[adrian_b]

The main difference between functions and subroutines in Fortran and other ancient programming languages is not the fact that subroutines do not have a return value.

The functions of Fortran are what would be called pure functions in C (which can be marked as such with compilers that support C language extensions, like gcc).

The pure functions cannot modify any of their arguments or any global variable, and they must be idempotent, which is important in program optimization.

[pklausler]

This is complete misinformation and you should stop posting it.

One can explicitly declare a function (or subroutine) to be PURE in Fortran, but it is not the default and never has been.