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Multiplying even and odd columns separately?

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Joe Smith
Joe Smith on 6 Mar 2015
Edited: Stephen on 22 May 2015
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?

Answers (2)

Stephen
Stephen on 6 Mar 2015
Edited: Stephen on 22 May 2015
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));

Giorgos Papakonstantinou
Giorgos Papakonstantinou on 6 Mar 2015
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;

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