How to grow a vector in a loop?

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.

Answer 1

[Matt J]

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);
                

[Saurabh Tyagi]

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]

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

Answer 2

[Scott MacKenzie]

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)];

[Saurabh Tyagi]

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]

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

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

Answer 3

[Matt J]

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

[Saurabh Tyagi]

Oh that "mm" thing was such an elegant fix to this problem. I have been breaking my head over it for more than a day.