Why does Python start at index -1 when indexing a list from the end?





.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ height:90px;width:728px;box-sizing:border-box;
}







45















list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"

print(list[-4]) # Number -4 is "a"









share|improve this question









New contributor




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
















  • 28





    Don't use list as a variable name, it's the name of a standard class.

    – Barmar
    10 hours ago






  • 8





    It isn't starting at 1, it's starting at -1. ?!?

    – Thomas Weller
    6 hours ago








  • 1





    mod arithmetic should really be mentioned on this page somewhere...

    – Nacht
    2 hours ago


















45















list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"

print(list[-4]) # Number -4 is "a"









share|improve this question









New contributor




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
















  • 28





    Don't use list as a variable name, it's the name of a standard class.

    – Barmar
    10 hours ago






  • 8





    It isn't starting at 1, it's starting at -1. ?!?

    – Thomas Weller
    6 hours ago








  • 1





    mod arithmetic should really be mentioned on this page somewhere...

    – Nacht
    2 hours ago














45












45








45


11






list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"

print(list[-4]) # Number -4 is "a"









share|improve this question









New contributor




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












list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"

print(list[-4]) # Number -4 is "a"






python list






share|improve this question









New contributor




abraham 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




abraham 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 43 mins ago









Aaron Hall

185k53309264




185k53309264






New contributor




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









asked 18 hours ago









abrahamabraham

22126




22126




New contributor




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





New contributor





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






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








  • 28





    Don't use list as a variable name, it's the name of a standard class.

    – Barmar
    10 hours ago






  • 8





    It isn't starting at 1, it's starting at -1. ?!?

    – Thomas Weller
    6 hours ago








  • 1





    mod arithmetic should really be mentioned on this page somewhere...

    – Nacht
    2 hours ago














  • 28





    Don't use list as a variable name, it's the name of a standard class.

    – Barmar
    10 hours ago






  • 8





    It isn't starting at 1, it's starting at -1. ?!?

    – Thomas Weller
    6 hours ago








  • 1





    mod arithmetic should really be mentioned on this page somewhere...

    – Nacht
    2 hours ago








28




28





Don't use list as a variable name, it's the name of a standard class.

– Barmar
10 hours ago





Don't use list as a variable name, it's the name of a standard class.

– Barmar
10 hours ago




8




8





It isn't starting at 1, it's starting at -1. ?!?

– Thomas Weller
6 hours ago







It isn't starting at 1, it's starting at -1. ?!?

– Thomas Weller
6 hours ago






1




1





mod arithmetic should really be mentioned on this page somewhere...

– Nacht
2 hours ago





mod arithmetic should really be mentioned on this page somewhere...

– Nacht
2 hours ago












7 Answers
7






active

oldest

votes


















101














To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.





If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:



arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c


The typical usages for ~ are like "swap mirror node" or "find median in a sort list":



def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]

def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2


~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.





Discussion about whether should use python tricks like ~:



In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.



For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.



def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums





share|improve this answer


























  • Comments are not for extended discussion; this conversation has been moved to chat.

    – Samuel Liew
    2 hours ago



















90














list[-1]


Is short hand for:



list[len(list)-1]


The len(list) part is implicit. That's why the -1 is the last element. That goes for any negative index - the subtraction from len(list) is always implicit






share|improve this answer



















  • 5





    In my opinion this answer is better than the accepted one.

    – NicolasB
    14 hours ago






  • 3





    Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

    – plugwash
    10 hours ago



















11














Because -0 in Python is 0.

With 0 you get first element of list and
with -1 you get the last element of the list.



list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"





share|improve this answer


























  • I think -0 is 0 pretty much everywhere.

    – Koray Tugay
    11 hours ago






  • 2





    @KorayTugay Except in floating point.

    – Barmar
    10 hours ago











  • I guess that is not 0 anymore. @Barmar

    – Koray Tugay
    10 hours ago






  • 1





    @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

    – curiousdannii
    3 hours ago



















5














This is the mnemonic method I use. It is just an approach of what is happening, but it works.





