CODEWITHSUNDEEP
functionfortrandefinition
Alej
Alej

Reputation: 75

How to define my function from a string?

This is normal definition of some function as I know:

real function f(x)
  real x
  f = (sin(x))**2*exp(-x)
end function f

But I want to define a function from some string, for example the program will ask me to write it, and then it will define the function f in a program. Is this possible in Fortran?

Upvotes: 4

Views: 232

Answers (2)

Alexander Vogt
Alexander Vogt

Reputation: 18098

I worked on a project once that tried to achieve something similar. We read in a string that contained a string with named variables and mathematical operations (a function if you will). In this string the variables then got replaced by their numerical values and the terms were evaluated.

The basic idea is not to too difficult, but it requires a lot of string manipulations - and it is not a function in the context of a programming language.

We did it like this:

  • Recursively divide the string at +,-,/,*, but remember to honor brackets
  • If this is not possible (without violating bracketing), evaluate the remaining string:
    • Does it contain a mathematical expression like cos? Yes => recurse into arguments
    • No => evaluate the mathematical expression (no variables allowed, but they got replaced)

This works quite well, but it requires:

  • Splitting strings
  • Matching in strings
  • Replacing strings with other strings, etc.

This is not trivial to do in Fortran, so if you have other options (like calling an external tool/script that returns the value), I would look into that - especially if you are new to Fortran!

Upvotes: 1

milancurcic
milancurcic

Reputation: 6241

What you are looking for is possible in reflective programming languages, and is not possible in Fortran.

Quote from the link above:

A language supporting reflection provides a number of features available at runtime that would otherwise be very obscure to accomplish in a lower-level language. Some of these features are the abilities to:

  • Discover and modify source code constructions (such as code blocks, classes, methods, protocols, etc.) as a first-class object at runtime.

  • Convert a string matching the symbolic name of a class or function into a reference to or invocation of that class or function.

  • Evaluate a string as if it were a source code statement at runtime.

  • Create a new interpreter for the language's bytecode to give a new meaning or purpose for a programming construct.

Upvotes: 4

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