# Plotting contours of a probability density

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Niki 2014-12-10

How do you create a script file to plot contours of a probability density in the y=0 plane? This is what I have done, but it's obviously wrong. Please help!
z=[-6:0.01:6];
x=[-6:0.01:6];
r=sqrt(x.^2+z.^2);
u1=(8*pi)^(-0.5)*(1-(r./2)).*exp(-r./2);
u2=(8*pi)^(-0.5)*(r./2).*(z/r).*exp(-r./2);
v1=(u1-u2).^(2).*0.5;
contour(x,z,v1)
xlabel('x')
zlabel('z')
I have attached the plot I am supposed to get. ### 采纳的回答

Star Strider 2014-12-10
You’re missing a meshgrid call and some element-wise dot-operator division
z=[-6:0.01:6];
x=[-6:0.01:6];
[X,Z] = meshgrid(x,z);
r=sqrt(X.^2+Z.^2);
u1=sqrt(8*pi)*(1-(r./2)).*exp(-r./2);
u2=sqrt(8*pi)*(r./2).*(Z./r).*exp(-r./2);
v1=(u1-u2).^(2).*0.5;
figure(1)
contour(X,Z,v1,20)
xlabel('x')
zlabel('z')
This isn’t exactly like the plot you posted, but it’s close! I’ll leave it to you to supply the necessary refinements to get it looking the way you want. You may want to adjust the number of contours the plot draws (I opted for 20 here).
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Star Strider 2014-12-10
My pleasure!

### 更多回答（1 个）

Youssef Khmou 2014-12-10
You have to use two dimensional arrays of x and z to produce two dimensional pdf, try :
[x,z]=meshgrid(-6:0.01:6);
r=sqrt(x.^2+z.^2);
u1=(8*pi)^(-0.5)*(1-(r./2)).*exp(-r./2);
u2=(8*pi)^(-0.5)*(r./2).*(z./r).*exp(-r./2);
v1=(u1-u2).^(2).*0.5;
contour(x,z,v1)
xlabel('x')
zlabel('z')
Niki 2014-12-10

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