Don't think of those as indexes. Think of them as offsets on a circular list.



Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:



enter image description here



You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).



If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)






share|improve this answer


























  • "Think about a[2] and a[-2]" is a the list, or an element of the list?

    – detly
    4 hours ago











  • "a" is an hypothetical list which contains the values a-h! I'll clarify!

    – T. Sar
    38 mins ago





















2














Another explanation:



Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.



Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.






share|improve this answer








New contributor




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




























    0














    This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i elements. -1 referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.



    For any list xs and index i positive or negative, the expression



    xs[i]


    will either have the same value as the expression below or produce an IndexError:



    xs[i % len(xs)]


    The index of the last element is -1 + len(xs) which is congruent to -1 mod len(xs). For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.



    In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs) or smaller than -1-len(xs) are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.






    share|improve this answer































      -4














      In a comment, abraham asked




      I wish python has another mechanism to get list backward.




      For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:



      lst = ["a", "b", "c", "d"]
      print(lst[::-1][0]) # "d"
      print(lst[::-1][1]) # "c"





      share|improve this answer










      New contributor




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
















      • 2





        True, but not really answer the question "why".

        – Valentino
        15 hours ago






      • 4





        It's a waste of ressources to create a whole new list in order to retrieve a single element.

        – Eric Duminil
        14 hours ago











      • What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

        – Sam
        11 hours ago













      • @Sam Why use C when we got Assembly?

        – mid
        11 hours ago












      Your Answer






      StackExchange.ifUsing("editor", function () {
      StackExchange.using("externalEditor", function () {
      StackExchange.using("snippets", function () {
      StackExchange.snippets.init();
      });
      });
      }, "code-snippets");

      StackExchange.ready(function() {
      var channelOptions = {
      tags: "".split(" "),
      id: "1"
      };
      initTagRenderer("".split(" "), "".split(" "), channelOptions);

      StackExchange.using("externalEditor", function() {
      // Have to fire editor after snippets, if snippets enabled
      if (StackExchange.settings.snippets.snippetsEnabled) {
      StackExchange.using("snippets", function() {
      createEditor();
      });
      }
      else {
      createEditor();
      }
      });

      function createEditor() {
      StackExchange.prepareEditor({
      heartbeatType: 'answer',
      autoActivateHeartbeat: false,
      convertImagesToLinks: true,
      noModals: true,
      showLowRepImageUploadWarning: true,
      reputationToPostImages: 10,
      bindNavPrevention: true,
      postfix: "",
      imageUploader: {
      brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
      contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
      allowUrls: true
      },
      onDemand: true,
      discardSelector: ".discard-answer"
      ,immediatelyShowMarkdownHelp:true
      });


      }
      });






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










      draft saved

      draft discarded


















      StackExchange.ready(
      function () {
      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55684960%2fwhy-does-python-start-at-index-1-when-indexing-a-list-from-the-end%23new-answer', 'question_page');
      }
      );

      Post as a guest















      Required, but never shown

























      7 Answers
      7






      active

      oldest

      votes








      7 Answers
      7






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      101














      To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.





      If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:



      arr = ["a", "b", "c", "d"]
      print(arr[~0]) # d
      print(arr[~1]) # c


      The typical usages for ~ are like "swap mirror node" or "find median in a sort list":



      def reverse(arr):
      for i in range(len(arr)//2):
      arr[i], arr[~i] = arr[~i], arr[i]

      def median(arr):
      mid = len(arr) // 2
      return (arr[mid] + arr[~mid]) / 2


      ~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.





      Discussion about whether should use python tricks like ~:



      In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.



      For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.



      def findStrobogrammatic(self, n):
      nums = n % 2 * list('018') or ['']
      while n > 1:
      n -= 2
      # n < 2 is so genius here
      nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
      return nums





      share|improve this answer


























      • Comments are not for extended discussion; this conversation has been moved to chat.

