How to grow a vector in a loop?

Question

Saurabh Tyagi 2021-10-15

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https://ww2.mathworks.cn/matlabcentral/answers/1564441-how-to-grow-a-vector-in-a-loop

评论： Saurabh Tyagi 2021-10-15

So I have two nested loops

for ii = 1:ldiv+1
    for jj = 1:sdiv+1
        
        x_m = sb_panel(ii,jj).xm;
        y_m = sb_panel(ii,jj).ym;
        z_m = sb_panel(ii,jj).zm;
        
        for kk = 1:ldiv+1          
            for ll = 1:sdiv+1
                
                x_a = sb_panel(kk,ll).xa;	
                y_a = sb_panel(kk,ll).ya;
                z_a = sb_panel(kk,ll).za;
                
                x_b = sb_panel(kk,ll).xb;	
                y_b = sb_panel(kk,ll).yb;
                z_b = sb_panel(kk,ll).zb;
                
                
                aa = 1/(((x_m-x_a)*(y_m-y_b)-(x_m-x_b)*(y_m-y_a)));
                bb = ((x_b-x_a)*(x_m-x_a)+(y_b-y_a)*(y_m-y_a))/sqrt((x_m-x_a)^2+(y_m-y_a)^2);
                cc = ((x_b-x_a)*(x_m-x_b)+(y_b-y_a)*(y_m-y_b))/sqrt((x_m-x_b)^2+(y_m-y_b)^2);
                dd = (1/(y_a-y_m))*(1+(x_m-x_a)/sqrt((x_m-x_a)^2+(y_m-y_a)^2));
                ee = (1/(y_b-y_m))*(1+(x_m-x_b)/sqrt((x_m-x_b)^2+(y_m-y_b)^2));
                
                coeff = [(aa*(bb-cc)+dd-ee)];
                
            end
        end
        
    end
end

The problem is the innermost loop runs as many times as it's supposed to and assigns the last value to 'coeff' but what I want is that each time it runs, it assigns a value to coeff, and then assign the value in next cell when it runs again.

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Answer 1

Matt J 2021-10-15

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https://ww2.mathworks.cn/matlabcentral/answers/1564441-how-to-grow-a-vector-in-a-loop#answer_809281

编辑：Matt J 2021-10-15

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You could just vectorize everything.

        x_m = [sb_panel.xm];
        y_m = [sb_panel.ym];
        z_m = [sb_panel.zm];
        
     
        x_a = [sb_panel.xa].';	
        y_a = [sb_panel.ya].';
        z_a = [sb_panel.za].';
        
        x_b = [sb_panel.xb].';	
        y_b = [sb_panel.yb].';
        z_b = [sb_panel.zb].';
        
        
        aa = 1./(((x_m-x_a).*(y_m-y_b)-(x_m-x_b).*(y_m-y_a)));
        bb = ((x_b-x_a).*(x_m-x_a)+(y_b-y_a).*(y_m-y_a))./sqrt((x_m-x_a).^2+(y_m-y_a).^2);
        cc = ((x_b-x_a).*(x_m-x_b)+(y_b-y_a).*(y_m-y_b))./sqrt((x_m-x_b).^2+(y_m-y_b).^2);
        dd = (1./(y_a-y_m)).*(1+(x_m-x_a)./sqrt((x_m-x_a).^2+(y_m-y_a).^2));
        ee = (1./(y_b-y_m)).*(1+(x_m-x_b)./sqrt((x_m-x_b).^2+(y_m-y_b).^2));
        
        coeff = reshape( aa.*(bb-cc)+dd-ee  , [],1);
                

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Saurabh Tyagi 2021-10-15

Yikes, this worked PERFECTLY. Thank you so much for you time. Can you point out my mistakes and more insight into what you did?

Matt J 2021-10-15

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Can you point out my mistakes

In your original approach? My other answer describes how to get that to work.

and more insight into what you did?

we first collected the fields of your stucture into vectors

s(1).a=10; s(2).a=20; s(3).a=30;
v=[s.a]
v = 1×3
    10    20    30

Then, you just use implicit expansion for all the arithmetic operations, e.g.,

v+(1:4).'
ans = 4×3
    11    21    31
    12    22    32
    13    23    33
    14    24    34

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Answer 2

Scott MacKenzie 2021-10-15

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One approach is to declare coeff as an empty array before the first for-statement:

coeff = [];

then add new values to the end of the coeff array as follows:

coeff = [coeff, (aa*(bb-cc)+dd-ee)];

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Saurabh Tyagi 2021-10-15

I tried doing that, didn't work. I do get correct answers, but in reverse order. Like if it should be [A B C D], I get [D C B A]

Scott MacKenzie 2021-10-15

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In that case, just reverse the order in the assignment:

coeff = [(aa*(bb-cc)+dd-ee), coeff];

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Answer 3

Matt J 2021-10-15

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1564441-how-to-grow-a-vector-in-a-loop#answer_809306

编辑：Matt J 2021-10-15

在 MATLAB Online 中打开

coef=nan((ldiv+1)^2*(sdiv+1)^2,1);  %PRE-ALLOCATE
mm=0;
for ii = 1:ldiv+1
    for jj = 1:sdiv+1
        
   ...
        
        for kk = 1:ldiv+1          
            for ll = 1:sdiv+1
                
                  ....
               
                mm=mm+1;  
                coeff(mm) = [(aa*(bb-cc)+dd-ee)];
                
            end
        end
        
    end
end