Number of Elements Between 2 Elements in a matrix.

Say I have a 3x5 matrix of zeros.

M = zeros(3,5)
00000
00000
00000

I change two elements in the matrix so the following happens.

M(1,3) = 1
M(3,3) = 2
00100
00000
00200

How do I count the number of elements between "1" and "2"?

Like below, it would be 11 elements within the brackets.

00[100
00000
002]00

As little lines of code as possible would be appreciated! Thank you.

Answer 1

[Star Strider]

MATLAB uses column-major ordering, so it ‘counts’ elements column-wise, and it is necessary to transpose ‘M’ to do what you want.

M = zeros(3,5);
M(1,3) = 1;
M(3,3) = 2;

idx = find(M');
Result = diff(idx)+1

Result =
    11

These can be combined into a one-line assignment:

Result = diff(find(M'))+1

It is necessary to add 1, because diff does not take into account the initial or final element when it does its subtraction.

Answer 2

[James Tursa]

Assuming the matrix has exactly two nonzero entries:

M = your matrix
f = find(M'); % Transpose to get the row data into column order
result = f(2) - f(1) + 1;

If you can't guarantee that there are exactly two nonzero entries, then you will have to add in appropriate checks etc and tell us what you would want for a result in that case.