        – Samuel Liew
        2 hours ago
















      101














      To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.





      If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:



      arr = ["a", "b", "c", "d"]
      print(arr[~0]) # d
      print(arr[~1]) # c


      The typical usages for ~ are like "swap mirror node" or "find median in a sort list":



      def reverse(arr):
      for i in range(len(arr)//2):
      arr[i], arr[~i] = arr[~i], arr[i]

      def median(arr):
      mid = len(arr) // 2
      return (arr[mid] + arr[~mid]) / 2


      ~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.





      Discussion about whether should use python tricks like ~:



      In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.



      For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.



      def findStrobogrammatic(self, n):
      nums = n % 2 * list('018') or ['']
      while n > 1:
      n -= 2
      # n < 2 is so genius here
      nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
      return nums





      share|improve this answer


























      • Comments are not for extended discussion; this conversation has been moved to chat.

        – Samuel Liew
        2 hours ago














      101












      101








      101







      To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.





      If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:



      arr = ["a", "b", "c", "d"]
      print(arr[~0]) # d
      print(arr[~1]) # c


      The typical usages for ~ are like "swap mirror node" or "find median in a sort list":



      def reverse(arr):
      for i in range(len(arr)//2):
      arr[i], arr[~i] = arr[~i], arr[i]

      def median(arr):
      mid = len(arr) // 2
      return (arr[mid] + arr[~mid]) / 2


      ~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.





      Discussion about whether should use python tricks like ~:



      In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.



      For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.



      def findStrobogrammatic(self, n):
      nums = n % 2 * list('018') or ['']
      while n > 1:
      n -= 2
      # n < 2 is so genius here
      nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
      return nums





      share|improve this answer















      To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.





      If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:



      arr = ["a", "b", "c", "d"]
      print(arr[~0]) # d
      print(arr[~1]) # c


      The typical usages for ~ are like "swap mirror node" or "find median in a sort list":



      def reverse(arr):
      for i in range(len(arr)//2):
      arr[i], arr[~i] = arr[~i], arr[i]

      def median(arr):
      mid = len(arr) // 2
      return (arr[mid] + arr[~mid]) / 2


      ~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.





      Discussion about whether should use python tricks like ~:



      In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.



      For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.



      def findStrobogrammatic(self, n):
      nums = n % 2 * list('018') or ['']
      while n > 1:
      n -= 2
      # n < 2 is so genius here
      nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
      return nums






      share|improve this answer














      share|improve this answer



      share|improve this answer








      edited 9 hours ago

























      answered 18 hours ago









      recnacrecnac

      1,9222524




      1,9222524













      • Comments are not for extended discussion; this conversation has been moved to chat.

        – Samuel Liew
        2 hours ago



















      • Comments are not for extended discussion; this conversation has been moved to chat.

        – Samuel Liew
        2 hours ago

















      Comments are not for extended discussion; this conversation has been moved to chat.

      – Samuel Liew
      2 hours ago





      Comments are not for extended discussion; this conversation has been moved to chat.

      – Samuel Liew
      2 hours ago













      90














      list[-1]


      Is short hand for:



      list[len(list)-1]


      The len(list) part is implicit. That's why the -1 is the last element. That goes for any negative index - the subtraction from len(list) is always implicit






      share|improve this answer



















      • 5





        In my opinion this answer is better than the accepted one.

        – NicolasB
        14 hours ago






      • 3





        Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

        – plugwash
        10 hours ago
















      90














      list[-1]


      Is short hand for:



      list[len(list)-1]


      The len(list) part is implicit. That's why the -1 is the last element. That goes for any negative index - the subtraction from len(list) is always implicit






      share|improve this answer



















      • 5





        In my opinion this answer is better than the accepted one.

