Index of element to remove exceeds matrix dimensions

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   

[Dyuman Joshi]

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]

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.

Answer 1

[Voss]

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