A loop within a loop (for or if)

Hi

I want to create a 3x3 matrix.I have a question regarding changing a value in an equation when i=the lenght of the column, which is 3.

The code so far:

Cm = [Cm1 Cm2 Cm3];
P = 3000

for i=1:3

        D2(i) = -(D1(i)+0.5); %mm
        Cg(i) = (D1(i)*D2(i))/(D1(i)+D2(i));

        a1(i) = 0.721*(P*Cg(i)*Cm(1)).^(1/3); %mm
        a2(i) = 0.721*(P*Cg(i)*Cm(2)).^(1/3); %mm
        a3(i) = 0.721*(P*Cg(i)*Cm(3)).^(1/3); %mm
        end

It works, if i manually change Cm. By doing it like this i can get a 3x3 matrix from:

a = [a1(:) a2(:) a3(:)]

But I want the Cm to change automatically. What to do? :S

Regards Tim

[KSSV]

Undefined function or variable 'D1'.

Error in first (line 4) D2(i) = -(D1(i)+0.5); %mm

Answer 1

[Ganesh Hegade]

Hi,

I didn't get exactly what you are trying to do. Hope this works.

Cm = [Cm1 Cm2 Cm3];
P = 3000
for i=1:3 % Better to use length(Cm) 

          D2(i) = -(D1(i)+0.5); %mm
          Cg(i) = (D1(i)*D2(i))/(D1(i)+D2(i));

          a1(i) = 0.721*(P*Cg(i)*Cm(i)).^(1/3); %mm
          a2(i) = 0.721*(P*Cg(i)*Cm(i)).^(1/3); %mm
          a3(i) = 0.721*(P*Cg(i)*Cm(i)).^(1/3); %mm
   end
a = [a1', a2', a3' ];

[Guillaume]

%Better to use length(Cm)

Actually, better to use numel(Cm). length is dangerous if the input is not a vector.

But of course, even better is not to use a loop...

Answer 2

[Guillaume]

I don't particularly understand your question. All I know is you don't need a loop to generate your final a.

I assume your D1 that is not shown is a 1x3 row vector. Then,

D2 = -(D1 + 0.5);  %note that numbering variables is never a good idea
Cg = D1 .* D2 ./ (D1 + D2);
%if using R2016b:
a = 0.721 * (P * Cg.' * Cm) .^ (1/3);
%if using an earlier version:
a = 0.721 * (P * bsxfun(@times, Cg.', Cm)) .^ (1/3);

Note that if D1 is a 3x1 column vector, then you don't need to transpose Cg in the a calculation.

If you want to calculate a for different Cm vectors, again you don't need a loop. Simply concatenate all these Cm vectors in the third dimension and the above will still work.