Hcfyk

I am trying to generate a binary matrix (whose elements are 0 or 1) of dimension 20x20. To do this, I want to supply as input the number of matrix elements that will take value 1. After, I want to draw randomly distinct position of those elements (values 1). I thought of doing this:

n = 20; (*matrix dimension*)
d = 300; (*number of matrix elements that will assume value 1*)
rules = RandomInteger[1, n, d, 2]; (*defines the position of the matrix elements*)
rules2 = Table[rules[[i]] -> 1, i, Length[rules]]; (*applies the list of random positions the value 1*)
s = SparseArray[rules2] (*creates the binary random matrix*)

However, this method is not efficient because it does not create 300 different random positions (some are repeated). For example, the result appears 212 filled matrix elements (many of the positions contains summed values).

SparseArray[<212>,20,20]

I would like to know if anyone could help me solve this problem of generating 300 numbers 1 in random positions in a 20x20 dimension matrix.

Thanks in advance

This might work:

n = 20;
d = 300;
s = Join[ConstantArray[1, d], ConstantArray[0, n^2 - d]];
a = Partition[RandomSample[s], n]

Total[a, 2]

300

r = RandomSample[Range[400], 300];
q = Array[0 &, 400];
q[[r]] = 1;
Q = ArrayReshape[q, 20, 20] // MatrixForm

In a situation where generating all admissible matrix indices can get prohibitive (e.g. the result of Tuples[] having too many elements), here is an approach that generates just the needed nonzero indices:

(* random k-subset *)
rs[n_, k_] := Take[PermutationList[RandomPermutation[n]], k]

BlockRandom[SeedRandom[1023, Method -> "ExtendedCA"]; (* for reproducibility *)
 With[n = 20, p = 300,
 Block[k = 1, idl, id,
 idl = rs[n, 2];
 While[k < p, id = rs[n, 2];
 If[! MemberQ[idl, id], k++; AppendTo[idl, id]]];
 mat = SparseArray[idl -> 1, n, n]]]];

Check:

Total[mat, 2]
 300

Here is a variation of JM's comment to mjw's answer that will be much faster when large matrices (e.g., 10^5 by 10^5) are to be created:

randomBinary[dim_, count_] := ArrayReshape[
 SparseArray[Thread[RandomSample[1;;dim^2, count]->1], dim^2],
 dim, dim
] 

The key idea is that one can use Span (e.g., 1;;max) as the first argument of RandomSample. For example:

mat = randomBinary[10^5, 300]; //RepeatedTiming
Total[mat, Infinity]

0.000327, Null

300

Of the other answers, only JM's answer will be able to produce a result, and does so about 4 orders of magnitude more slowly.

sa = SparseArray[RandomSample[Tuples[Range@20, 2], 300] -> 1, 20, 20]

Total[sa, 2]

300

Alternatively, without Tuples:

a2 = Unitize @ Threshold[RandomReal[1, 20, 20], "LargestValues", 300]

a3 = Partition[SparseArray[Partition[RandomSample[Range[20^2], 300], 1] -> 1, 20^2], 20]

a4 = SparseArray[(1 + QuotientRemainder[RandomSample[Range[20^2 - 1], 300], 20]) -> 1, 
 20, 20]

Total[#, 2] & /@ a2, a3, a4

300, 300, 300