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8. Procedures & subprograms

8. Procedures & subprograms

A procedure is a reusable piece of computation. FORTRAN 66 has four kinds (§8): statement functions, intrinsic functions, external functions, and external subroutines. The first three are functions — they return a value into an expression; the last is invoked by CALL. External functions and subroutines, plus the BLOCK DATA unit, are written as separate subprograms.

Statement functions

A statement function is a one-line function defined inside the program unit that uses it (§8.1). Its definition looks like an assignment: f(a1,…,an) = e. The arguments are dummy names; the expression e may use those dummies, constants, variables, intrinsic functions, previously defined statement functions, and external functions.

      COMMON /O/ R(1)
      REAL R
      SQ(X) = X*X
      R(1) = SQ(3.0) + SQ(4.0)
C     -> 9.0 + 16.0 = 25.0

Statement function definitions must come after the specification statements and before the first executable statement (§8.1.1).

Intrinsic functions

The intrinsic functions are built in and listed in the standard's Table 3 (§8.2) — the full table is reproduced in Appendix A. They cover absolute value, truncation, remaindering, choosing the largest/smallest, type conversion, and the complex-number helpers:

      COMMON /O/ R(5)
      REAL R
      R(1) = ABS(-3.0)
C     -> 3.0
      R(2) = MOD(7, 3)
C     -> 1   (remainder)
      R(3) = MAX0(2, 9, 5)
C     -> 9
      R(4) = FLOAT(4)
C     -> 4.0  (integer -> real)
      R(5) = AINT(3.9)
C     -> 3.0  (truncate, result still real)

Each intrinsic has a fixed argument type and result type (Table 3); pick the spelling that matches your data — ABS for real, IABS for integer, DABS for double precision.

External functions

An external function is a separate subprogram headed by a FUNCTION statement (§8.3):

      FUNCTION CUBE(X)
      CUBE = X*X*X
      RETURN
      END

Inside the subprogram, the function name (CUBE) is used as a variable; its value at the RETURN becomes the function's result (§8.3.1). A function subprogram must contain at least one RETURN. You reference it like any function:

      COMMON /O/ R(1)
      REAL R
      R(1) = CUBE(2.0)
C     -> 8.0

The optional type prefix on the FUNCTION statement (INTEGER FUNCTION …, DOUBLE PRECISION FUNCTION …) sets the result type explicitly; otherwise it follows the implicit naming rule.

Basic external functions

The standard also requires a set of basic external functions — the mathematical library — listed in Table 4 (§8.3.3): EXP, ALOG, ALOG10, SIN, COS, TANH, SQRT, ATAN, ATAN2, and their double precision (D…) and complex (C…) variants. They are referenced exactly like any external function:

      COMMON /O/ R(2)
      REAL R
      R(1) = SQRT(16.0)
C     -> 4.0
      R(2) = EXP(0.0)
C     -> 1.0

External subroutines

An external subroutine is a subprogram headed by a SUBROUTINE statement and invoked by CALL (§8.4). Unlike a function, it returns nothing directly; it communicates through its arguments and through common:

      SUBROUTINE DBL(A, B)
      B = 2.0*A
      RETURN
      END

      COMMON /O/ R(1)
      REAL R
      CALL DBL(5.0, Y)
      R(1) = Y
C     -> 10.0

A subroutine must contain at least one RETURN. A Hollerith constant may be passed as an actual argument even though it has no type of its own — the one exception to the rule that actual and dummy argument types must agree (§8.4.2).

Block data subprograms

A block data subprogram exists solely to give initial values to variables in labeled common (§8.5). It begins with BLOCK DATA, contains only DIMENSION, COMMON, EQUIVALENCE, type, and DATA statements, and has no executable statements:

      BLOCK DATA
      COMMON /C/ A, B
      DATA A, B /1.5, 2.5/
      END

Now A and B in common block /C/ start out as 1.5 and 2.5 everywhere they appear. This is the only way to initialize labeled common (§7.2.2).

forterp notes.

  • forterp supplies the full Table 3 intrinsic set and Table 4 mathematical library, and adds a number of DEC FORTRAN-10 intrinsics beyond the standard (collected in Appendix C). The standard set is the portable one.
  • Recursion is prohibited by the standard: a procedure subprogram may not be referenced a second time before a RETURN from the first has intervened (§10.2.1) — so a subprogram must not, directly or indirectly, reference itself. forterp rejects re-entry by default rather than give a silently wrong answer; declare a procedure with the F90 RECURSIVE keyword (or flip the recursion knob globally) to permit correct recursion. See Appendix C.
  • An external function or subroutine may live in a separate source file; forterp links the units of a program together (see Command-line tools).