![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/596620/image.png)
I am trying to plot a contour graph with with x,y and z points
2 visualizaciones (últimos 30 días)
Mostrar comentarios más antiguos
Hi I am a newbie in matlab. I tried plotting x,y and z values (which are column vectors) with contourf function but the I get an error message that "Z must be at least a 2x2 matrix". How do I make z a 2x2 matrice?
2 comentarios
DGM
el 25 de Abr. de 2021
Editada: DGM
el 25 de Abr. de 2021
If Z is a function of both X and Y, then one would expect it to be a 2D array. The fact that it's a vector leaves me to question how it was calculated and what it means conceptually. Without knowing that, there's no way to say how to fix it.
Consider the example:
x = linspace(-1,1,10);
y = linspace(-1,1,10);
z1 = x.^2 + y.^2; % this is a vector
z2 = x.^2 + y'.^2; % this is a 2D array
In this case, z2 is a 2D array describing a paraboloid over a rectangular domain. On the other hand, z1 describes the same paraboloid along the diagonal of the same domain (the orange line).
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/596620/image.png)
The second case (z2) is what you'd want to use with contourf().
contourf(x,y',z2)
Whereas z1 doesn't contain enough information for a contour plot. You'd normally just plot it.
plot(x,z1)
Adam Danz
el 25 de Abr. de 2021
> How do I make z a 2x2 matrice
Impossible to answer without more info.
z(i,j) defines the z-value at x(i) and y(j).
Respuestas (1)
Clayton Gotberg
el 26 de Abr. de 2021
Editada: Clayton Gotberg
el 26 de Abr. de 2021
It sounds like the problem is that you haven't made these functions into a grid.
You started with x and y, then found z as a function of x and y. However, I'd guess that when you did that, you just said z = f(x,y).
% What I think you have:
x = [1 2 3 4];
y = [4 3 2 1];
z = x+y; % This equals [5 5 5 5]
% This matches up each element in x with the element in the same location in y
%what I think you want:
z = x+y; % Except now it equals [5 6 7 8; 4 5 6 7; 3 4 5 6; 2 3 4 5].
% Now, every element in x is matched with every element in y.
% How to get what you want:
[X,Y] = meshgrid(x,y); % Now X = [1 2 3 4; 1 2 3 4; 1 2 3 4; 1 2 3 4]
% and Y = [4 4 4 4; 3 3 3 3; 2 2 2 2; 1 1 1 1]
Z = X+Y;
3 comentarios
Clayton Gotberg
el 26 de Abr. de 2021
I glanced over the documentation to see if that would work but I missed the section that says it does! I'm also in the habit of using meshgrid but I definitely see your point about a use case for leaving the x and y vectors alone.
DGM
el 26 de Abr. de 2021
Yeah, prior to R2016b, trying to do things with orthogonal vectors often devolves into a complicated mess requiring bsxfun(). Since I'm usually running R2015b still, meshgrid is a big convenience, even if it can be slow.
Ver también
Categorías
Más información sobre Contour Plots en Help Center y File Exchange.
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!