Matlab code would not plot part of the function

Question

I tried recently to plot some velocicty fields for a tutorial problem in my fluids problem set. I wrote the following Matlab code

clear;

h_theta_multiple=0.01;
h_theta=h_theta_multiple*2*pi;
h_rho=0.1;

[theta,rho] = meshgrid(0:h_theta:2*pi,0:h_rho:5);
[x1,x2] = pol2cart(theta,rho);

N=[1 2 3 -1 -2 -3];

figure
for i=1:length(N)
n=N(i);
u1 = rho.^n .* cos(n.*theta); 
u2 = rho.^n .* sin(n.*theta); 
subplot(2,3,i);
quiver(x1,x2,u1,u2);
end

figure
for i=1:length(N)
n=N(i);
u1 = -rho.^n .* sin(n.*theta); 
u2 = rho.^n .* cos(n.*theta); 
subplot(2,3,i);
quiver(x1,x2,u1,u2);
end

It gives me the following

for the first and second functions respectively. I can't quite figure out why it doesn't plot the ones for n is negative... I tried to isolate everything but couldn't quite debug it in the end.

Interesting, the data is indeed there, but you can not see it. quiver(x1,x2,u1,u2,'o') will show it. — Ander Biguri
– Ander Biguri, Commented Nov 21, 2016 at 15:02
@AnderBiguri Yeah I just tried that and it works but I do want the vector form. It is so weird. — thephysicsguy
– thephysicsguy, Commented Nov 21, 2016 at 15:11

Luis Mendo · Accepted Answer · 2016-11-21 15:43:08Z

6

The problem is that for negative n the matrices u1 and u2 contain infinite values in some entries. quiver auto-scales the values, so everything gets compressed down to zero and thus does not show in the graph.

A solution is to replace infinite values by NaN:

clear;

h_theta_multiple=0.01;
h_theta=h_theta_multiple*2*pi;
h_rho=0.1;

[theta,rho] = meshgrid(0:h_theta:2*pi,0:h_rho:5);
[x1,x2] = pol2cart(theta,rho);

N=[1 2 3 -1 -2 -3];

figure
for i=1:length(N)
n=N(i);
u1 = rho.^n .* cos(n.*theta); 
u2 = rho.^n .* sin(n.*theta); 
u1(isinf(u1)) = NaN; % replace infinite values by NaN
u2(isinf(u2)) = NaN;
subplot(2,3,i);
quiver(x1,x2,u1,u2);
end

figure
for i=1:length(N)
n=N(i);
u1 = -rho.^n .* sin(n.*theta); 
u2 = rho.^n .* cos(n.*theta); 
u1(isinf(u1)) = NaN; % replace infinite values by NaN
u2(isinf(u2)) = NaN;
subplot(2,3,i);
quiver(x1,x2,u1,u2);
end

This gives

edited Nov 21, 2016 at 15:43

answered Nov 21, 2016 at 15:13

Luis Mendo

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1 Comment

thephysicsguy Over a year ago

Oh yes of course, for the first iteration it is finding 1/0^n and hence the Inf values. Don't know why I didn't just have a look at the u1 and u2 arrays. Thanks a lot!

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