Another question on eliminating for loops....

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.

Answer 1

[John D'Errico]

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

Answer 2

[Steven Lord]

Take a look at the ndgrid function.

[Howard Wilton]

I just can't see ndgrid doing it Showing the first 30 of 81 rows...

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

0 1 0 1

0 1 0 2

0 1 1 0

0 1 1 1

0 1 1 2

0 1 2 0

0 1 2 1

0 1 2 2

0 2 0 0

0 2 0 1

0 2 0 2

0 2 1 0

0 2 1 1

0 2 1 2

0 2 2 0

0 2 2 1

0 2 2 2

1 0 0 0

1 0 0 1

1 0 0 2

[Torsten]

Ok, the order of the rows within the matrix has changed, but does it matter ?

[Howard Wilton]

i didn't see it, but yes... sortrows and all is good.