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Question

I have a matrix z (say 200x5) where only one element is non-zero in each row. What would be the most efficient way of finding that index without using for loops.

For example:

z=[1 0 0;0 0 1];
a=findRow(z)

where a should display:

[1 3]

Answer 1

Turns out that 'find' is the solution, except that I have to transpose z:

[rowidx,~]=find(z');

edit: Have to transpose it first because matlab finds things column first. Therefore the order of row indices is off if you don't transpose.

Answer 2

If you have a matrix with mostly 0 elements, you could consider usingsparse matrices which are designed to be spatially efficient in such cases:

z = sparse([1,0,0;0,0,1]);

You can still just use find to get your answer

[~,idx] = find(z);