multiply all numbers above a threshold in a matrix by n, but only on certain indices, and keep original matrix dimension

Hi,

I want to multiply all numbers above a threshold in a matrix by n, but only on certain indices, and keep the original matrix dimension

so consider I have a 201x201 matrix, but only want to do my multiplication where y=1:115, I can do something like

matrix = rand(201,201);
matrix(matrix(:, 115) > 0.5) = 0; 

which works great; but I want to something like

matrix(matrix(:, 115) > 0.5) = matrix .* 100; 

but i can't of course because because the left and right sides have a different number of elements

I know this is super simple but please help me because I have a mind block

I think I need to make a logical mask, or something - but I can't think :-(

[madhan ravi]

y?

Answer 1

[Jan]

index              = (matrix(:, 115) > 0.5);  % logical mask
matrix(index, 115) = matrix(index, 115) * 100; 

[cgenes]

thanks!