FORTRAN 77 - External Functions and Subroutines External Functions and Subroutines Besides the main program and the BLOCK DATA subprogram there are two other kinds of complete program units: the external function and the subroutine. Any FORTRAN 77 statement (except BLOCK DATA and PROGRAM) may appear in these procedures and two statements, RETURN and SAVE, may only appear in them. Both must end with an END statement. The external function returns one value via the name of the function and the subroutine may return zero or more values via an argument list. In a sense, the external function is a specialised form of the subroutine.

Program units are independent in that they have their own symbolic names and labels. They may call each other but cannot call themselves, either directly or indirectly. (This is called recursion and it is not allowed in FORTRAN 77.) Information is passed to and from external procedures via three methods: • argument lists • COMMON blocks • external files External Functions The first line of an external function declares the type and name of the function, as well as listing the dummy arguments. Type FUNCTION fname( dummy 1, dummy 2,, dummy n) The data type of the function type designates the data type of the value returned by the external function. If it is omitted, then the data type returned is determined by the first letter of the symbolic name fname. If the external function returns a CHARACTER value, then the length must be specified or given as CHARACTER*(*). In this second case, the length will be as specified in the invoking program unit.

The name fname may be used within the external function as a variable but it must be assigned a value before the end of the program unit. The last value assigned to the name fname is the value returned to the invoking program unit.

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A function is invoked by using its name fname followed by parentheses and an optional list of arguments called actual arguments which correspond to the dummy arguments in the FUNCTION statement. There may be zero or more dummy arguments of any data type which are declared (if necessary) in the body of the external function. However, the parentheses must be present even if there are no arguments. Example Consider the following program consisting of a main program and an external function. PROGRAM MAIN INTEGER I,N,NMAX PARAMETER(NMAX=10) REAL COEF(0:NMAX),HORNER,X 10 CONTINUE WRITE(*,*)'Enter the degree of the polynomial' READ(*,*)N IF (N.GT. NMAX) THEN WRITE(*,*)'Degree too large. Choose smaller value.'

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GO TO 10 END IF WRITE(*,*)'Enter the coefficients in ascending order' DO 20, I = 0,N WRITE(*,*)'Enter the value for coefficient ',I READ(*,*)COEF(I) 20 CONTINUE WRITE(*,*)'Enter the value of X' READ(*,*)X WRITE(*,*)'The value of the polynomial at ',X,' is ', $ HORNER(COEF,N,X) STOP 'End of program' END REAL FUNCTION HORNER(A,N,X) C This function returns the value of the polynomial C y = a_0 + a_1 x + a_2 x^2 + + a_n x^n C using Horner's method. INTEGER I,N REAL A(0:N),X HORNER = A(N) D0 10 I = N-1,0,-1 HORNER = A(I) + HORNER*X 10 CONTINUE END In this example, COEF, N and X are the actual arguments in the function reference in the main program and A, N and X are the dummy arguments in the FUNCTION statement. The main program invokes the function in a WRITE statement. Side Effects An external function is supposed to return exactly one value via the function name. However, it is permitted for the function to have 'side effects', that is, for the function to alter the values of the actual arguments or of variables appearing in COMMON blocks. However, there are many restrictions on how a function with side effects can be used in a program so if it is necessary to alter arguments or variables in COMMON blocks, it is best to use a subroutine instead. Subroutines The first line of a subroutine gives the name of the program unit and a listing of the dummy arguments.