Question

I have one file with the following code:

function fx=ff(x)
fx=x;

I have another file with the following code:

function g = LaplaceTransform(s,N)
g = ff(x)*exp(-s*x);

a=0; 
b=1;

If=0;
h=(b-a)/N;
If=If+g(a)*h/2+g(b)*h/2;
for i=1:(N-1)
    If=If+g(a+h*i)*h;
end;
If

Whenever I run the second file, I get the following error:

Undefined function or variable ‘x’.

What I am trying to do is integrate the function g between 0 and 1 using trapezoidal approximations. However, I am unsure how to deal with x and that is clearly causing problems as can be seen with the error.

Any help would be great. Thanks.

Answer 1

Looks like what you’re trying to do is create a function in the variable g. That is, you want the first line to mean,

"Let g(x) be a function that is calculated like this: ff(x)*exp(-s*x)",

rather than

"calculate the value of ff(x)*exp(-s*x) and put the result in g".

Solution

You can create a subfunction for this

function result = g(x)
  result = ff(x) * exp(-s * x);
end

Or you can create an anonymous function

g = @(x) ff(x) * exp(-s * x);

Then you can use g(a), g(b), etc to calculate what you want.