Functions with Side-effects

  The rules of Fortran allow functions to have side-effects, that is to alter their actual arguments or to change other variables within common blocks. Functions with side-effects cannot be used in expressions where any of the other operands of the expression would be affected, nor can they be used in subscript or substring references when any other expression used in the same references would be affected. This rule ensures that the value of an expression cannot depend arbitrarily on the way in which the computer chooses to evaluate it.

There are also restrictions on functions which make use of input/output statements even on internal files: these cannot be used in expressions in other I/O statements. This is to avoid the I/O system being used recursively.

By far the best course is to use the subroutine form for any procedure with side-effects.