. searching for those below threshold.




          Alternative implementation



          #include <vector>
          #include <algorithm>
          #include <type_traits>

          using index_type = std::make_signed_t<std::size_t>;

          struct interval {
          index_type first;
          index_type last;
          };

          bool operator==(const interval lhs, const interval rhs) {
          return lhs.first == rhs.first && lhs.last == rhs.last;
          }

          std::vector<interval> find_suitable_intervals(const std::vector<int>& input,
          const int threshold,
          const index_type min_length) {
          auto predicate = [threshold](int x) {
          return x <= threshold;
          };
          std::vector<interval> intervals;
          auto first = input.begin();
          auto previous = input.begin();
          auto current = first;
          while (current != input.end()) {
          previous = current;
          current = std::find_if_not(current, input.end(), predicate);
          if (current - previous >= min_length) {
          intervals.push_back({previous - first, current - first});
          }
          if (current == input.end()) {
          break;
          }
          ++current;
          }

          return intervals;
          }

          int main() {
          const int min_length = 3;
          const int threshold = 3;
          const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

          const auto actual = find_suitable_intervals(v, threshold, min_length);
          const std::vector<interval> expected { {1, 4} };

          return actual != expected;
          }


          Wandbox Demo.



          The logic got more "flat", but there are culprits of bridging STL style with more traditional style. Also, since incrementing iterator beyond end will cause undefined behavior, I had to put in the condition to check if the loop reached end. Mixing std::size_t and std::distance/difference will cause a warning and will require a cast to get rid of the warning, since one is unsigned and the other is not, thus I created index_type. There are rumors of std::index, but I wouldn't expect it in near future.



          One could also make the condition an input into the function, e.g. predicate. Then it would look like this:



          find_suitable_intervals(data, min_length, [threshold](auto x) { 
          x < threshold;
          });


          Which is I believe is a bit more readable.






          share|improve this answer











          $endgroup$





















            6












            $begingroup$

            If you want to write concise, idiomatic C++, the best way is to rely on the STL as much as possible, as a tool as well as an inspiration.



            So how would this algorithm be implemented in the STL?




            • It probably wouldn't implemented so specifically. It would be more abstract: for instance, being under a threshold is a particular case of a satisfying a predicate; iterating over a vector is a particular case of iterating over a sequence (i.e a pair of iterators).


            • It would also be separated into orthogonal components: finding ranges whose elements satisfy a predicate is a thing, filtering those ranges which aren't long enough another.


            • Finally, complex algorithms are broken into simpler parts when possible (some say that the whole <algorithm> header is a patient construction of std::sort from its parts).



            In the light of all this, I suggest:




            • function signatures based on iterators


            • an intermediate algorithm to find consecutive elements satisfying a predicate


            • an algorithm to find all sequences of consecutive elements satisfying a predicate


            • composing the latter algorithm with known STL algorithm to customize its behavior.



            For instance:



            #include <algorithm>
            #include <functional>
            #include <vector>
            #include <iostream>

            // the intermediate algorithm
            template <typename Iterator, typename Pred>
            std::pair<Iterator, Iterator> find_range_satisfying(Iterator first, Iterator last, Pred pred) {
            auto f = std::find_if(first, last, pred);
            if (f == last) return {last, last}; // representation of failure. std::optional would have been a good choice also
            return {f, std::find_if(std::next(f), last, std::not_fn(pred))};
            }

            template <typename Iterator, typename Pred>
            auto find_all_ranges_satisfying(Iterator first, Iterator last, Pred pred) {
            std::vector<std::pair<Iterator, Iterator>> result;
            while (first != last) {
            auto [b, e] = find_range_satisfying(first, last, pred);
            if (b == last) break;
            result.push_back({b, e});
            first = e;
            }
            return result;
            }

            int main() {
            const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };
            const auto threshold = 3;
            auto test = find_all_ranges_satisfying(v.begin(), v.end(), (auto elem) { return elem < 3; });
            // composing with remove_if to obtain the desired behavior
            test.erase(std::remove_if(test.begin(), test.end(), [threshold](auto rng) {
            return std::distance(rng.first, rng.second) < threshold;
            }));
            for (auto [b, e] : test) {
            std::for_each(b, e, (auto elem) { std::cout << elem << ' '; });
            std::cout << std::endl;
            }
            }





            share|improve this answer









            $endgroup$













            • $begingroup$
              I like this implementation. But unfortunately the project I am working on is only C++11
              $endgroup$
              – Michael Hall
              14 hours ago










