Multiplying even and odd columns separately?

I have a matrix called 'displacements'

It has 50 columns

I want to multiply all the even columns by 5 and all the odd columns by 8

Anyone know a quick way to do this?

Answer 1

[Stephen23]

You could do this directly using matrix indexing :

>> A = rand(4,50);
>> A(:,1:2:end) = A(:,1:2:end) * 8;
>> A(:,2:2:end) = A(:,2:2:end) * 5;

Or using bsxfun and repmat:

>> B = bsxfun(@times, A, repmat([8,5],1,25));

Answer 2

[Giorgos Papakonstantinou]

Here is one way:

A =rand(100, 50);

idx = mod(1:size(A,2), 2) ==1; % logical indexing of the odd numbers

theodd  = A(:, idx)*8; 
theeven =   A(:, ~idx)*5;