How to check for a zero in any row of a particular column in an [50000, 50000]?

So I have an array of [50000, 50000] full of zeros. I gradually add elements to it by replacing the zeros with the elements. I want to add elements to a randomly picked column such that the element is placed at a lowest position of row available with zero.

Answer 1

[DGM]

Depends what you mean by "lowest position of row".

Consider the example:

firstzerorow = find(A(:,thiscolumn)==0,1,'first');
A(firstzerorow,thiscolumn) = mynumber;

or

lastzerorow = find(A(:,thiscolumn)==0,1,'last');
A(lastzerorow,thiscolumn) = mynumber;