        – NicolasB
        14 hours ago






      • 3





        Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

        – plugwash
        10 hours ago














      90












      90








      90







      list[-1]


      Is short hand for:



      list[len(list)-1]


      The len(list) part is implicit. That's why the -1 is the last element. That goes for any negative index - the subtraction from len(list) is always implicit






      share|improve this answer













      list[-1]


      Is short hand for:



      list[len(list)-1]


      The len(list) part is implicit. That's why the -1 is the last element. That goes for any negative index - the subtraction from len(list) is always implicit







      share|improve this answer












      share|improve this answer



      share|improve this answer










      answered 18 hours ago









      DroidX86DroidX86

      2,271919




      2,271919








      • 5





        In my opinion this answer is better than the accepted one.

        – NicolasB
        14 hours ago






      • 3





        Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

        – plugwash
        10 hours ago














      • 5





        In my opinion this answer is better than the accepted one.

        – NicolasB
        14 hours ago






      • 3





        Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

        – plugwash
        10 hours ago








      5




      5





      In my opinion this answer is better than the accepted one.

      – NicolasB
      14 hours ago





      In my opinion this answer is better than the accepted one.

      – NicolasB
      14 hours ago




      3




      3





      Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

      – plugwash
      10 hours ago





      Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.

      – plugwash
      10 hours ago











      11














      Because -0 in Python is 0.

      With 0 you get first element of list and
      with -1 you get the last element of the list.



      list = ["a", "b", "c", "d"]
      print(list[0]) # "a"
      print(list[-1]) # "d"





      share|improve this answer


























      • I think -0 is 0 pretty much everywhere.

        – Koray Tugay
        11 hours ago






      • 2





        @KorayTugay Except in floating point.

        – Barmar
        10 hours ago











      • I guess that is not 0 anymore. @Barmar

        – Koray Tugay
        10 hours ago






      • 1





        @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

        – curiousdannii
        3 hours ago
















      11














      Because -0 in Python is 0.

      With 0 you get first element of list and
      with -1 you get the last element of the list.



      list = ["a", "b", "c", "d"]
      print(list[0]) # "a"
      print(list[-1]) # "d"





      share|improve this answer


























      • I think -0 is 0 pretty much everywhere.

        – Koray Tugay
        11 hours ago






      • 2





        @KorayTugay Except in floating point.

        – Barmar
        10 hours ago











      • I guess that is not 0 anymore. @Barmar

        – Koray Tugay
        10 hours ago






      • 1





        @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

        – curiousdannii
        3 hours ago














      11












      11








      11







      Because -0 in Python is 0.

      With 0 you get first element of list and
      with -1 you get the last element of the list.



      list = ["a", "b", "c", "d"]
      print(list[0]) # "a"
      print(list[-1]) # "d"





      share|improve this answer















      Because -0 in Python is 0.

      With 0 you get first element of list and
      with -1 you get the last element of the list.



      list = ["a", "b", "c", "d"]
      print(list[0]) # "a"
      print(list[-1]) # "d"






      share|improve this answer














      share|improve this answer



      share|improve this answer








      edited 18 hours ago

























      answered 18 hours ago









      AshishAshish

      1,457630




      1,457630













      • I think -0 is 0 pretty much everywhere.

        – Koray Tugay
        11 hours ago






      • 2





        @KorayTugay Except in floating point.

        – Barmar
        10 hours ago











      • I guess that is not 0 anymore. @Barmar

        – Koray Tugay
        10 hours ago






      • 1





        @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

        – curiousdannii
        3 hours ago



















      • I think -0 is 0 pretty much everywhere.

        – Koray Tugay
        11 hours ago






      • 2





        @KorayTugay Except in floating point.

        – Barmar
        10 hours ago











      • I guess that is not 0 anymore. @Barmar

        – Koray Tugay
        10 hours ago






      • 1





        @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

        – curiousdannii
        3 hours ago

















      I think -0 is 0 pretty much everywhere.

      – Koray Tugay
      11 hours ago





      I think -0 is 0 pretty much everywhere.

      – Koray Tugay
      11 hours ago




      2




      2





      @KorayTugay Except in floating point.

