11. Control statements

By default a program runs straight down, one statement after the next. Control statements change that — they branch, loop, call out, and stop (§11). FORTRAN 77 has both the modern structured IF/DO and the older GO TO forms you'll meet in vintage code.

GO TO

Unconditional — jump to a labelled statement:

      GO TO 100
      ...
  100 CONTINUE

Computed GO TO — pick the i-th label by an integer selector:

      GO TO (10, 20, 30) K       ! K=1 -> 10, K=2 -> 20, K=3 -> 30

If the selector is out of range (here, not 1–3), the GO TO does nothing and falls through to the next statement (§11.2).

Assigned GO TO — jump to a label previously stored with ASSIGN (Chapter 10):

      ASSIGN 200 TO LBL
      GO TO LBL

The computed and assigned forms are legacy; prefer the block IF and DO below.

The arithmetic IF

A three-way branch on the sign of an expression (§11.4) — negative, zero, positive:

      IF (X) 10, 20, 30      ! X<0 -> 10,  X=0 -> 20,  X>0 -> 30

Also legacy, but common in old numerical code.

The logical IF

Run one statement when a condition is true (§11.5):

      IF (N .LT. 0) N = -N           ! a single statement after the condition
      IF (DONE) GO TO 900

The block IF

The structured conditional (§11.6–11.9) — the workhorse of readable F77:

      IF (K .EQ. 1) THEN
        result = 10
      ELSE IF (K .EQ. 2) THEN
        result = 20
      ELSE
        result = 30
      END IF

ELSE IF and ELSE are optional; you may nest block IFs. END IF may also be written ENDIF.

The DO loop

A counted loop (§11.10):

      DO 10 I = 1, 10            ! I = 1, 2, ..., 10
        S = S + A(I)
   10 CONTINUE

• The form is DO label var = start, stop [, step]. The label marks the loop's last statement (conventionally a CONTINUE); the step defaults to 1 and may be negative.
• The trip count is computed once, up front, as MAX(INT((stop − start + step) / step), 0). If that is zero or negative, the loop body runs zero times — a DO 1 K = 5, 1 does not execute at all. (This zero-trip rule is the defining change from FORTRAN 66, whose loops always ran at least once.)
• After the loop, the control variable holds the value that failed the test — one step past the last used value. DO I = 1, 10 leaves I = 11; a zero-trip DO K = 5, 1 leaves K = 5.
• The control variable may be integer, real, or double precision.

      M = 0
      DO 1 K = 5, 1             ! start > stop with step +1 -> zero trips
        M = M + 1
    1 CONTINUE
C     M = 0 (body never ran), K = 5

CONTINUE, STOP, PAUSE, END

• CONTINUE does nothing; it is a convenient labelled placeholder, typically a DO terminator.
• STOP halts the program; an optional 1–5-digit number or character constant is displayed: STOP, STOP 99, STOP 'NO DATA'.
• PAUSE suspends until the operator resumes it (a relic of batch/interactive terminals); like STOP it may carry a number or string.
• END ends the program unit; in the main program it acts as STOP, in a subprogram as RETURN (Chapter 15).

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forterp notes. All control forms are implemented: the block IF family (ENDIF and END IF), the zero-trip DO with the correct post-loop control-variable value, computed-GO TO fall-through on an out-of-range selector, the arithmetic/logical/assigned forms, and CONTINUE/STOP/PAUSE/END. (DO WHILE is a DEC/Fortran-90 extension, not F77 — it is available under the FORTRAN10 dialect, rejected under F77.)