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 v

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.