      – Barmar
      10 hours ago





      @KorayTugay Except in floating point.

      – Barmar
      10 hours ago













      I guess that is not 0 anymore. @Barmar

      – Koray Tugay
      10 hours ago





      I guess that is not 0 anymore. @Barmar

      – Koray Tugay
      10 hours ago




      1




      1





      @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

      – curiousdannii
      3 hours ago





      @KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.

      – curiousdannii
      3 hours ago











      5














      This is the mnemonic method I use. It is just an approach of what is happening, but it works.





      Don't think of those as indexes. Think of them as offsets on a circular list.



      Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:



      enter image description here



      You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).



      If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)






      share|improve this answer


























      • "Think about a[2] and a[-2]" is a the list, or an element of the list?

        – detly
        4 hours ago











      • "a" is an hypothetical list which contains the values a-h! I'll clarify!

        – T. Sar
        38 mins ago


















      5














      This is the mnemonic method I use. It is just an approach of what is happening, but it works.





      Don't think of those as indexes. Think of them as offsets on a circular list.



      Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:



      enter image description here



      You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).



      If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)






      share|improve this answer


























      • "Think about a[2] and a[-2]" is a the list, or an element of the list?

        – detly
        4 hours ago











      • "a" is an hypothetical list which contains the values a-h! I'll clarify!

        – T. Sar
        38 mins ago
















      5












      5








      5







      This is the mnemonic method I use. It is just an approach of what is happening, but it works.





      Don't think of those as indexes. Think of them as offsets on a circular list.



      Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:



      enter image description here



      You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).



      If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)






      share|improve this answer















      This is the mnemonic method I use. It is just an approach of what is happening, but it works.





      Don't think of those as indexes. Think of them as offsets on a circular list.



      Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:



      enter image description here



      You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).



      If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)







      share|improve this answer














      share|improve this answer



      share|improve this answer








      edited 37 mins ago

























      answered 8 hours ago









      T. SarT. Sar

      181110




      181110













      • "Think about a[2] and a[-2]" is a the list, or an element of the list?

        – detly
        4 hours ago











      • "a" is an hypothetical list which contains the values a-h! I'll clarify!

        – T. Sar
        38 mins ago





















      • "Think about a[2] and a[-2]" is a the list, or an element of the list?

        – detly
        4 hours ago











      • "a" is an hypothetical list which contains the values a-h! I'll clarify!

        – T. Sar
        38 mins ago



















      "Think about a[2] and a[-2]" is a the list, or an element of the list?

      – detly
      4 hours ago





      "Think about a[2] and a[-2]" is a the list, or an element of the list?

      – detly
      4 hours ago













      "a" is an hypothetical list which contains the values a-h! I'll clarify!

      – T. Sar
      38 mins ago







      "a" is an hypothetical list which contains the values a-h! I'll clarify!

      – T. Sar
      38 mins ago













      2














      Another explanation:



      Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.



      Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.






      share|improve this answer








      New contributor




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

























        2














        Another explanation:



        Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.



        Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.






        share|improve this answer








        New contributor




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























          2












          2








          2







          Another explanation:



          Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.



          Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.






          share|improve this answer








          New contributor




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










          Another explanation:



          Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.



          Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.







          share|improve this answer








          New contributor




          Oscar 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 answer



          share|improve this answer






          New contributor




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









          answered 13 hours ago









          OscarOscar

          213




          213




          New contributor




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





          New contributor





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






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























              0














              This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i elements. -1 referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.



              For any list xs and index i positive or negative, the expression



              xs[i]


              will either have the same value as the expression below or produce an IndexError:



              xs[i % len(xs)]


              The index of the last element is -1 + len(xs) which is congruent to -1 mod len(xs). For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.



              In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs) or smaller than -1-len(xs) are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.






              share|improve this answer




























                0














                This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i elements. -1 referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.



