avoiding nested for loops in MATLAB

Question

I have a scenario in MATLAB where I would like to evaluate a function for certain parameter values. The parameters are extracted from an arbitrary number of arrays, and each array can have an arbitrary number of elements. I know the number of arrays and the number of elements in them before I call the function.

For example, say I have arrays A = [a1 a2 ... aL], B = [b1 b2 ... bM] and C = [c1 c2 ... cN].

for i = 1:length(A)
    for j = 1:length(B)
        for k = 1:length(C)
            myfunc(A(i), B(j), C(k))
        end
     end
end

I am considering taking the L elements of A, M elements of B and N elements of C, and flatenning them into a cell array, and iterating with a single for loop over this cell array.

I was wondering if there was a MATLAB function that does something like this ... The result does not have to be a cell array. I want a way to avoid having multiple nested for loops. It is fine for a small number of loops, but as this number grows, it is pretty difficult to read and maintain.

Luis Mendo · Accepted Answer · 2014-11-26 20:31:53Z

4

ndgrid can be used to flatten several nested loops into one. It generates all combinations of values (variables aa, bb, cc in the code below), so that a single index (k below) can be used to traverse all combinations. But note that generating all combinations may required a lot of memory, depending on your L, M, N.

[cc, bb, aa] = ndgrid(C, B, A); %// outermost variable should be rightmost here
for k = 1:numel(aa)
     myfunc(aa(k), bb(k), cc(k));
end

edited Nov 26, 2014 at 20:31

answered Nov 25, 2014 at 23:27

Luis Mendo

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endbegin · Accepted Answer · 2014-12-01 20:16:10Z

0

Thanks to direction from the accepted answer, I made a function that generalizes to any number of arrays. The result is a 2D array of N-tuples - where N is the number of input arrays.

function [Result] = makeTuples(varargin)

nInputArgs = length(varargin);
b = cell(1, nInputArgs);
a = flip(varargin);

[b{:}] = ndgrid(a{:});
bb = flip(b);
nOutputs = length(bb);
N = numel(bb{1});

Result = zeros(N, nInputArgs);
arr = zeros(1,nInputArgs);
for j = 1:N
    for k = 1:nOutputs
        arr(k) = bb{k}(j);
    end
    Result(j,:) = arr;
    arr = zeros(1,nInputArgs);
end
end

answered Dec 1, 2014 at 20:16

endbegin

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