            • $begingroup$
              @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
              $endgroup$
              – Incomputable
              14 hours ago












            • $begingroup$
              std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
              $endgroup$
              – Michael Hall
              14 hours ago










            • $begingroup$
              @papagaga, by the way, there is std::erase_if coming in C++20.
              $endgroup$
              – Incomputable
              14 hours ago










            • $begingroup$
              @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
              $endgroup$
              – Incomputable
              14 hours ago













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            2 Answers
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            2 Answers
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            active

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            active

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            active

            oldest

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            7












            $begingroup$

            Code Review





            1. This piece



                      Interval interval;
              interval.start = start_position;
              interval.end = end_position;
              intervals.push_back(interval);


              can be transformed into



              intervals.emplace_back(start_position, end_position);


            2. Don't accept small objects by reference for read-only purposes. Although it usually doesn't hurt, in most implementations reference (which is implemented as pointer) will take up more space (compiler will probably inline the function or just pass by value though).



            3. Algorithm. When there is a state which is represented by combination of flags and some metadata, flags usually go out of hand quickly. I would instead implement something like this:



              1. Set previous, current to start of the input
              2. previous = current
              3. current = first index of element that is higher than threshold
              4. if current - previous >= minlength, add to result
              5. increment current
              6. Go to 2


              One could also create it the other way around, e.g. searching for those below threshold.




            Alternative implementation



            #include <vector>
            #include <algorithm>
            #include <type_traits>

            using index_type = std::make_signed_t<std::size_t>;

            struct interval {
            index_type first;
            index_type last;
            };

            bool operator==(const interval lhs, const interval rhs) {
            return lhs.first == rhs.first && lhs.last == rhs.last;
            }

            std::vector<interval> find_suitable_intervals(const std::vector<int>& input,
            const int threshold,
            const index_type min_length) {
            auto predicate = [threshold](int x) {
            return x <= threshold;
            };
            std::vector<interval> intervals;
            auto first = input.begin();
            auto previous = input.begin();
            auto current = first;
            while (current != input.end()) {
            previous = current;
            current = std::find_if_not(current, input.end(), predicate);
            if (current - previous >= min_length) {
            intervals.push_back({previous - first, current - first});
            }
            if (current == input.end()) {
            break;
            }
            ++current;
            }

            return intervals;
            }

            int main() {
            const int min_length = 3;
            const int threshold = 3;
            const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

            const auto actual = find_suitable_intervals(v, threshold, min_length);
            const std::vector<interval> expected { {1, 4} };

            return actual != expected;
            }


            Wandbox Demo.



            The logic got more "flat", but there are culprits of bridging STL style with more traditional style. Also, since incrementing iterator beyond end will cause undefined behavior, I had to put in the condition to check if the loop reached end. Mixing std::size_t and std::distance/difference will cause a warning and will require a cast to get rid of the warning, since one is unsigned and the other is not, thus I created index_type. There are rumors of std::index, but I wouldn't expect it in near future.



            One could also make the condition an input into the function, e.g. predicate. Then it would look like this:



            find_suitable_intervals(data, min_length, [threshold](auto x) { 
            x < threshold;
            });


            Which is I believe is a bit more readable.






            share|improve this answer











            $endgroup$


















              7












              $begingroup$

              Code Review





              1. This piece



                        Interval interval;
                interval.start = start_position;
                interval.end = end_position;
                intervals.push_back(interval);


                can be transformed into



                intervals.emplace_back(start_position, end_position);


              2. Don't accept small objects by reference for read-only purposes. Although it usually doesn't hurt, in most implementations reference (which is implemented as pointer) will take up more space (compiler will probably inline the function or just pass by value though).



              3. Algorithm. When there is a state which is represented by combination of flags and some metadata, flags usually go out of hand quickly. I would instead implement something like this:



                1. Set previous, current to start of the input
                2. previous = current
                3. current = first index of element that is higher than threshold
                4. if current - previous >= minlength, add to result
                5. increment current
                6. Go to 2


                One could also create it the other way around, e.g. searching for those below threshold.




