Matlab loop closes for no reason at all

I have a loop as follows:

g=zeros(dim,dim);
for np=1:dim
    b=2*pxHz*phantom(dim);b=b(:,np);
    b=dct(b);
    g(:,np)=dctCoeffs1d*dctGradVect(b,np);
    np
end
figure, imagesc(g),colorbar

and dctGradVect function is as follows:

function g = dctGradVect(b,np)
global imgg imgDistupg imgDistdowng dctCoeffs1d
imgVect=imgg(:,np);distUpVect=imgDistupg(:,np);distDownVect=imgDistdowng(:,np);
fieldVect=dctCoeffs1d*b;
[Ku,Kd]=Kv(fieldVect);
[Ku2,Kd2]=Kv2(fieldVect);
for p=1:length(b)
    for mp=1:length(imgVect)
        matu(p,mp)=trace(diag(dctCoeffs1d(:,p)).*diag(Ku2(mp,:)).*diag(imgVect));
        matd(p,mp)=trace(diag(dctCoeffs1d(:,p)).*diag(Kd2(mp,:)).*diag(imgVect));
    end
end

g=2*real(matu*conj(distUpVect-Ku*imgVect))+2*real(matd*conj(distDownVect-Kd*imgVect));

The loop crashes at some values of np. i.e. crashes at np=10. This crashing point changes every time. Also it gives an error message, some index exceeding some value. However, for a number of iterations this is not the case. What could be the proble? Thx in advance

Edit: imgg, imgDistupg and imgDistdowng are complex dim by dim matrices. dim is fixed and 32 for my case. dctCoeffs1d is a dim by dim matrix again and represents 1d discrete cosine transform.

Edit 2: The loop works fine at first. I mean g is calculated and everything is ok. However, if I run the loop again, it get stuck and MATLAB crashes

[Walter Roberson]

You have not posted enough to reproduce the problem.

Your function includes global variables, which makes debugging harder.

http://www.mathworks.com/help/matlab/math/parameterizing-functions.html

[Adam Danz]

Which line is producing the indexing error message? It must be one of these

• g(:,np)=dctCoeffs1d*dctGradVect(b,np);
• imgVect=imgg(:,np);distUpVect=imgDistupg(:,np);distDownVect=imgDistdowng(:,np);

If the error happens in the 2nd line above, break it up into 3 separate lines and tell us which part is causing the error.

One of the variables does not have space for the 10th index. It may be helpful for you to review the basics on indexing: https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

[VBBV]

What is dim when you create

%if true
  % code
%end
g = zeros(dim,dim)

Is it fixed value or changing ?

May be it's exceeding the limit of one of the vectors in your function

%if true
  % code
%end
function g = dctGradvect(b,np)

[Image Analyst]

Looks like you blew right past the posting guidelines that would have enabled a fast answer. Anyway, give us code where we can reproduce it. This means ALL code, even the code that defined the global variables. Also, give the exact error message - ALL THE RED TEXT - but only after you read this link.