Skipping even indices in a for-loop and subsequently avoiding the odd values to appear in the output

Question

Jigsaw 2019-4-15

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/456478-skipping-even-indices-in-a-for-loop-and-subsequently-avoiding-the-odd-values-to-appear-in-the-output

回答： Dennis 2019-4-15

Through a nested-for-loop for n=1 and m=1,3 i want to acquire a matrix A with elements a1 and a2

m_length = 3;
n_length = 1;
for n=1:1:n_length
    for m=1:2:m_length
                                 
        a1(n,m) = ((n-1)+m);
        a2(n,m) = ((n-1)+m);
                        
    end
end
A = [a1 a2];

The above would output A as

A = [1,0,3,1,0,3]

It takes into account the even values of m too (and equates them to 0).

But what i require is purely the odd values of m (i.e. m=1,3) in the matrix.

What i would like to see is:

A = [1,3,1,3]

PS: In the original problem, n = 0,1,2,3,4...n_length and m = 1,3,5,7,9...m_length.

I shortened m and n and their total lengths for ease of explaining the problem.

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Answer 1

Dennis 2019-4-15

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https://ww2.mathworks.cn/matlabcentral/answers/456478-skipping-even-indices-in-a-for-loop-and-subsequently-avoiding-the-odd-values-to-appear-in-the-output#answer_370722

在 MATLAB Online 中打开

When you assign a value to A(3,3)=1, Matlab will create a Matrix of the size 3,3 and will fill empty fields with 0.

A(3,3)=1
A =
     0     0     0
     0     0     0
     0     0     1         

This is what is happening in your code. One possible solution might be to fix the index of your matrix:

m_length = 3;
n_length = 1;
for n=1:1:n_length
    for m=1:2:m_length
                                 
        a1(n,(m+1)/2) = ((n-1)+m);
        a2(n,(m+1)/2) = ((n-1)+m);
                        
    end
end
A = [a1 a2];