Matrices question

Hi,

I have a script with two 'for' loops. Both of the loops involve matrices generated with 'linspace'-the first has N increments, and the second has P increments.

In the first loop, I have generated variables of the type A(N), B(N), etc. For the second loop, I want to craete a new variable of the type F(P,N). I am looking for F to be a matrix of P rows and N columns (or vise versa): however, I seem to be getting F to be a matrix of just one column. Does anybody have any advice?

EDIT: Sorry about the confusion, I'll post the code:

for Phi=linspace(Phimin, Phimax, N)
  G(N)=Phi + BOC;
  C2(N)=Rcon ./ ((Ccon .* cos(G(N))) + (Scon .* sin(G(N))));
  C1(N)=Rmax^2 + (C2(N) .* cos(G(N) + Thetamax));
  A(N)= (sqrt(abs(C1(N)+C2(N))) + sqrt(abs(C1(N)-C2(N))))./2;
  B(N)=C2(N) ./ (2.*A(N));
  for Theta=linspace(Thetamin, Thetamax, P)
    F(N,P)= abs(((R(P).*(M*g*L)) .*cos(Theta)) /((A(N).*B(N)) .* sin(G(N)+Theta)));
    disp(F(N,P))
  end
end

BOC, Rcon. Ccon and Scon are all constants.

[EDITED, JSimon, 06-Oct-2011 09:27 UTC: Code formatted]

[Jan]

Without seeing the code, only wild guessing is possible.

Please, Nathan, post the code instead of describing it.

[Sean de Wolski]

Well my attempt at a wild guess. You overwrite the first column on each iteration

for ii ...

for kk ...

d(1,:) = f(ii,kk)

end

end

Answer 1

[Jan]

You are writing to the element with index N and P, but their values are static and not effected by the loop. I assume, you want this:

vN = linspace(Phimin, Phimax, N)
for iN = 1:length(vN)
  Phi = vN(iN);
  G(iN)=Phi + BOC;
  C2(iN)=Rcon ./ ((Ccon .* cos(G(iN))) + (Scon .* sin(G(iN))));
  C1(iN)=Rmax^2 + (C2(iN) .* cos(G(iN) + Thetamax));
  A(iN)= (sqrt(abs(C1(iN)+C2(iN))) + sqrt(abs(C1(iN)-C2(iN))))./2;
  B(iN)=C2(iN) ./ (2.*A(iN));

    vP = linspace(Thetamin, Thetamax, P);
    for iP = 1:length(vP)
      Theta = vP(iP);
      F(iN,iP)= abs(((R(iP).*(M*g*L)) .*cos(Theta)) / ...
                    ((A(iN).*B(iN)) .* sin(G(iN)+Theta)));
      disp(F(iN,iP))
    end
  end

If this solves your problem, it can be made much faster: 1. Pre-allocate the vectors by zeros(1, length(vN)) and zeros(length(vN), length(vP)). And move all repeated calculation before the loops. E.g. the creation of vP, M*g*L.*cos(Theta) etc. But I'm not going to optimize your code, as long as I do not know, if it matchs your needs at all.

[Nathan Edge]

Thank you very much for your answer, but I'm still struggling slightly. Can I just ask, is it possible for MATLAB to take a matrix like this:

X= 1

2

3

4

5

6...

And turn it into this:

Y= 1 2

3 4

5 6...

-where I can specify the number of rows and columns?

[Andrei Bobrov]

doc reshape

y = reshape(X,2,[])'