Hi, is it possible to count the number of ones in a matrix ?

Example: Suppose we a have a matrix A=[1 0 0 1 0 0; 1 0 0 0 1 1]; how do I count the number of ones in this matrix??

Answer 1

[Ahmet Cecen]

sum(A(:) == 1);

[nafila aytija]

Thank you Ahmet....This works perfectly fine. Thank you very much

[Yong-Joo Kwon]

more simply sum(A)

[Walter Roberson]

sum(A) does not quite work as that would produce one sum per column. sum(A(:)) would work, provided that the values are only 0 and 1.

If we know that the values are only 0 and 1, then we can use nnz(A) which is equivalent (but faster) to sum(A(:) ~= 0)

[Shilpi Mishra]

Is there a simple way to get the indices of those 1s in the matrix too, when matrix is symmetric?

[Harsh Agarwal]

find(A==1)

[Walter Roberson]

A = randi([0 1], 5, 5); A = xor(A,A.'); %random symmetric
A

A = 5×5 logical array
   0   0   0   1   1
   0   0   0   0   1
   0   0   0   0   1
   1   0   0   0   0
   1   1   1   0   0

[r,c] = find(A);
mask = r>=c;
r = r(mask); c = c(mask);
[r,c]

ans = 4×2

4 1 5 1 5 2 5 3

or

[r,c] = find(tril(A))

r = 4×1

4 5 5 5

c = 4×1

1 1 2 3