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Another question on eliminating for loops....

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I would like to eliminate for loops in the following code,
L=3; N=L; R=L^2 * N^2;
mat = int8(zeros(R,4));
ind = 1;
for n = 0:N-1
for p = 0:N-1
for l = 0:L-1
for q = 0:L-1
mat(ind,:) = [n,p,l,q];
ind = ind + 1;
end
end
end
end
Would welcome any insights.

Accepted Answer

John D'Errico
John D'Errico on 23 Nov 2022
Edited: John D'Errico on 23 Nov 2022
Try it. If you can't see ndgrid doing it, then look harder. :) (By the way, lower case l (L) is a really bad variable name to use. DID I write a lower case L there, or was it the number 1, or perhaps an upper case I (i)?) Depending on the font, all of those characters/numbers can easily be confused. Your code will suffer from nasty bugs one day, due to typos, that you will never be able to find.
Anyway...
L=3; N=L;
[x,y,z,w] = ndgrid(0:N-1,0:N-1,0:L-1,0:L-1);
xyzw = [x(:),y(:),z(:),w(:)]
xyzw = 81×4
0 0 0 0 1 0 0 0 2 0 0 0 0 1 0 0 1 1 0 0 2 1 0 0 0 2 0 0 1 2 0 0 2 2 0 0 0 0 1 0
If you don't like the sequence generated by ndgrid with the first column moving fastest, then a sort will fix that.
xyzw = sortrows(xyzw,[1 2 3 4])
xyzw = 81×4
0 0 0 0 0 0 0 1 0 0 0 2 0 0 1 0 0 0 1 1 0 0 1 2 0 0 2 0 0 0 2 1 0 0 2 2 0 1 0 0

More Answers (1)

Steven Lord
Steven Lord on 23 Nov 2022
Take a look at the ndgrid function.
  3 Comments
Torsten
Torsten on 23 Nov 2022
Edited: Torsten on 23 Nov 2022
Ok, the order of the rows within the matrix has changed, but does it matter ?
Howard Wilton
Howard Wilton on 23 Nov 2022
i didn't see it, but yes... sortrows and all is good.

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