create a variable number of nested for loops? select elements in matrix with variables dimensions?

hi,

i have a fucntion with a matrix that has variables dimensions (depends on a parameter given in input) and i would like to scan all the elements inside, but the fact is that i don't know how to nest many for loops.

let´s make an example: M is a matrix 3 dimensional with length [2,4,6]

i would like to do:

for i1:1=2
for i2:1=4
for i3:1=6

M(i1,i2,i3) = 0;

end
end
end

as you can notice, i need 3 loops, but how to use N loops, with N the number of dimensions of M (N is fixed once that function is called)? i thought about using eval, but it seems not working..

another related question, i would like to access to element M(i1,i2,i3) if N=3 or M(i1,i2,i3,i4,i5) if N=5, should i use eval also in this case, or is there any better solution?

thanks in advance

Answer 1

[Walter Roberson]

The particular example you give can be written as

M(1:2,1:4,1:6) = 0;

With regards to the nested loop: turn them into a single loop using an "odometer" type mechanism. See my code in https://groups.google.com/group/comp.lang.c/browse_thread/thread/d98842738fa27a55/2f6d5ff877063de1 and Chris Torek's useful modification there.

With regard to variable number of subscripts:

subs = [i1, i2, i3, i4];
subscell = num2cell(subs);
M(subscell{:})

[Tom]

Would it be easier in this case to run through the variables sequentially?

for i=1:numel(M)

M(i)=0

end

[Walter Roberson]

If all of the elements are to be set to 0 without disturbing the size or shape, then M(:) = 0 is easier.

[Tom]

Indeed, though in that case it would seem pointless generating the matrix in the first place (or just use the zeros function), so I assume there's some other stuff later in the loop.

Answer 2

[Andrea]

thanks, i didnt have the time to check your discussion..

of course there is much code inside the loops, i wrote m= 0 only to write something random inside

Answer 3

[Henry Charlesworth]

So you can do it with eval although not sure it's particularly good practice:

    % 
nLoops=3;
str = '';
for i=1:nLoops
    str = sprintf('%s \n for i%d=1:5',str,i);
end
str = sprintf('%s \n [i1 i2 i3]',str);
for i=1:nLoops
    str = sprintf('%s \n end',str);
end
eval(str)

[Walter Roberson]

eval() is not recommended at all. It is fairly prone to mistakes, and always inefficient.