How to fix this error?

 function [u,x,t] = wave(a,xf,T,f,i1t0,bx0,bxf,M,N)
%solve a u_xx = u_tt for 0<=x<=xf, 0<=t<=T
% Initial Condition: u(x,0) = it0(x), u_t(x,0) = i1t0(x)
% Boundary Condition: u(0,t)= bx0(t), u(xf,t) = bxf(t)
% M = # of subintervals along x axis
% N = # of subintervals along t axis
%solve_wave
a = 1, L=0.64, h=0.005;
d = input('d = ');

 if 'x' >= 0 && 'x' <= d
   f = h*'x'./d;
elseif 'x' >= d && 'x' <= L
   f = h*(L-'x')./(L-d);
end
i1t0 = inline('0'); %initial condition
bx0 = inline('0'); bxf = inline('0'); %boundary condition
xf = 1; M = 20; T = 2; N = 50;

 dx = xf/M; x = [0:M]'*dx;  %space step size
dt = T/N; t = [0:N]*dt;    %time step size
for i = 1:M + 1, u(i,1) = f(x(i));
end
for k = 1:N + 1
u([1 M + 1],k) = [bx0(t(k)); bxf(t(k))];
end
r = a*(dt/dx)^ 2; r1 = r/2; r2 = 2*(1 - r);
u(2:M,2) = r1*u(1:M - 1,1) + (1 - r)*u(2:M,1) + r1*u(3:M + 1,1) ...
+ dt*i1t0(x(2:M)); 
for k = 3:N + 1
u(2:M,k) = r*u(1:M - 1,k - 1) + r2*u(2:M,k-1) + r*u(3:M + 1,k - 1)...
- u(2:M,k - 2); 
end

 figure(1), clf
mesh(t,x,u)
figure(2), clf
xlabel('x')
ylabel('u')
for n = 1:N %dynamic picture
plot(x,u(:,n)), axis([0 xf -0.3 0.3]), pause(0.2)
end

The error is: ??? Input argument "f" is undefined.

Error in ==> pdehypwave at 24 for i = 1:M + 1, u(i,1) = f(x(i));

How to fix this error?Thanks!

[Walter Roberson]

Exactly how are you running the function?