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7. Statements

7. Statements

This is the largest part of the language: every statement FORTRAN 66 provides (§7). Statements divide into two classes — executable statements specify action (§7.1), nonexecutable statements describe data, editing, and arrangement (§7.2). This chapter is organized exactly that way; use it as a catalogue.

§7.1 Executable statements

There are three groups: assignment, control, and input/output (§7.1).

Assignment statements

Arithmetic assignment — v = e

Evaluate the arithmetic expression e and store it in variable or array element v (v of any type except logical) (§7.1.1.1). When the types differ, the value is converted per the standard's Table 1 — the important cases are:

  • assigning a real value to an integer v truncates toward zero (I = 3.9 gives 3);
  • assigning an integer value to a real v floats it (X = 3 gives 3.0).
      COMMON /O/ N(1)
      N(1) = 7.9
C     -> integer N(1) = 7 (real value truncated, Table 1 "Fix")

Logical assignment — v = e

v is a logical variable or array element and e a logical expression (§7.1.1.2):

      LOGICAL BIG
      BIG = X .GT. 100.0

ASSIGN — ASSIGN k TO i

Assign the statement label k to the integer variable i, for later use by an assigned GO TO (§7.1.1.3). While i holds a label it must not be used as an ordinary integer.

Control statements

GO TO

Three forms (§7.1.2.1):

      GO TO 100
C     -> unconditional: jump to label 100

      GO TO (10,20,30), I
C     -> computed: jump to the I-th label (I=2 -> label 20)

      ASSIGN 20 TO K
      GO TO K, (10,20)
C     -> assigned: jump to the label currently in K (-> label 20)

Arithmetic IF — IF (e) k1,k2,k3

A three-way branch on the sign of arithmetic expression e: to k1 if e<0, k2 if e=0, k3 if e>0 (§7.1.2.2):

      COMMON /O/ N(1)
      J = -5
      IF (J) 10, 20, 30
10    N(1) = 1
C     -> J<0, so control goes to label 10

Logical IF — IF (e) S

If logical expression e is true, execute statement S (any executable statement except a DO or another logical IF); if false, S is skipped (§7.1.2.3):

      IF (X .LT. 0.0) X = -X
C     -> absolute value of X

DO — DO n i = m1, m2, m3

A counting loop (§7.1.2.8). The range is every statement from the one after the DO through the terminal statement labeled n. The control variable i starts at m1; after the range runs, i is incremented by m3 (default 1 if omitted) and the range repeats while i does not pass m2. m1, m2, m3 are integer constants or variables.

      COMMON /O/ N(1)
      K = 0
      DO 1 I = 1, 5
1     K = K + I
      N(1) = K
C     -> 1+2+3+4+5 = 15

The FORTRAN 66 DO is one-trip: the range always executes at least once, even when m1 > m2, because the termination test happens after the range (§7.1.2.8.1). FORTRAN 77 later changed this to a zero-trip test. This is a real behavioral difference between the dialects — see the forterp notes at the end of the chapter.

CONTINUE

Does nothing; it is most often the terminal statement of a DO (§7.1.2.6).

CALL and RETURN

CALL s(a1,...,an) (or CALL s) invokes subroutine s; RETURN returns control from a subprogram to its caller (§7.1.2.4, §7.1.2.5). See Chapter 8.

STOP and PAUSE

STOP (optionally STOP n) terminates the program; PAUSE (optionally PAUSE n) suspends it (§7.1.2.7). The n is an octal digit string of one to five digits.

Input/output statements

READ and WRITE

Formatted transfer names a format f; unformatted transfer omits it (§7.1.3.2–§7.1.3.2.5):

      READ  (5, 100) A, B
      WRITE (6, 100) A, B
      READ  (5) A, B
C     -> unformatted (binary) read

The list after the format names the variables and array elements to transfer. A DO-implied list embeds a loop in the list (§7.1.3.2.1):

      DIMENSION A(3)
      WRITE (6,100) (A(I), I=1,3)
C     -> writes A(1), A(2), A(3)

Auxiliary I/O — REWIND, BACKSPACE, ENDFILE

REWIND u repositions unit u to its start, BACKSPACE u backs up one record, ENDFILE u writes an end-of-file mark (§7.1.3.3).

§7.2 Nonexecutable statements

Five kinds: specification statements, the DATA statement, the FORMAT statement, and the function-defining and subprogram statements (the last two are in Chapter 8) (§7.2).

