'n' number of nested for loops

Question

Fawad Farooq Ashraf 2021-3-14

1
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/772268-n-number-of-nested-for-loops

编辑： Matt J 2021-3-14

采纳的回答： Matt J

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I have a cell array M with 4 cell elements and each cell element has a 8x2 matrix.

I'm trying to making different combinations using nested for loops as

n = length(M);
[s,d]  = cellfun(@size,M);
ncombs = prod(s);
P = cell(1,ncombs);
k = 0;
for i = 1:size(M{1},1)
    for j = 1:size(M{2},1)
        for x = 1:size(M{3},1)
            for y = 1:size(M{4},1)
                k = k+1;
                P{k} = [M{1}(i,:);M{2}(j,:);M{3}(x,:);M{4}(y,:)];
            end
        end
    end
end

Now in this case, I have n = 4 and hence 4 nested loops. Can I generalize this thing with any n? For example like for n = 2, 2 nested loops and correspondingly P{k} becomes [M{1}(i,:),M{2}(j,:)]

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Matt J 2021-3-14

I have a cell array M with 4 cell elements and each cell element has a 8x2 matrix.

It doesn't make much sense for them to be a cell array in that case. You could have an 8x2x4 matrix.

Fawad Farooq Ashraf 2021-3-14

You're right, they could be a 3-dimensional matrix but I pass the cell element to another library function which uses it as elements of a cell array.

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Answer 1

Matt J 2021-3-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/772268-n-number-of-nested-for-loops#answer_647293

编辑：Matt J 2021-3-14

在 MATLAB Online 中打开

[c{1:n}]=ndgrid(1:8);
c=reshape(cat(n+1,c{:})  [],n);
K=size(c,1);
Q=cell(n,1);
for i=1:n
    Q{i}=reshape(M{i}( c(:,n+1-i),: ), 1,[],K);
    
end
P=num2cell(vertcat(Q{:}), [1,2]);

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Fawad Farooq Ashraf 2021-3-14

在 MATLAB Online 中打开

Sorry to bother you again @Matt J. But I think I'm still not getting the correct results. It'd be a great help if you could take a look at the test script I'm trying to run.

%% 
M{1} = rand(8,2);
M{2} = rand(8,2);
%% Method 01 
n = length(M);
[s,d]  = cellfun(@size,M);
ncombs = prod(s);
P = cell(ncombs,1);
k = 0;
for i = 1:size(M{1},1)
    for j = 1:size(M{2},1)
                k = k+1;
                P{k} = [M{1}(i,:);M{2}(j,:)];
    end
end
%% Method 02
c = cell(1,n);
[c{1:n}] = ndgrid(1:size(M{1},1));
c = reshape(cat(n+1,c{:}),  [],n);
K = size(c,1);
Q = cell(n,1);
for i=1:n
    Q{i} = reshape(M{i}( c(:,n+1-i) , : ), 1,[],K);
end
P2 = num2cell(vertcat(Q{:}), [1,2]);