how to use MATLAB function? T^T

function [x, y] = rk4 (f, tspan, y0, h)

a = tspan(1); b = tspan(2); n = (b-a) / h;
x = (a+h: h : b);
k1 = h * feval(f, a, y0);
k2 = h * feval(f, a + h / 2, y0 + k1 / 2);
k3 = h * feval(f, a + h / 2, y0 + k2 / 2);
k4 = h * feval(f, a + h, y0 + k3);
y(1) = y0 + k1/6 + k2/3 + k3/3 + k4/6;
for i= 1 : n-1
    k1 = h * feval(f, x(i), y(i) );
    k2 = h * feval(f, x(i) + h/2, y(i) + k1 / 2);
    k3 = h * feval(f, x(i) + h/2, y(i) + k2 / 2);
    k4 = h * feval(f, x(i) + h, y(i) + k3);
    y(i+1) = y(i) + k1/6 + k2/3 + k3/3 + k4/6;
end
x = [ a x ];
y = [ y0 y];







function y = f(p, q)
y = exp(p) + q;
end







rk4.m, and f.m...
Use RK 4th order numerical method, solve that function..



I type at command window

??? Input argument "p" is undefined.

Error in ==> f at 2
y = exp(p) + q;



and,

??? Error using ==> feval
Argument must contain a string or function_handle.

Error in ==> rk4 at 6
k1 = h * feval(f, a, y0);



Initial value is (0,0), and original function is y'=y+e^t

how can i solve this problem? please help me!