                For any list xs and index i positive or negative, the expression



                xs[i]


                will either have the same value as the expression below or produce an IndexError:



                xs[i % len(xs)]


                The index of the last element is -1 + len(xs) which is congruent to -1 mod len(xs). For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.



                In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs) or smaller than -1-len(xs) are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.






                share|improve this answer


























                  0












                  0








                  0







                  This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i elements. -1 referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.



                  For any list xs and index i positive or negative, the expression



                  xs[i]


                  will either have the same value as the expression below or produce an IndexError:



                  xs[i % len(xs)]


                  The index of the last element is -1 + len(xs) which is congruent to -1 mod len(xs). For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.



                  In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs) or smaller than -1-len(xs) are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.






                  share|improve this answer













                  This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i elements. -1 referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.



                  For any list xs and index i positive or negative, the expression



                  xs[i]


                  will either have the same value as the expression below or produce an IndexError:



                  xs[i % len(xs)]


                  The index of the last element is -1 + len(xs) which is congruent to -1 mod len(xs). For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.



                  In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs) or smaller than -1-len(xs) are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.







                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 2 hours ago









                  Gregory NisbetGregory Nisbet

                  3,24321334




                  3,24321334























                      -4














                      In a comment, abraham asked




                      I wish python has another mechanism to get list backward.




                      For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:



                      lst = ["a", "b", "c", "d"]
                      print(lst[::-1][0]) # "d"
                      print(lst[::-1][1]) # "c"





                      share|improve this answer










                      New contributor




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
















                      • 2





                        True, but not really answer the question "why".

                        – Valentino
                        15 hours ago






                      • 4





                        It's a waste of ressources to create a whole new list in order to retrieve a single element.

                        – Eric Duminil
                        14 hours ago











                      • What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                        – Sam
                        11 hours ago













                      • @Sam Why use C when we got Assembly?

                        – mid
                        11 hours ago
















                      -4














                      In a comment, abraham asked




                      I wish python has another mechanism to get list backward.




                      For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:



                      lst = ["a", "b", "c", "d"]
                      print(lst[::-1][0]) # "d"
                      print(lst[::-1][1]) # "c"





                      share|improve this answer










                      New contributor




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
















                      • 2





                        True, but not really answer the question "why".

                        – Valentino
                        15 hours ago






                      • 4





                        It's a waste of ressources to create a whole new list in order to retrieve a single element.

                        – Eric Duminil
                        14 hours ago











                      • What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                        – Sam
                        11 hours ago













                      • @Sam Why use C when we got Assembly?

                        – mid
                        11 hours ago














                      -4












                      -4








                      -4







                      In a comment, abraham asked




                      I wish python has another mechanism to get list backward.




                      For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:



                      lst = ["a", "b", "c", "d"]
                      print(lst[::-1][0]) # "d"
                      print(lst[::-1][1]) # "c"





                      share|improve this answer










                      New contributor




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










                      In a comment, abraham asked




                      I wish python has another mechanism to get list backward.




                      For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:



                      lst = ["a", "b", "c", "d"]
                      print(lst[::-1][0]) # "d"
                      print(lst[::-1][1]) # "c"






                      share|improve this answer










                      New contributor




                      Michael Engen 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 answer



                      share|improve this answer








                      edited 14 hours ago





















                      New contributor




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









                      answered 16 hours ago









                      Michael EngenMichael Engen

                      12




                      12




                      New contributor




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






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








                      • 2





                        True, but not really answer the question "why".

                        – Valentino
                        15 hours ago






                      • 4





                        It's a waste of ressources to create a whole new list in order to retrieve a single element.

                        – Eric Duminil
                        14 hours ago











                      • What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                        – Sam
                        11 hours ago













                      • @Sam Why use C when we got Assembly?

                        – mid
                        11 hours ago














                      • 2





                        True, but not really answer the question "why".

                        – Valentino
                        15 hours ago






                      • 4





                        It's a waste of ressources to create a whole new list in order to retrieve a single element.

