A function is invoked by its appearance in an expression at the place where its result value is needed,
total = total + largest(a,b,c)
The function is defined as follows,
INTEGER FUNCTION largest(i,j,k)
INTEGER :: i, j, k
largest = i
IF (j .GT. largest) largest = j
IF (k .GT. largest) largest = k
END FUNCTION largest
or equivalently as,
FUNCTION largest(i,j,k)
INTEGER :: i, j, k
INTEGER :: largest
...
END FUNCTION largest
The largest value of i, j and k will be substituted at the place where the function call is made. As with subroutines, the dummy and actual arguments must match in type kind and rank.
More than one function (or the same function more than once) may be invoked in a single statement. For example,
rezzy = funky1(a,b,c) + funky2(a,b,c)
Care must be taken that the order of execution will not alter the result; aside from operator precedence, the order of evaluation of a statement in a Fortran 90 program is undefined. It is not specified whether funky1 or funky2 is evaluated first; they could even be executed in parallel!
If a function invocation has side-effects and it is called twice in the same
statement then problems may occur. To safeguard against this a function
invocation should not assign
to its arguments, modify any global variables or perform I/O! A function
which obeys
these restrictions is termed PURE
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