How to plot function from m file with vectors

Hello I have this function in my m file

function F = A(x)
F = 2 * x(1)^2 - x(2)^2;
end

and I want to plot this function but I have no idea how to do this. I tried fplot(@(x)A(x)) but I got this error

Warning: Function behaves unexpectedly on array inputs. To improve performance,
properly vectorize your function to return an output with the same size and shape as
the input arguments. 
> In matlab.graphics.function.FunctionLine>getFunction
  In matlab.graphics.function.FunctionLine/updateFunction
  In matlab.graphics.function.FunctionLine/set.Function_I
  In matlab.graphics.function.FunctionLine/set.Function
  In matlab.graphics.function.FunctionLine
  In fplot>singleFplot (line 234)
  In fplot>@(f)singleFplot(cax,{f},limits,extraOpts,args) (line 193)
  In fplot>vectorizeFplot (line 193)
  In fplot (line 163) 
Warning: Error updating FunctionLine.

   The following error was reported evaluating the function in FunctionLine update:
   Index exceeds array bounds.

Answer 1

[Star Strider]

First, vectorizing will help:

F = 2 * x(1).^2 - x(2).^2;

Second, you are confusing fplot, since it does not know what to do with a bivariate function. A solution is to use fsurf (or fmesh), and create ‘A’ as a function of two variables (which it actually is):

figure(1)
fsurf( @(x1,x2) A([x1,x2]), [-1 1  -1 1] )

The plot then works. The warning continues to appear for some reason. You can safely ignore it.

[Michael Urban]

thank you it works :)

[Star Strider]

My pleasure.

If my Answer helped you solve your problem, please Accept it!

[phoenix]

Hello Star Strider, Your comments are not showing.Please tell why 2^18 is used in freqz(nz,dz,2^18)?

Answer 2

[Abraham Boayue]

If x is a vector (1×N), then F must be a function of x, write F = x.^2 in this case. Else if x is a matrix (2×N) containing two vectors, x1 and x2, then F must be a function of two variables. You should write F = 2x1-x2;

Plotting F in case 1
N = 200; 
x = -2:4/(N-1):2;
F = A(x); 
Plot (x,F,'linewidth ',2);
Plotting F for case 2

First modify your function as 
function F = A (x1, x2)
F = 2*x1.^2-x2.^2; 
end
N = 200; 
x1 = 0:2/(N-1):2; 
x2 = -2:4/(N-1):2;
[x1,x2]= meshgrid(x1,x2);
F = A (x1, x2); 
mesh  (x1, x2, F)