Index of element to remove exceeds matrix dimensions

Question

ahmed mer 2023-3-14

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1928815-index-of-element-to-remove-exceeds-matrix-dimensions

回答： Voss 2023-3-14

Can you tell me why this erreer massage is popping up in this program (Index of element to remove exceeds matrix dimensions.)

% Mutation and Crossover
    for i = 1:Np
        idx = 1:Np;
        idx(i) = []; % Remove current solution from list
        a = idx(randi(Np-1));
        idx(a) = []; % Remove a from list
        b = idx(randi(Np-2));
        idx(b) = []; % Remove b from list
        c = idx(randi(Np-3));
        V = P(a,:) + F*(P(b,:) - P(c,:)); % Mutation
        jrand = randi(D,1); % Random index for crossover
        for j = 1:D
            if (rand <= CR) || (j == jrand)
                U(i,j) = V(j);
            else
                U(i,j) = P(i,j);
            end
        end
    end   

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Dyuman Joshi 2023-3-14

There are undefined variables in your code and thus we can't run the code.

Please update your code. Also, mentioned the full error message, that means all the red text.

John D'Errico 2023-3-14

编辑：John D'Errico 2023-3-14

Use the debugger. Set a breakpoint that will trigger on an error. Then look carefully at the variables involved in that line. As has been said, we can't debug code we can't run.

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Answer 1

Voss 2023-3-14

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Let's examine what happens on the first iteration of that for loop.

for i = 1:Np

First time through the loop, i is 1.

idx = 1:Np;

idx is 1:Np.

idx(i) = []; % Remove current solution from list

First time through the loop, the 1st element of idx is removed, so idx is now 2:Np.

a = idx(randi(Np-1));

randi(Np-1) generates a random integer between 1 and Np-1, so a is a random element of idx. That is, a is one of 2,3,4,...,Np, which are the elements of idx.

idx(a) = []; % Remove a from list

This removes element #a fom idx, i.e., element #2 if a is 2, element #3 if a is 3, and so on. But if a happens to be Np, then there is no element #a in idx, and you get the error you got. Remember idx is 2:Np at this point, which is a vector of length Np-1. There is no element #Np.

    b = idx(randi(Np-2));
    idx(b) = []; % Remove b from list

The same error can happen when removing element #b, for the same reason.

    c = idx(randi(Np-3));
    % more stuff
end

I suspect that what you meant to do is:

for i = 1:Np
    idx = 1:Np;
    idx(i) = [];     % Remove current solution from list
    a = randi(Np-1); % random integer between 1 and Np-1 (not 2 to Np)
    idx(a) = [];     % Remove element #a from idx
    b = randi(Np-2); % random integer between 1 and Np-2
    idx(b) = [];     % Remove element #b from idx
    c = randi(Np-3); % random integer between 1 and Np-3
    % more stuff
end