I am trying to plot a contour graph with with x,y and z points

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?

[DGM]

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).

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]

> 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).

Answer 1

[Clayton Gotberg]

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;

[DGM]

It's worth pointing out that it's not necessary to have x and y as orthogonal grids. Two orthogonal vectors contain the same information (though they are sometimes cumbersome to deal with). Functions like surf() and contour() will take x and y as either grids or as vectors, but it's z which must be 2D.

[xx yy] = meshgrid(1:10);
zz = xx+yy;
contourf(xx,yy,zz)

is equivalent to

x = 1:10;
y = 1:10;
zz = x+y';
contourf(x,y,zz)

Personally, I have a habit of using meshgrid as you suggest, but it can be costly.

[Clayton Gotberg]

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]

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.