              Alternative implementation



              #include <vector>
              #include <algorithm>
              #include <type_traits>

              using index_type = std::make_signed_t<std::size_t>;

              struct interval {
              index_type first;
              index_type last;
              };

              bool operator==(const interval lhs, const interval rhs) {
              return lhs.first == rhs.first && lhs.last == rhs.last;
              }

              std::vector<interval> find_suitable_intervals(const std::vector<int>& input,
              const int threshold,
              const index_type min_length) {
              auto predicate = [threshold](int x) {
              return x <= threshold;
              };
              std::vector<interval> intervals;
              auto first = input.begin();
              auto previous = input.begin();
              auto current = first;
              while (current != input.end()) {
              previous = current;
              current = std::find_if_not(current, input.end(), predicate);
              if (current - previous >= min_length) {
              intervals.push_back({previous - first, current - first});
              }
              if (current == input.end()) {
              break;
              }
              ++current;
              }

              return intervals;
              }

              int main() {
              const int min_length = 3;
              const int threshold = 3;
              const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

              const auto actual = find_suitable_intervals(v, threshold, min_length);
              const std::vector<interval> expected { {1, 4} };

              return actual != expected;
              }


              Wandbox Demo.



              The logic got more "flat", but there are culprits of bridging STL style with more traditional style. Also, since incrementing iterator beyond end will cause undefined behavior, I had to put in the condition to check if the loop reached end. Mixing std::size_t and std::distance/difference will cause a warning and will require a cast to get rid of the warning, since one is unsigned and the other is not, thus I created index_type. There are rumors of std::index, but I wouldn't expect it in near future.



              One could also make the condition an input into the function, e.g. predicate. Then it would look like this:



              find_suitable_intervals(data, min_length, [threshold](auto x) { 
              x < threshold;
              });


              Which is I believe is a bit more readable.






              share|improve this answer











              $endgroup$
















                7












                7








                7





                $begingroup$

                Code Review





                1. This piece



                          Interval interval;
                  interval.start = start_position;
                  interval.end = end_position;
                  intervals.push_back(interval);


                  can be transformed into



                  intervals.emplace_back(start_position, end_position);


                2. Don't accept small objects by reference for read-only purposes. Although it usually doesn't hurt, in most implementations reference (which is implemented as pointer) will take up more space (compiler will probably inline the function or just pass by value though).



                3. Algorithm. When there is a state which is represented by combination of flags and some metadata, flags usually go out of hand quickly. I would instead implement something like this:



                  1. Set previous, current to start of the input
                  2. previous = current
                  3. current = first index of element that is higher than threshold
                  4. if current - previous >= minlength, add to result
                  5. increment current
                  6. Go to 2


                  One could also create it the other way around, e.g. searching for those below threshold.




                Alternative implementation



                #include <vector>
                #include <algorithm>
                #include <type_traits>

                using index_type = std::make_signed_t<std::size_t>;

                struct interval {
                index_type first;
                index_type last;
                };

                bool operator==(const interval lhs, const interval rhs) {
                return lhs.first == rhs.first && lhs.last == rhs.last;
                }

                std::vector<interval> find_suitable_intervals(const std::vector<int>& input,
                const int threshold,
                const index_type min_length) {
                auto predicate = [threshold](int x) {
                return x <= threshold;
                };
                std::vector<interval> intervals;
                auto first = input.begin();
                auto previous = input.begin();
                auto current = first;
                while (current != input.end()) {
                previous = current;
                current = std::find_if_not(current, input.end(), predicate);
                if (current - previous >= min_length) {
                intervals.push_back({previous - first, current - first});
                }
                if (current == input.end()) {
                break;
                }
                ++current;
                }

                return intervals;
                }

                int main() {
                const int min_length = 3;
                const int threshold = 3;
                const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

                const auto actual = find_suitable_intervals(v, threshold, min_length);
                const std::vector<interval> expected { {1, 4} };

                return actual != expected;
                }


                Wandbox Demo.



                The logic got more "flat", but there are culprits of bridging STL style with more traditional style. Also, since incrementing iterator beyond end will cause undefined behavior, I had to put in the condition to check if the loop reached end. Mixing std::size_t and std::distance/difference will cause a warning and will require a cast to get rid of the warning, since one is unsigned and the other is not, thus I created index_type. There are rumors of std::index, but I wouldn't expect it in near future.



                One could also make the condition an input into the function, e.g. predicate. Then it would look like this:



                find_suitable_intervals(data, min_length, [threshold](auto x) { 
                x < threshold;
                });


                Which is I believe is a bit more readable.






                share|improve this answer











                $endgroup$



                Code Review





                1. This piece



                          Interval interval;
                  interval.start = start_position;
                  interval.end = end_position;
                  intervals.push_back(interval);


                  can be transformed into



                  intervals.emplace_back(start_position, end_position);


                2. Don't accept small objects by reference for read-only purposes. Although it usually doesn't hurt, in most implementations reference (which is implemented as pointer) will take up more space (compiler will probably inline the function or just pass by value though).



