trouble using quiver and contour plots.

Here is a portion of my code:

dx=0.05;
dy=0.05;
im=41;
jm=21;

x=[-1:dx:1];
y=[0:dy:1];

for i=1:1:im

      for j=1:1:jm
          u(i,j)=2*y(j)*(1-x(i)^2);
          v(i,j)=-2*x(i)*(1-y(j)^2);
          psi(i,j)=x(i)^2-(x(i)^2)*y(j)^2+y(j)^2;
      end
  end

figure
axes('position',[.1 .3 .8 .6]);

contour(x,y,psi);
axes('position',[.1 .1 .8 .1]);
pcolor([0:100;0:100]);

figure
quiver(x,y,u,v);

I get the following error message for both plots: 'The size of X must match the size of Z or the number of columns of Z.'

Using the meshgrid function reverses the dimensions for some reason (it gives (21X41) for x and y when the actual size is x(41), y(21)). If I add this dimension by hand to get x and y of (41X21) and adding zeros to all other rows/columns, my plots are incorrect. Why don't the dimensions/assignment of (x,y,u,v) work for these plots?

I am clearly not understanding how the contour and quiver functions interpret the input.

Please help if you can. Thank you very much!

Answer 1

[Nicole Hill]

I found that reversing the x and y vectors when calling meshgrid() solves the problem. I still do not understand what is going on here, but this seems to work.

[ypl,xpl] = meshgrid(y,x);

figure

contour(xpl,ypl,psi), hold on

quiver(xpl,ypl,u,v), hold off, axis image

Answer 2

[Jonathan Epperl]

Does this code achieve what you want:

dx=0.05;
dy=0.05;
x=[-1:dx:1];
y=[0:dy:1];
[X Y] = meshgrid(x,y);
P = X.^2 - (X.*Y).^2+Y.^2;
U = 2*Y.*(1-X.^2);
V = -2*X.*(1-Y.^2);
contour(X,Y,P); hold on;
quiver(X,Y,U,V);
hold off
colorbar

If yes, then some words of explanation:

• The contour command expects 3 matrices of equal size from you, and will interpret it as the height at the point [X(i,j), Y(i,j)] being Z(i,j) for all i and j. In my version of Matlab, contour is even smart enough to make the x,y grid itself, and all that was missing in your code was a prime: contour(x,y,psi'); works, at least in 2012b.
• quiver works similarly, and again only 2 primes were missing from your code
• [X,Y] = meshgrid(x,y) generates the matrices such that X is constant along the 1st dimension (columns), and Y is constant along the 2nd dimension (rows). I don't think it matters, but your for-loop atrocity just happens to create the transpose of what's needed.
• Lastly, is there a reason why you use pcolor and not colorbar? If so, tell us, I would be surprised if it wasn't easier to achieve what you want using the latter.