MATLAB: Element replacement of matrix.

matrix manipulation

How do I replace each element in third row of the 5*4 matrix with its corresponding column number using for loop?

If using a for loop is not actually a requirement you can do this very simply as such:

myMatrix = rand(5,4); % create 5x4 matrix

myMatrix(3,:) = 1:4; % replace values

If the for loop is a requirement:

myMatrix = rand(5,4); % create 5x4 matrix
for n = 1:4
    myMatrix(3,n) = n;
end

You can do that with simple vector multiplication:

A = ones(5, 1) * (1:5);

a(a(:,3)>5,:)=[];