                3. Algorithm. When there is a state which is represented by combination of flags and some metadata, flags usually go out of hand quickly. I would instead implement something like this:



                  1. Set previous, current to start of the input
                  2. previous = current
                  3. current = first index of element that is higher than threshold
                  4. if current - previous >= minlength, add to result
                  5. increment current
                  6. Go to 2


                  One could also create it the other way around, e.g. searching for those below threshold.




                Alternative implementation



                #include <vector>
                #include <algorithm>
                #include <type_traits>

                using index_type = std::make_signed_t<std::size_t>;

                struct interval {
                index_type first;
                index_type last;
                };

                bool operator==(const interval lhs, const interval rhs) {
                return lhs.first == rhs.first && lhs.last == rhs.last;
                }

                std::vector<interval> find_suitable_intervals(const std::vector<int>& input,
                const int threshold,
                const index_type min_length) {
                auto predicate = [threshold](int x) {
                return x <= threshold;
                };
                std::vector<interval> intervals;
                auto first = input.begin();
                auto previous = input.begin();
                auto current = first;
                while (current != input.end()) {
                previous = current;
                current = std::find_if_not(current, input.end(), predicate);
                if (current - previous >= min_length) {
                intervals.push_back({previous - first, current - first});
                }
                if (current == input.end()) {
                break;
                }
                ++current;
                }

                return intervals;
                }

                int main() {
                const int min_length = 3;
                const int threshold = 3;
                const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };

                const auto actual = find_suitable_intervals(v, threshold, min_length);
                const std::vector<interval> expected { {1, 4} };

                return actual != expected;
                }


                Wandbox Demo.



                The logic got more "flat", but there are culprits of bridging STL style with more traditional style. Also, since incrementing iterator beyond end will cause undefined behavior, I had to put in the condition to check if the loop reached end. Mixing std::size_t and std::distance/difference will cause a warning and will require a cast to get rid of the warning, since one is unsigned and the other is not, thus I created index_type. There are rumors of std::index, but I wouldn't expect it in near future.



                One could also make the condition an input into the function, e.g. predicate. Then it would look like this:



                find_suitable_intervals(data, min_length, [threshold](auto x) { 
                x < threshold;
                });


                Which is I believe is a bit more readable.







                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited 15 hours ago









                Toby Speight

                26.6k742118




                26.6k742118










                answered 15 hours ago









                IncomputableIncomputable

                6,79021753




                6,79021753

























                    6












                    $begingroup$

                    If you want to write concise, idiomatic C++, the best way is to rely on the STL as much as possible, as a tool as well as an inspiration.



                    So how would this algorithm be implemented in the STL?




                    • It probably wouldn't implemented so specifically. It would be more abstract: for instance, being under a threshold is a particular case of a satisfying a predicate; iterating over a vector is a particular case of iterating over a sequence (i.e a pair of iterators).


                    • It would also be separated into orthogonal components: finding ranges whose elements satisfy a predicate is a thing, filtering those ranges which aren't long enough another.


                    • Finally, complex algorithms are broken into simpler parts when possible (some say that the whole <algorithm> header is a patient construction of std::sort from its parts).



                    In the light of all this, I suggest:




                    • function signatures based on iterators


                    • an intermediate algorithm to find consecutive elements satisfying a predicate


                    • an algorithm to find all sequences of consecutive elements satisfying a predicate


                    • composing the latter algorithm with known STL algorithm to customize its behavior.



                    For instance:



                    #include <algorithm>
                    #include <functional>
                    #include <vector>
                    #include <iostream>

                    // the intermediate algorithm
                    template <typename Iterator, typename Pred>
                    std::pair<Iterator, Iterator> find_range_satisfying(Iterator first, Iterator last, Pred pred) {
                    auto f = std::find_if(first, last, pred);
                    if (f == last) return {last, last}; // representation of failure. std::optional would have been a good choice also
                    return {f, std::find_if(std::next(f), last, std::not_fn(pred))};
                    }

                    template <typename Iterator, typename Pred>
                    auto find_all_ranges_satisfying(Iterator first, Iterator last, Pred pred) {
                    std::vector<std::pair<Iterator, Iterator>> result;
                    while (first != last) {
                    auto [b, e] = find_range_satisfying(first, last, pred);
                    if (b == last) break;
                    result.push_back({b, e});
                    first = e;
                    }
                    return result;
                    }

