Question

I am trying to plot the function

f(x, y) = (x – 3).^2 – (y – 2).^2.

x is a vector from 2 to 4, and y is a vector from 1 to 3, both with increments of 0.2. However, I am getting the error:

“Subscript indices must either be real positive integers or logicals”.

What do I do to fix this error?

Answer 1

I (think) I see what you are trying to achieve. You are writing your syntax like a mathematical function definition. Matlab is interpreting f as a 2-dimensional data type and trying to assign the value of the expression to data indexed at x,y. The values of x and y are not integers, so Matlab complains.

If you want to plot the output of the function (we’ll call it z) as a function of x and y, you need to define the function quite differently . . .

f = @(x,y)(x-3).^2 - (y-2).^2;
x=2:.2:4;
y=1:.2:3; 
z = f(  repmat(x(:)',numel(y),1)  , repmat(y(:),1,numel(x) ) );

surf(x,y,z); 
xlabel('X'); ylabel('Y'); zlabel('Z');

This will give you an output like this . . .

The f = @(x,y) part of the first line states you want to define a function called f taking variables x and y. The rest of the line is the definition of that function.

If you want to plot z as a function of both x and y, then you need to supply all possible combinations in your range. This is what the line containing the repmat commands is for.

EDIT

There is a neat Matlab function meshgrid that can replace the repmat version of the script as suggested by @bas (welcome bas, please scroll to bas’ answer and +1 it!) …

f = @(x,y)(x-3).^2 - (y-2).^2;
x=2:.2:4;
y=1:.2:3;
[X,Y] = meshgrid(x,y);
surf(x,y,f(X,Y)); 
xlabel('x'); ylabel('y'); zlabel('z');