Undefined function or variable 'S', help!

I have this function

function z = funct(t,S)
z = 0.4*S-((0.4*S.^2)./10);

and

function euler(func,S0,dt,t0,tf)

% Time interval
t=t0:dt:tf;

% Loop using Euler's method
for i = 1:length(t)-1
    S(i+1) = S(i) + dt*(feval(func,t(i),S(i)));
end

t=t'
S=S'

plot(t,S)
xlabel('Time')
ylabel ('S')

when I put this into the command window:

euler(@funct,7,.001,0,25)

I get this error:

Undefined function or variable 'S'.

Error in euler (line 8)
    S(i+1) = S(i) + dt*(feval(func,t(i),S(i)));

[Walter Roberson]

Instead of using

feval(func, t(i), S(i))

use

func(t(i), S(i))

unless you specifically want to allow the user to pass a function name as a string instead of as a function handle.

Answer 1

[jgg]

You never define S in your function. You probably want something like this:

 function euler(func,S0,dt,t0,tf)
% Time interval
t=t0:dt:tf;
 S = zeros(1,length(t)-1)
 S(1) = S0;
% Loop using Euler's method
for i = 1:length(t)-1
    S(i+1) = S(i) + dt*(feval(func,t(i),S(i)));
end
t=t'
S=S'
plot(t,S)
xlabel('Time')
ylabel ('S')