how to plot 3D curves with for loop

Hello! I'm kind of beginner with matlab, however I need to plot a 3D curve. My problem is that I use a for loop to get the points because I don't see another way to proceed to avoid a problem with vectors dimensions. Here is what I do:

clear all; close all; clc;

U=400.0;
S=120000;
Phi=acos(1.0); 
P=S*cos(Phi);
Q=S*sin(Phi);

I=S/(U*sqrt(3));
Zn=(U^2)/S;
delta=20.0;

for v=0:1:20
Xd=(1/Zn)*repmat(v,100,1);              %abscisses
Xq=(1/Zn)*repmat(v',1,100);             %ordonnées
[Xd,Xq]=meshgrid(v,v); 

E1=sqrt(((U+Xq*I*sin(Phi))^2)+(Xq*I*cos(Phi))^2);
Eaf=(1/(2-Xq))*(E1+U*cos(delta)-(Xq/Xd)*U*cos(delta));
F=(-(3/2)*(U*Eaf/Xd)*sin(delta)+((U*U*(Xd-Xq))/(2*Xd*Xq))*sin(2*delta))-P;

%%%Tracé de la surface associée%%%
figure(1)
surf(Xd,Xq,F)
axis([3 6 1 6 0 10000000000]) %définition des limites des axes x,y et z.

end

And what I get is as you can see a picture with nothing on it. So what do you think I could do to solve it? Thank you for your answers.

Answer 1

[Roger Stafford]

In the 'for' command you write:

for v=0:1:20

which means that on each trip through the loop 'v' will be a scalar quantity - that is, a single number. When you do

[Xd,Xq]=meshgrid(v,v);

that destroys the Xd and Xq you computed just before that and creates scalar values for Xd and Xq instead, both equal to the scalar v. Therefore the command

surf(Xd,Xq,F)

has only one point which is not a very effective surface plot. Moreover, since you are not activating the "hold on" command, each plot overwrites the previous one.

If you want to plot a surface using 'surf' (which is different from plotting a 3D curve) you should not be using the for-loop. Instead just write

v = 0:20;
[Xd,Xq] = meshgrid(v,v); % <-- You can also use just meshgrid(v)
% Calculate F using Xd and Xq (which will be 41 x 41 matrices)
surf(Xd,Xq,F)

If on the other hand you really do want to plot a 3D curve, you should be using the 'plot3' command, not 'surf', and you should generate F in terms of a single vector, presumably 'v'.

Answer 2

[Thierry]

I know but if v is not a scalar quantity, then I have the error "Matrix dimensions must agree" by the way that's the same error that I get when trying to operate your suggested changes , try this code :

clear all; close all; clc;

U=400.0; S=120000; Phi=acos(1.0); P=S*cos(Phi); Q=S*sin(Phi);

I=S/(U*sqrt(3)); Zn=(U^2)/S; delta=20.0;

v=0:20; [Xd,Xq]=meshgrid(v);

E1=sqrt(((U+Xq*I*sin(Phi))^2)+(Xq*I*cos(Phi))^2); Eaf=(1/(2-Xq))*(E1+U*cos(delta)-(Xq/Xd)*U*cos(delta)); F=(-(3/2)*(U*Eaf/Xd)*sin(delta)+((U*U*(Xd-Xq))/(2*Xd*Xq))*sin(2*delta))-P;

%%%drawing of the surface%%% figure(1) surf(Xd,Xq,F) axis([3 6 1 6 0 10000000000]) %définition des limites des axes x,y et z.

Answer 3

[Thierry]

I found the problem.. that was just about using the matrix ops here is the code that works:

clear all; close all; clc; U=400.0; S=120000; Phi=acos(1.0); P=S*cos(Phi); Q=S*sin(Phi);

I=S/(U*sqrt(3)); Zn=(U^2)/S; delta=20.0;

%%%Formulation de l'équation%%% v=0:10; %Xd=repmat(v,100,1); %abscisses %Xq=repmat(v',1,100); %ordonnées [Xd,Xq]=meshgrid(v); %autre possibilité pour créer X,Y

E1=sqrt(((U+Xq.*I*sin(Phi))^2)+(Xq.*I*cos(Phi))^2); Eaf=(1./(2-Xq)).*(E1+U.*cos(delta)-(Xq./Xd).*U*cos(delta)); F=(-(3/2)*(U*Eaf./Xd).*sin(delta)+((U*U*(Xd-Xq))./(2*Xd.*Xq)).*sin(2*delta))-P;

%F=exp(-0.5*(Xd.^2+Xq.^2))/(2*pi); %valeurs de la fonctions aux points (X,Y) contour(Xd,Xq,F,20) %représentation de 20 courbes de niveau colorbar %affichage du code couleur des valeurs de F

%%%Tracé de la surface associée%%% figure; surf(Xd,Xq,F) axis([3 20 1 20 0 1000000]) %définition des limites des axes x,y et z. figure surfl(Xd,Xq,F); shading interp; colormap('copper'); axis([3 20 1 20 0 1000000])