                        – Eric Duminil
                        14 hours ago











                      • What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                        – Sam
                        11 hours ago













                      • @Sam Why use C when we got Assembly?

                        – mid
                        11 hours ago








                      2




                      2





                      True, but not really answer the question "why".

                      – Valentino
                      15 hours ago





                      True, but not really answer the question "why".

                      – Valentino
                      15 hours ago




                      4




                      4





                      It's a waste of ressources to create a whole new list in order to retrieve a single element.

                      – Eric Duminil
                      14 hours ago





                      It's a waste of ressources to create a whole new list in order to retrieve a single element.

                      – Eric Duminil
                      14 hours ago













                      What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                      – Sam
                      11 hours ago







                      What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?

                      – Sam
                      11 hours ago















                      @Sam Why use C when we got Assembly?

                      – mid
                      11 hours ago





                      @Sam Why use C when we got Assembly?

                      – mid
                      11 hours ago










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










                      draft saved

                      draft discarded


















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













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












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
















                      Thanks for contributing an answer to Stack Overflow!


                      • Please be sure to answer the question. Provide details and share your research!

                      But avoid



                      • Asking for help, clarification, or responding to other answers.

                      • Making statements based on opinion; back them up with references or personal experience.


                      To learn more, see our tips on writing great answers.




                      draft saved


                      draft discarded














                      StackExchange.ready(
                      function () {
                      StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55684960%2fwhy-does-python-start-at-index-1-when-indexing-a-list-from-the-end%23new-answer', 'question_page');
                      }
                      );

                      Post as a guest















                      Required, but never shown





















































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown

































                      Required, but never shown














                      Required, but never shown












                      Required, but never shown







                      Required, but never shown







                      Popular posts from this blog

                      Færeyskur hestur Heimild | Tengill | Tilvísanir | LeiðsagnarvalRossið - síða um færeyska hrossið á færeyskuGott ár hjá færeyska hestinum

                      He _____ here since 1970 . Answer needed [closed]What does “since he was so high” mean?Meaning of “catch birds for”?How do I ensure “since” takes the meaning I want?“Who cares here” meaningWhat does “right round toward” mean?the time tense (had now been detected)What does the phrase “ring around the roses” mean here?Correct usage of “visited upon”Meaning of “foiled rail sabotage bid”It was the third time I had gone to Rome or It is the third time I had been to Rome

                      Slayer Innehåll Historia | Stil, komposition och lyrik | Bandets betydelse och framgångar | Sidoprojekt och samarbeten | Kontroverser | Medlemmar | Utmärkelser och nomineringar | Turnéer och festivaler | Diskografi | Referenser | Externa länkar | Navigeringsmenywww.slayer.net”Metal Massacre vol. 1””Metal Massacre vol. 3””Metal Massacre Volume III””Show No Mercy””Haunting the Chapel””Live Undead””Hell Awaits””Reign in Blood””Reign in Blood””Gold & Platinum – Reign in Blood””Golden Gods Awards Winners”originalet”Kerrang! Hall Of Fame””Slayer Looks Back On 37-Year Career In New Video Series: Part Two””South of Heaven””Gold & Platinum – South of Heaven””Seasons in the Abyss””Gold & Platinum - Seasons in the Abyss””Divine Intervention””Divine Intervention - Release group by Slayer””Gold & Platinum - Divine Intervention””Live Intrusion””Undisputed Attitude””Abolish Government/Superficial Love””Release “Slatanic Slaughter: A Tribute to Slayer” by Various Artists””Diabolus in Musica””Soundtrack to the Apocalypse””God Hates Us All””Systematic - Relationships””War at the Warfield””Gold & Platinum - War at the Warfield””Soundtrack to the Apocalypse””Gold & Platinum - Still Reigning””Metallica, Slayer, Iron Mauden Among Winners At Metal Hammer Awards””Eternal Pyre””Eternal Pyre - Slayer release group””Eternal Pyre””Metal Storm Awards 2006””Kerrang! 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