                    int main() {
                    const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };
                    const auto threshold = 3;
                    auto test = find_all_ranges_satisfying(v.begin(), v.end(), (auto elem) { return elem < 3; });
                    // composing with remove_if to obtain the desired behavior
                    test.erase(std::remove_if(test.begin(), test.end(), [threshold](auto rng) {
                    return std::distance(rng.first, rng.second) < threshold;
                    }));
                    for (auto [b, e] : test) {
                    std::for_each(b, e, (auto elem) { std::cout << elem << ' '; });
                    std::cout << std::endl;
                    }
                    }





                    share|improve this answer









                    $endgroup$













                    • $begingroup$
                      I like this implementation. But unfortunately the project I am working on is only C++11
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                      $endgroup$
                      – Incomputable
                      14 hours ago












                    • $begingroup$
                      std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @papagaga, by the way, there is std::erase_if coming in C++20.
                      $endgroup$
                      – Incomputable
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                      $endgroup$
                      – Incomputable
                      14 hours ago


















                    6












                    $begingroup$

                    If you want to write concise, idiomatic C++, the best way is to rely on the STL as much as possible, as a tool as well as an inspiration.



                    So how would this algorithm be implemented in the STL?




                    • It probably wouldn't implemented so specifically. It would be more abstract: for instance, being under a threshold is a particular case of a satisfying a predicate; iterating over a vector is a particular case of iterating over a sequence (i.e a pair of iterators).


                    • It would also be separated into orthogonal components: finding ranges whose elements satisfy a predicate is a thing, filtering those ranges which aren't long enough another.


                    • Finally, complex algorithms are broken into simpler parts when possible (some say that the whole <algorithm> header is a patient construction of std::sort from its parts).



                    In the light of all this, I suggest:




                    • function signatures based on iterators


                    • an intermediate algorithm to find consecutive elements satisfying a predicate


                    • an algorithm to find all sequences of consecutive elements satisfying a predicate


                    • composing the latter algorithm with known STL algorithm to customize its behavior.



                    For instance:



                    #include <algorithm>
                    #include <functional>
                    #include <vector>
                    #include <iostream>

                    // the intermediate algorithm
                    template <typename Iterator, typename Pred>
                    std::pair<Iterator, Iterator> find_range_satisfying(Iterator first, Iterator last, Pred pred) {
                    auto f = std::find_if(first, last, pred);
                    if (f == last) return {last, last}; // representation of failure. std::optional would have been a good choice also
                    return {f, std::find_if(std::next(f), last, std::not_fn(pred))};
                    }

                    template <typename Iterator, typename Pred>
                    auto find_all_ranges_satisfying(Iterator first, Iterator last, Pred pred) {
                    std::vector<std::pair<Iterator, Iterator>> result;
                    while (first != last) {
                    auto [b, e] = find_range_satisfying(first, last, pred);
                    if (b == last) break;
                    result.push_back({b, e});
                    first = e;
                    }
                    return result;
                    }

                    int main() {
                    const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };
                    const auto threshold = 3;
                    auto test = find_all_ranges_satisfying(v.begin(), v.end(), (auto elem) { return elem < 3; });
                    // composing with remove_if to obtain the desired behavior
                    test.erase(std::remove_if(test.begin(), test.end(), [threshold](auto rng) {
                    return std::distance(rng.first, rng.second) < threshold;
                    }));
                    for (auto [b, e] : test) {
                    std::for_each(b, e, (auto elem) { std::cout << elem << ' '; });
                    std::cout << std::endl;
                    }
                    }





                    share|improve this answer









                    $endgroup$













                    • $begingroup$
                      I like this implementation. But unfortunately the project I am working on is only C++11
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                      $endgroup$
                      – Incomputable
                      14 hours ago












                    • $begingroup$
                      std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @papagaga, by the way, there is std::erase_if coming in C++20.
                      $endgroup$
                      – Incomputable
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                      $endgroup$
                      – Incomputable
                      14 hours ago
















                    6












                    6








                    6





                    $begingroup$

                    If you want to write concise, idiomatic C++, the best way is to rely on the STL as much as possible, as a tool as well as an inspiration.



                    So how would this algorithm be implemented in the STL?




                    • It probably wouldn't implemented so specifically. It would be more abstract: for instance, being under a threshold is a particular case of a satisfying a predicate; iterating over a vector is a particular case of iterating over a sequence (i.e a pair of iterators).


