# two - loop with GPU

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Anh on 3 Apr 2013
I just have GPU on my desktop with matlab R2012b I plotted a Gaussian profile using two - loop on CPUas below:
for n=1:1:N
for m=1:1:N
x(n)=n*ds-L/2;
y(m)=m*ds-L/2;
G(n,m)=exp(-((x(n)^2)+(y(m)^2)))/(w^2);
end
end
I read somewhere that GPU does not support index like that, I even tried but it not work either. Is there any way possible to do the same thing on GPU?
Thank so much
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Anh on 15 Apr 2013
All variables were on GPU (N, ds, N). I got the message: Undefined function 'colon' for input arguments of type 'gpuArray' Even one "For loop", if N is on GPU, it not work either. I know there is one way we can deal with "For loop" of an array by using gpuArray.colon.(...), but I have loop inside the loop to create a matrix G(n,m), so I don't know how to solve this problem.

James Lebak on 23 Apr 2013
Yes, 1:gpuArray(N) for scalar N doesn't work in MATLAB R2013a. Try the following:
x=gpuArray.colon(1,N)*ds-L/2;
y=gpuArray.colon(1,N)*ds-L/2;
G = exp(-((x.*x).'*gpuArray.ones(1,N)+gpuArray.ones(N,1)*(y.*y)))./(w^2);
This vectorizes the calculation of x and y and calculates all the elements in the matrix at once, which should be much more efficient on the GPU than using a loop.