You provide the CHAR vector 'q3func' as input, but you want a handle to the function instead:
fd = forwarddiff(@q3func, -2:2, 0.25)
If func is the vector 'q3func', the expression (i+stepsize) is interpreted as index, which must be a positive integer.
Hint: In your loop the function is evaluated twice for all inner points. q3func accepts vectors as inputs. You can obtain the output of the function once only and use diff() to calculate the difference:
function dx = forwarddiff(func, inter, stepsize)
x = inter(1):stepsize:inter(end);
dx = diff(func(x)) ./ stepsize;
end
