MATLAB: Creating a Variable Number of Nested ‘for’ Loops

for loopMATLAB

Hi all,

I am trying to iterate down every column of array x, and extract all combinations when the sum of these elements is equal to one.

At the moment this is being made possible by having a for loop iterate down each individual column, and then using an if statement to pause the loop and store these combinations in the array 'Combinations'.

N = 3;
x = zeros(6, N);
x(1, :) = 0;
x(2, :) = 0.2;
x(3, :) = 0.4;
x(4, :) = 0.6;
x(5, :) = 0.8;
x(6, :) = 1;
a = 0;
A = 1;
for i = 1:size(x, 1)
    for j = 1:size(x, 1)
        for k = 1:size(x, 1)
            X = x(i, 1) + x(j, 1) + x(k, 1);
            if X == 1
                Combinations(A, 1) = x(i, 1);
                Combinations(A, 2) = x(j, 2);
                Combinations(A, 3) = x(k, 3);
                a = a + 1;
                A = A + 1;
                continue
            end
        end
    end
end

However, if the number of columns in x were to change, I would need to be able to vary the number of for loops depending on the value of N. Is this possible/is there an alternative method of achieving a similar result?

Any help would be appreciated. Thanks!

Best Answer

Use https://www.mathworks.com/matlabcentral/fileexchange/10064-allcomb-varargin- allcomb() from the File Exchange, and then throw away the entries that do not match your constraint.

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In the line comb(n+1,X,A); the function comb is called, but this does not change the value of X, because the output of this function is not assigned. Do you mean x = comb(n+1,X,A)?

A recursive function might not be the best choice. Either use a method from the FEX, e.g. FEX: Combinator or FEX: VChooseKRO.

Or create a loop:

A       = [3 4 0;2 3 7;45 7 0];
[nRow, nCol] = size(A);
nResult = nCol ^ nRow;
index   = ones(1, nRow);
Result  = zeros(nRow, nResult);
for k = 1:nResult
  Result(:, k) = A(sub2ind(size(A), 1:nRow, index));
    % Update the index vector:
    for k = nRow:-1:1
      index(k) = index(k) + 1;
      if index(k) <= nCol
        break;       % index(k) increased successfully, leave "for k" loop
      end
      index(k) = 1;  % index(k) reached the limit, reset it and iterate index(k-1)
    end
  end

This is equivalent to nRow nested for loops, which count from 1 to nCol and insert the corresponding element into the output. Hard coding this does not work, because the number of loops is flexible. There instead of a bunch of for loops, this has one loop and the list of for loop indices is store in the vector indices. Each index value is incremented and if it exceeds nCol, it is reset to 1 and the next index element is processed.

MATLAB: Hi isnt do I get this error ‘ operator ‘*’ is not supported for operands of type struct’

The problem is with the line

t(K + 1) = t(K) * Time.In;

I have no idea what you're trying to even do here. Both t and Time are structs.

I don't know why you would even be trying to use structs for any of these variables. Do you mean to be using structs at all, or are you just using the . as a space?

 TimeIn = 0.7;
 rho =1.5; 
 A =1; 
 m = 1700; 
 Vfstream = 96.639; 
 a = 4.1202; 
 v = 0; 
 T = 7004.34; 
 t=0;
 for K = 1 : 50 
     v(K + 1) = v(K) + a(K) * TimeIn; 
     T(K + 1) = 1/2 * rho * A * (Vfstream ^ 2 - v(K) .^2); 
     a(K + 1) = T(K) / m; 
     b(K + 1) = v(K) * TimeIn; 
     t(K + 1) = t(K) * TimeIn; 
 end
 % the 'all' option is too recent for my version
 distance = sum(b(:))

This at least gives an answer. Of course, I have no idea if it's correct. I've done no analysis of the math or anything.

Assuming that's right, since the loop is a fixed length, it would be good practice to preallocate the vectors instead of growing them like this.

Best Answer

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MATLAB: Hi isnt do I get this error ‘ operator ‘*’ is not supported for operands of type struct’

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