                    • It would also be separated into orthogonal components: finding ranges whose elements satisfy a predicate is a thing, filtering those ranges which aren't long enough another.


                    • Finally, complex algorithms are broken into simpler parts when possible (some say that the whole <algorithm> header is a patient construction of std::sort from its parts).



                    In the light of all this, I suggest:




                    • function signatures based on iterators


                    • an intermediate algorithm to find consecutive elements satisfying a predicate


                    • an algorithm to find all sequences of consecutive elements satisfying a predicate


                    • composing the latter algorithm with known STL algorithm to customize its behavior.



                    For instance:



                    #include <algorithm>
                    #include <functional>
                    #include <vector>
                    #include <iostream>

                    // the intermediate algorithm
                    template <typename Iterator, typename Pred>
                    std::pair<Iterator, Iterator> find_range_satisfying(Iterator first, Iterator last, Pred pred) {
                    auto f = std::find_if(first, last, pred);
                    if (f == last) return {last, last}; // representation of failure. std::optional would have been a good choice also
                    return {f, std::find_if(std::next(f), last, std::not_fn(pred))};
                    }

                    template <typename Iterator, typename Pred>
                    auto find_all_ranges_satisfying(Iterator first, Iterator last, Pred pred) {
                    std::vector<std::pair<Iterator, Iterator>> result;
                    while (first != last) {
                    auto [b, e] = find_range_satisfying(first, last, pred);
                    if (b == last) break;
                    result.push_back({b, e});
                    first = e;
                    }
                    return result;
                    }

                    int main() {
                    const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };
                    const auto threshold = 3;
                    auto test = find_all_ranges_satisfying(v.begin(), v.end(), (auto elem) { return elem < 3; });
                    // composing with remove_if to obtain the desired behavior
                    test.erase(std::remove_if(test.begin(), test.end(), [threshold](auto rng) {
                    return std::distance(rng.first, rng.second) < threshold;
                    }));
                    for (auto [b, e] : test) {
                    std::for_each(b, e, (auto elem) { std::cout << elem << ' '; });
                    std::cout << std::endl;
                    }
                    }





                    share|improve this answer









                    $endgroup$



                    If you want to write concise, idiomatic C++, the best way is to rely on the STL as much as possible, as a tool as well as an inspiration.



                    So how would this algorithm be implemented in the STL?




                    • It probably wouldn't implemented so specifically. It would be more abstract: for instance, being under a threshold is a particular case of a satisfying a predicate; iterating over a vector is a particular case of iterating over a sequence (i.e a pair of iterators).


                    • It would also be separated into orthogonal components: finding ranges whose elements satisfy a predicate is a thing, filtering those ranges which aren't long enough another.


                    • Finally, complex algorithms are broken into simpler parts when possible (some say that the whole <algorithm> header is a patient construction of std::sort from its parts).



                    In the light of all this, I suggest:




                    • function signatures based on iterators


                    • an intermediate algorithm to find consecutive elements satisfying a predicate


                    • an algorithm to find all sequences of consecutive elements satisfying a predicate


                    • composing the latter algorithm with known STL algorithm to customize its behavior.



                    For instance:



                    #include <algorithm>
                    #include <functional>
                    #include <vector>
                    #include <iostream>

                    // the intermediate algorithm
                    template <typename Iterator, typename Pred>
                    std::pair<Iterator, Iterator> find_range_satisfying(Iterator first, Iterator last, Pred pred) {
                    auto f = std::find_if(first, last, pred);
                    if (f == last) return {last, last}; // representation of failure. std::optional would have been a good choice also
                    return {f, std::find_if(std::next(f), last, std::not_fn(pred))};
                    }

                    template <typename Iterator, typename Pred>
                    auto find_all_ranges_satisfying(Iterator first, Iterator last, Pred pred) {
                    std::vector<std::pair<Iterator, Iterator>> result;
                    while (first != last) {
                    auto [b, e] = find_range_satisfying(first, last, pred);
                    if (b == last) break;
                    result.push_back({b, e});
                    first = e;
                    }
                    return result;
                    }

                    int main() {
                    const std::vector<int> v { 4, 2, 1, 1, 4, 1, 2, 4 };
                    const auto threshold = 3;
                    auto test = find_all_ranges_satisfying(v.begin(), v.end(), (auto elem) { return elem < 3; });
                    // composing with remove_if to obtain the desired behavior
                    test.erase(std::remove_if(test.begin(), test.end(), [threshold](auto rng) {
                    return std::distance(rng.first, rng.second) < threshold;
                    }));
                    for (auto [b, e] : test) {
                    std::for_each(b, e, (auto elem) { std::cout << elem << ' '; });
                    std::cout << std::endl;
                    }
                    }






                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 15 hours ago









                    papagagapapagaga

                    4,712421




                    4,712421












                    • $begingroup$
                      I like this implementation. But unfortunately the project I am working on is only C++11
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                      $endgroup$
                      – Incomputable
                      14 hours ago












                    • $begingroup$
                      std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @papagaga, by the way, there is std::erase_if coming in C++20.
                      $endgroup$
                      – Incomputable
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                      $endgroup$
                      – Incomputable
                      14 hours ago




















                    • $begingroup$
                      I like this implementation. But unfortunately the project I am working on is only C++11
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                      $endgroup$
                      – Incomputable
                      14 hours ago












                    • $begingroup$
                      std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                      $endgroup$
                      – Michael Hall
                      14 hours ago










                    • $begingroup$
                      @papagaga, by the way, there is std::erase_if coming in C++20.
                      $endgroup$
                      – Incomputable
                      14 hours ago










                    • $begingroup$
                      @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                      $endgroup$
                      – Incomputable
                      14 hours ago


















                    $begingroup$
                    I like this implementation. But unfortunately the project I am working on is only C++11
                    $endgroup$
                    – Michael Hall
                    14 hours ago




                    $begingroup$
                    I like this implementation. But unfortunately the project I am working on is only C++11
                    $endgroup$
                    – Michael Hall
                    14 hours ago












                    $begingroup$
                    @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                    $endgroup$
                    – Incomputable
                    14 hours ago






                    $begingroup$
                    @MichaelHall, at first glance, other than structured bindings, I didn't find anything that C++11 capable compiler couldn't compile.
                    $endgroup$
                    – Incomputable
                    14 hours ago














                    $begingroup$
                    std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                    $endgroup$
                    – Michael Hall
                    14 hours ago




                    $begingroup$
                    std::not_fn is C++17 en.cppreference.com/w/cpp/utility/functional/not_fn
                    $endgroup$
                    – Michael Hall
                    14 hours ago












                    $begingroup$
                    @papagaga, by the way, there is std::erase_if coming in C++20.
                    $endgroup$
                    – Incomputable
                    14 hours ago




                    $begingroup$
                    @papagaga, by the way, there is std::erase_if coming in C++20.
                    $endgroup$
                    – Incomputable
                    14 hours ago












                    $begingroup$
                    @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                    $endgroup$
                    – Incomputable
                    14 hours ago






                    $begingroup$
                    @MichaelHall, one can replace std::find_if with std::find_if_not. I believe there is no expressive gained in C++14+ for this problem, but it might reduce the elegance significantly. It is good to include language version tag in the question, but sometimes it is ignored by reviewers.
                    $endgroup$
                    – Incomputable
                    14 hours ago












                    Michael Hall is a new contributor. Be nice, and check out our Code of Conduct.










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                    Michael Hall is a new contributor. Be nice, and check out our Code of Conduct.













                    Michael Hall is a new contributor. Be nice, and check out our Code of Conduct.












                    Michael Hall is a new contributor. Be nice, and check out our Code of Conduct.
















