Regular versus recursive functions in Fortran
This is a short post that contrasts recursive versus regular Fortran functions and gives an example factorial $n!$ recursive function.
Basics of a Fortran function
In Fortran, unlike many other languages, a user-defined function can only calculate a single value such as an integer, an array, or a character string. Also, the name of the function should also be the name of the value that it calculates by assigning it somewhere inside the function (see example below). Lastly, a return statement is not needed in a Fortran function unless the end of the function is always reached every time it is invoked.
What makes a recursive Fortran function different?
Recursive functions were added to Fortran 90 and later versions. The main syntactical difference with normal Fortran functions is the intrinsic "recursive" declaration and "result()" assignment that both go in the function declaration line. For example, using the factorial as an example: