Multiple loop variables Matlab

Question

In C++/C, we have multiple loop variables in a single loop, like for(int i=0; int j=0; i<5; j<5; i++; j++) Is there any facility in Matlab for multiple variables loop? And also, I'm very conscious in loop iterations computations, so does it effects the speed as I'v already a nested loop in Matlab.

Answer 1

MATLAB sort of supports multiple loop variables in that it supports a matrix as the loop expression. How does that work? Individual columns of the matrix are assigned to the loop variable at the beginning of each iteration.

Example code:

V = [1:1:5; 2:2:10]
for iv = V,
    fprintf('iv = [%d %d];\n',iv);
end 

Output:

V =
     1     2     3     4     5
     2     4     6     8    10

iv = [1 2];
iv = [2 4];
iv = [3 6];
iv = [4 8];
iv = [5 10];

We've achieved two loop variables here, iv(1) and iv(2), which are specified by the rows of the matrix used as the loop expression. Note that the array can be any type (e.g. string, cell, struct, etc.).

Summary

Pre-define each iteration of the loop variables, and store them as the rows of the matrix. Inside the loop, the loop variable will contain a column of the matrix.

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Side note

I'm guessing that this convention is a consequence of the fact that the colon operator produces an array by horizontal concatenation rather than vertical. Just consider what happens in the following case:

for ii = (1:3).', numel(ii), end

You might be expecting three iterations, each indicating numel(ii)=1, but you only get one iteration and the loop reports:

ans =
     3

The problem is clear if you are expecting ii to be a scalar.

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Terminology

for loop_variable = loop_expression, statement, ..., statement end

Answer 2

MATLAB doesn't have the capability of doing multiple loop variables, you will have to use nested for-loops. That said, one of MATLAB's greatest strengths is efficiently applying a function across an array.

For example:

a = zeros(1,5);
for i=1:5
    a(i) = sin(i);
end

b = sin(1:5);

In the above example a & b will be identical, but calculating b doesn't require an explicit for-loop. There are times when explicit for-loops (including nested loops) are necessary (like running a simulation via the sim command), but since you are concerned about the time to compute the loop iterations, my guess is you aren't running time-intensive tasks like a massive simulation.

So rather than using nested for-loops, I'd look into setting up your functions to work with arrays and input your "loop variables" as the arrays. Look into the commands meshgrid & griddata to help create those arrays.

Answer 3

As chappjc pointed out, and as MathWorks states in the documentation, each iteration of a for loop takes the next column of the iterator. Thus, to iterate through a column vector, for example, one must transpose it (i.e. for ii = [1; 1; 2; 3; 5]'), otherwise ii is equal to the column vector, all at once.

And merely to extend chappjc's excellent answer, you may take advantage of this behavior with cells, also, where you might have some differently sized strings, in addition to wanting a numeric iterator, and then you can deal them to variables so you don't have to do as much indexing. Here is a crude example:

figure(1)
imageList = {};
for ii = [{somePath; someDirListing; 1}, {anotherPath; anotherDirListing; 2}] % Each iteration takes one column

    [pathname, images, iPos] = deal(ii{:});

    for iImage = images
        img = imread(fullfile(pathname, iImage));
        imagesc(img)
        axis image
        if iPos == 1
            title(['This is a left image, titled ' iImage])
        else
            title(['This is a right image, titled ' iImage])
        end
        pause(1)
    end
end