How to create a separate function file for a piecewise function?

How do I create a function with the function:

f(x) = -ax+b for 0<x<b/a and f(x)=0 for x>b/a?

I want it in a seprate file.

How do I then use it in my main code? (call it)

[Stephen23]

What should be the value for x<=0 ?

Answer 1

[Torsten]

a = 3;
b = 12;
x = 0:0.01:12;
plot(x,f(a,b,x))

function pw = f(a,b,x)
  pw = NaN(size(x));
  idx = x>0 & x<b/a;
  jdx = x>=b/a;
  pw(idx) = -a*x(idx)+b;
  pw(jdx) = 0;
end