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

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##### 2 Comments

DGM
on 25 Apr 2021

Edited: DGM
on 25 Apr 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).

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
on 25 Apr 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).

### Answers (1)

Clayton Gotberg
on 26 Apr 2021

Edited: Clayton Gotberg
on 26 Apr 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 Comments

Clayton Gotberg
on 26 Apr 2021

DGM
on 26 Apr 2021

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