# Error using contour (Z must be at least a 2x2 matrix. What changes should I make to my code to make Z a 2x2 matrix?

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Azeem Singh Kahlon on 30 Jan 2020
Edited: Cris LaPierre on 31 Jan 2020
alpha=linspace(1,8);
beta=linspace(0,8);
a=(1/5).*(sqrt((2.*pi.*beta)./((alpha.^2)-1)));
u= sqrt((alpha.^2)+(beta.^2));
t= sqrt(1+(beta).^2);
G=real(a.*reallog((alpha+u)./(1+t)));
[X,Y] = meshgrid(alpha,beta);
contour(X,Y,G)
The final plot of the above code should resemble something like this:
I have double chekced, the formula is correct and the value of G at a given alpha and beta match with the experimental results. However, I face a problem with the dimensions of Z as it should be a 2x2 matrix.

#### 1 Comment

Cris LaPierre on 30 Jan 2020
Minor correction - the error message says it should be at least 2x2.

Cris LaPierre on 30 Jan 2020
Edited: Cris LaPierre on 31 Jan 2020
The solution is to ensure X, Y and G are all the same size. The meshgrid operation makes X and Y 100 x 100. The problem, then, is when you try to use contour, there is not a value in G for each X,Y pair.
The simplest way to do this is call the meshgrid function immediately after creating alpha and beta. That way, all the subsequent calculations are performed on the 100x100 variables, resulting in G being 100x100 as well.
alpha=linspace(1,8);
beta=linspace(0,8);
[alpha,beta] = meshgrid(alpha,beta);
a=(1/5).*(sqrt((2.*pi.*beta)./((alpha.^2)-1)));
u= sqrt((alpha.^2)+(beta.^2));
t= sqrt(1+(beta).^2);
G=real(a.*reallog((alpha+u)./(1+t)));
contour(alpha,beta,G)

#### 1 Comment

Azeem Singh Kahlon on 31 Jan 2020
Thank you so much Chris for the code and explaining me the reasin behind the problem.
Thanks a lot.