feval for beginner, evaluating a function at a specific fvalue

how do I evaluate the following function at 55/8, 9/2? I keep getting feval errors saying I need a string or handle. Any help?

>> c

c =

 55/8
  9/2

>> f

f =

- 4*q1^2 + 55*q1 - 15*q2^2 + 135*q2 - 100

>> y=feval(f,c)

Error using feval

Argument must contain a string or function_handle.

Answer 1

[Star Strider]

The easiest way is to simply create an anonymous function out of ‘f’ and a vector out of ‘c’:

f = @(q) - 4*q(1)^2 + 55*q(1) - 15*q(2).^2 + 135*q(2) - 100;
c = [55/8; 9/2];

fc = f(c)

produces:

fc =

     392.8125

Answer 2

[Adam]

f should be a function handle or the name of an actual function, not a string containing a raw function. Just convert your function to a function handle e.g.

f = @(q1,q2) - 4*q1^2 + 55*q1 - 15*q2^2 + 135*q2 - 100