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Hall Of Fame””Slayer Wins 'Best Metal' Grammy Award””Slayer Guitarist Jeff Hanneman Dies””Bullet-For My Valentine booed at Metal Hammer Golden Gods Awards””Unholy Aliance””The End Of Slayer?””Slayer: We Could Thrash Out Two More Albums If We're Fast Enough...””'The Unholy Alliance: Chapter III' UK Dates Added”originalet”Megadeth And Slayer To Co-Headline 'Canadian Carnage' Trek”originalet”World Painted Blood””Release “World Painted Blood” by Slayer””Metallica Heading To Cinemas””Slayer, Megadeth To Join Forces For 'European Carnage' Tour - Dec. 18, 2010”originalet”Slayer's Hanneman Contracts Acute Infection; Band To Bring In Guest Guitarist””Cannibal Corpse's Pat O'Brien Will Step In As Slayer's Guest Guitarist”originalet”Slayer’s Jeff Hanneman Dead at 49””Dave Lombardo Says He Made Only $67,000 In 2011 While Touring With Slayer””Slayer: We Do Not Agree With Dave Lombardo's Substance Or Timeline Of Events””Slayer Welcomes Drummer Paul Bostaph Back To The Fold””Slayer Hope to Unveil Never-Before-Heard Jeff Hanneman Material on Next Album””Slayer Debut New Song 'Implode' During Surprise Golden Gods Appearance””Release group Repentless by Slayer””Repentless - Slayer - Credits””Slayer””Metal Storm Awards 2015””Slayer - to release comic book "Repentless #1"””Slayer To Release 'Repentless' 6.66" Vinyl Box Set””BREAKING NEWS: Slayer Announce Farewell Tour””Slayer Recruit Lamb of God, Anthrax, Behemoth + Testament for Final Tour””Slayer lägger ner efter 37 år””Slayer Announces Second North American Leg Of 'Final' Tour””Final World Tour””Slayer Announces Final European Tour With Lamb of God, Anthrax And Obituary””Slayer To Tour Europe With Lamb of God, Anthrax And Obituary””Slayer To Play 'Last French Show Ever' At Next Year's Hellfst””Slayer's Final World Tour Will Extend Into 2019””Death Angel's Rob Cavestany On Slayer's 'Farewell' Tour: 'Some Of Us Could See This Coming'””Testament Has No Plans To Retire Anytime Soon, Says Chuck Billy””Anthrax's Scott Ian On Slayer's 'Farewell' Tour Plans: 'I Was Surprised And I Wasn't Surprised'””Slayer””Slayer's Morbid Schlock””Review/Rock; For Slayer, the Mania Is the Message””Slayer - Biography””Slayer - Reign In Blood”originalet”Dave Lombardo””An exclusive oral history of Slayer”originalet”Exclusive! Interview With Slayer Guitarist Jeff Hanneman”originalet”Thinking Out Loud: Slayer's Kerry King on hair metal, Satan and being polite””Slayer Lyrics””Slayer - Biography””Most influential artists for extreme metal music””Slayer - Reign in Blood””Slayer guitarist Jeff Hanneman dies aged 49””Slatanic Slaughter: A Tribute to Slayer””Gateway to Hell: A Tribute to Slayer””Covered In Blood””Slayer: The Origins of Thrash in San Francisco, CA.””Why They Rule - #6 Slayer”originalet”Guitar World's 100 Greatest Heavy Metal Guitarists Of All Time”originalet”The fans have spoken: Slayer comes out on top in readers' polls”originalet”Tribute to Jeff Hanneman (1964-2013)””Lamb Of God Frontman: We Sound Like A Slayer Rip-Off””BEHEMOTH Frontman Pays Tribute To SLAYER's JEFF HANNEMAN””Slayer, Hatebreed Doing Double Duty On This Year's Ozzfest””System of a Down””Lacuna Coil’s Andrea Ferro Talks Influences, Skateboarding, Band Origins + More””Slayer - Reign in Blood””Into The Lungs of Hell””Slayer rules - en utställning om fans””Slayer and Their Fans Slashed Through a No-Holds-Barred Night at Gas Monkey””Home””Slayer””Gold & Platinum - The Big 4 Live from Sofia, Bulgaria””Exclusive! Interview With Slayer Guitarist Kerry King””2008-02-23: Wiltern, Los Angeles, CA, USA””Slayer's Kerry King To Perform With Megadeth Tonight! - Oct. 21, 2010”originalet”Dave Lombardo - Biography”Slayer Case DismissedArkiveradUltimate Classic Rock: Slayer guitarist Jeff Hanneman dead at 49.”Slayer: "We could never do any thing like Some Kind Of Monster..."””Cannibal Corpse'S Pat O'Brien Will Step In As Slayer'S Guest Guitarist | The Official Slayer Site”originalet”Slayer Wins 'Best Metal' Grammy Award””Slayer Guitarist Jeff Hanneman Dies””Kerrang! Awards 2006 Blog: Kerrang! Hall Of Fame””Kerrang! Awards 2013: Kerrang! Legend”originalet”Metallica, Slayer, Iron Maien Among Winners At Metal Hammer Awards””Metal Hammer Golden Gods Awards””Bullet For My Valentine Booed At Metal Hammer Golden Gods Awards””Metal Storm Awards 2006””Metal Storm Awards 2015””Slayer's Concert History””Slayer - Relationships””Slayer - Releases”Slayers officiella webbplatsSlayer på MusicBrainzOfficiell webbplatsSlayerSlayerr1373445760000 0001 1540 47353068615-5086262726cb13906545x(data)6033143kn20030215029