MATLAB: Help with Basic 2D Contour Plot

contour

I am trying to get this contour plot to work. This is a simple example so I can figure out how to do it before applying it to trickier equations.
I have this function saved in a separate .m file:
function a = triarea(x,y)
a = x.^3-2.*x-5+y.^2;
end
The main script calls this function:
for s=[0:1:3],
    for t=[0:1:3],
        q=triarea(s,t)
    end
end
This displays the range of values for q (one after the other, ideally I'd like them as one matrix upon completion but don't know how to do that).
What I want to do is a contour plot where the x-axis is the s variable and the y-axis is the t variable and q is represented as the contour i.e. varying in colour depending on it's value at each s,t point.
How do I do this?
It says that the contour variable Z must be at least a 2×2 matrix but I have only one value at each s,t point.

Best Answer

Hi,
don't use a loop. Use meshgrid:
s=[0:1:3]
t=[0:1:3]
[X,Y]  = meshgrid(s,t)
Z =triarea(X,Y);
contour(X,Y,Z)
This works as long the function triarea accepts matrices as input like it does in your example. Also you don't need to care about s and t (they dont need the same length and the values also doesn't matter).
s=-pi:0.1:pi;
t = 10:0.1:20;
[X,Y]  = meshgrid(s,t)
Z =triarea(X,Y);
contour(X,Y,Z)
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