Indexing issue- using negative values in for loop calculations.

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Shrishti Yadav 2021-10-30

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https://ww2.mathworks.cn/matlabcentral/answers/1575478-indexing-issue-using-negative-values-in-for-loop-calculations

评论： Shrishti Yadav 2021-10-30

I would like to store the values Zx and Zy from the code below in Z but not sure how.

clc;
u = 4
Z = zeros(u*u,1);
cx = 1;
cy = 1;
r = 3;
for j = 1:(u*u)
    for x = -u:u
        for y = -u:u
            
            theta = atan2((y-cy) ,(x-cx));
            Zx = cx-r*cos(theta) 
            Zy = cy - r*sin(theta)
            Z(j,1) = (Zx);
            Z(j,2) = (Zy);
            
            
        end
    end 
end

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Answer 1

DGM 2021-10-30

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https://ww2.mathworks.cn/matlabcentral/answers/1575478-indexing-issue-using-negative-values-in-for-loop-calculations#answer_820423

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It's not clear why you're overwriting your results and then repeating the process 16 times anyway. If you want Z for all x and all y, then the outer loop doesn't do anything.

u = 4;
Z = zeros(u*u,1);
cx = 1;
cy = 1;
r = 3;
x = -u:u;
y = (-u:u).';
theta = atan2((y-cy) ,(x-cx));
Zx = cx - r*cos(theta)
Zx = 9×9
    3.1213    2.8741    2.5435    2.1142    1.5883    1.0000    0.4117   -0.1142   -0.5435
    3.3426    3.1213    2.8000    2.3416    1.7276    1.0000    0.2724   -0.3416   -0.8000
    3.5725    3.4000    3.1213    2.6641    1.9487    1.0000    0.0513   -0.6641   -1.1213
    3.7854    3.6833    3.4962    3.1213    2.3416    1.0000   -0.3416   -1.1213   -1.4962
    3.9417    3.9104    3.8460    3.6833    3.1213    1.0000   -1.1213   -1.6833   -1.8460
    4.0000    4.0000    4.0000    4.0000    4.0000   -2.0000   -2.0000   -2.0000   -2.0000
    3.9417    3.9104    3.8460    3.6833    3.1213    1.0000   -1.1213   -1.6833   -1.8460
    3.7854    3.6833    3.4962    3.1213    2.3416    1.0000   -0.3416   -1.1213   -1.4962
    3.5725    3.4000    3.1213    2.6641    1.9487    1.0000    0.0513   -0.6641   -1.1213
Zy = cy - r*sin(theta)
Zy = 9×9
    3.1213    3.3426    3.5725    3.7854    3.9417    4.0000    3.9417    3.7854    3.5725
    2.8741    3.1213    3.4000    3.6833    3.9104    4.0000    3.9104    3.6833    3.4000
    2.5435    2.8000    3.1213    3.4962    3.8460    4.0000    3.8460    3.4962    3.1213
    2.1142    2.3416    2.6641    3.1213    3.6833    4.0000    3.6833    3.1213    2.6641
    1.5883    1.7276    1.9487    2.3416    3.1213    4.0000    3.1213    2.3416    1.9487
    1.0000    1.0000    1.0000    1.0000    1.0000    1.0000    1.0000    1.0000    1.0000
    0.4117    0.2724    0.0513   -0.3416   -1.1213   -2.0000   -1.1213   -0.3416    0.0513
   -0.1142   -0.3416   -0.6641   -1.1213   -1.6833   -2.0000   -1.6833   -1.1213   -0.6641
   -0.5435   -0.8000   -1.1213   -1.4962   -1.8460   -2.0000   -1.8460   -1.4962   -1.1213

If you want Zx and Zy to be reshaped as column vectors, figure out how you want them reshaped:

Z = [Zx(:) Zy(:)]
Z = 81×2
1213    3.1213
3426    2.8741
5725    2.5435
7854    2.1142
9417    1.5883
0000    1.0000
9417    0.4117
7854   -0.1142
5725   -0.5435
8741    3.3426