Specification statements

DIMENSION — declare arrays

      DIMENSION A(10), B(2,3), C(4,4,4)
C     -> arrays of 1, 2, and 3 dimensions

An array declarator gives the array name and the size of each of its (one to three) dimensions (§7.2.1.1). A declarator may also appear in a type or COMMON statement.

COMMON — shared storage

      COMMON /BLK/ X, Y, A(10)
      COMMON I, J
C     -> /BLK/ is a named common block; I,J are in blank common

COMMON places variables and arrays in a storage block shared between program units (§7.2.1.3). A name between two slashes (/BLK/) names a block; entities before any block name, or with //, go in blank common. Corresponding positions in a block must match in type across program units (§7.2.1.3.1).

EQUIVALENCE — overlay storage

      DIMENSION A(2,2), B(4)
      EQUIVALENCE (A, B)
C     -> A and B name the same storage; B(1)=A(1,1), B(2)=A(2,1), ...

EQUIVALENCE makes two or more entities in the same program unit share storage (§7.2.1.4). It is for storage sharing, not for asserting mathematical equality.

EXTERNAL — pass a procedure as an argument

      EXTERNAL SIN
      CALL APPLY(SIN, X)
C     -> SIN is passed as a procedure argument

EXTERNAL declares that a name is an external procedure, so it can be passed as an argument (§7.2.1.5).

Type-statements — declare a name's type

      INTEGER COUNT
      REAL    MASS, FORCE
      DOUBLE PRECISION D
      COMPLEX Z
      LOGICAL FLAG

A type-statement overrides or confirms the implicit typing and may also supply dimension information (§7.2.1.6). The types are INTEGER, REAL, DOUBLE PRECISION, COMPLEX, LOGICAL.

The DATA statement — initial values

      COMMON /O/ N(4)
      DIMENSION M(4)
      DATA M /2*7, 3, 9/
C     -> M = 7, 7, 3, 9   (2*7 means "the value 7, twice")

DATA gives variables and array elements their initial values (§7.2.2). Names and constants correspond one-to-one; the form j*c repeats the constant c exactly j times. A variable in blank common may not be initialized; one in a labeled common block may be initialized only inside a BLOCK DATA subprogram (Chapter 8).

The FORMAT statement — editing for formatted I/O

A (labeled) FORMAT statement describes how values convert between their internal form and characters (§7.2.3). The common field descriptors are:

Descriptor Edits
Iw integer in a field w wide
Fw.d real, fixed point, d decimals
Ew.d real, exponential
Dw.d double precision
Gw.d general (chooses F or E)
Lw logical (T/F)
Aw Hollerith / character data
nH… literal text of n characters
nX n blank columns

A repeat count r before a descriptor (3F6.2) repeats it; a group in parentheses with a count (2(I3,F5.1)) repeats the group; the separators are , and / (a / ends a record). A scale factor nP shifts the decimal for F/E/G/D conversions.

      I = 42
      X = 3.14
      WRITE (6,100) I, X
100   FORMAT (1X, I3, F6.2)
C     -> " 42  3.14"

Carriage control for printed output

When a formatted record is sent to a printing device, its first character is not printed — it controls vertical spacing (§7.1.3.4):

First char Effect before printing
blank advance one line
0 advance two lines (blank line before)
1 advance to the top of the next page
+ no advance (overprint) 
      WRITE (6,100)
100   FORMAT (1H0, 5HHELLO)
C     -> "0" -> a blank line, then HELLO

This is why example output lines begin with 1X or 1H — that blank is the carriage control that keeps the text on its own line.

forterp notes.

  • One-trip DO. Under F66 (and FORTRAN10), a DO loop always executes its range at least once, faithfully reproducing X3.9-1966 (§7.1.2.8.1) and real FORTRAN-10. The F77 dialect uses the zero-trip test instead. If you need zero-trip behavior under F66, the zero_trip_do knob flips it; see Appendix C.
  • Carriage control applies because forterp treats unit 6 as a line printer by default, as FORTRAN-10 did. Output is collected on engine.out and the first column of each record is consumed as the control character. (In the F77 dialect, standard output defaults to a terminal device with no carriage control — see Appendix C.)
  • Apostrophe text in FORMAT (100 FORMAT(' HELLO')) is a FORTRAN-10 extension and is rejected under strict F66; use a Hollerith nH field, as above.