Flow control statements

CONTINUE statement

The CONTINUE statement has no effect on program execution. It is generally used to mark a place for a statement label, especially when it occurs as the terminal statement of a FORTRAN 77-style DO loop.

Syntax

CONTINUE

Execution logic

No action occurs.

Example

! find the 50th triangular number
   triangular_num = 0
   DO 10 i = 1, 50
       triangular_num = triangular_num + i
10 CONTINUE
   PRINT *, triangular_num

CYCLE statement

The CYCLE statement interrupts execution of the current iteration of a DO loop.

Syntax

CYCLE [ do-construct-name ]

Execution logic

Example

LOGICAL :: even
INTEGER :: number
 
loop: DO i = 1, 10
   PRINT *, ”Enter an integer: ”
   READ *, number
    IF (number == 0) THEN
      PRINT *, ”Must be nonzero.”
      CYCLE loop
    END IF
    even = (MOD(number, 2) == 0)
    IF (even) THEN
      PRINT *, ”Even”
    ELSE
      PRINT *, ”Odd”
    END IF
END DO loop

EXIT statement

The EXIT statement terminates a DO loop. If it specifies the name of a DO loop within a nest of DO loops, the EXIT statement terminates all loops by which it is enclosed, up to and including the named DO loop.

Syntax

EXIT [ do-construct-name ]

Execution logic

If do-construct-name is specified, execution terminates for all DO loops that are within range, up to and including the DO loop with that name. If no name is specified, execution terminates for the immediately enclosing DO loop.

Example

DO
  PRINT *, ”Enter a nonzero integer: ”
  READ *, number
  IF (number == 0) THEN
    PRINT *, ”Bye”
    EXIT
  END IF
  even_odd = MOD(number, 2)
  IF (even_odd == 0) THEN
    PRINT *, ”Even”
  ELSE
    PRINT *, ”Odd”
  END IF
END DO

Assigned GO TO statement

The assigned GO TO statement transfers control to the statement whose statement label was assigned to an integer variable by an ASSIGN statement.

Syntax

GO TO integer-variable [ , ( label-list ) ]

If label-list is present, then the label previously assigned to integer-variable must be in the list.

Execution logic

Control transfers to the executable statement at integer-variable.

Example

INTEGER int_label
   .
   .
   .
   ASSIGN 20 TO int_label
   .
   .
   .
GOTO int_label
.
.
.
20 ...

Computed GO TO statement

The computed GO TO statement transfers control to one of several labeled statements, as determined by the value of an arithmetic expression.

Syntax

GO TO ( label-list ) [ , ] integer-expression

Execution logic

Example

  DO
    PRINT *, ”Enter a number 1-3: ”
    READ *, k
    GO TO (20, 30, 40) k
    PRINT *, ”Number out of range.”
    EXIT
20  i = 20
    GO TO 100
30  i = 30
    GO TO 100
40  i = 40
100 print *, i
  END DO

Unconditional GO TO statement

The unconditional GO TO statement transfers control to the statement with the specified label.

Syntax

GO TO label

Execution logic

Control transfers to the statement at label.

Example

Older, “dusty-deck” Fortran programs often combine the GO TO statement with the logical IF statement to form a kind of leap-frog logic, as in the following:

   IF ( num1 /= num2) GO TO 10
   PRINT *, ”num1 and num2 are equal.”
   GO TO 30
10 IF ( num1 > num2 ) GO TO 20
   PRINT *, ”num1 is smaller than num2.”
   GO TO 30
20 PRINT *, ”num1 is greater than num2.”
30 CONTINUE

Arithmetic IF statement

The arithmetic IF transfers control to one of three labeled statements, as determined by the value of an arithmetic expression.

Syntax

IF ( arithmetic-expression ) label1, label2, label3

Execution logic

Example

As shown in this example, two or more labels in the label list can be the same.

i = MOD(total, 3) + 1
IF ( i ) 10, 20, 10

Logical IF statement

The logical IF statement executes a single statement, conditional upon the value of a logical expression. The statement it executes must not be:

Syntax

IF ( logical-expression ) executable-statement

Execution logic

Example

LOGICAL :: finished
.
.
.
IF ( finished ) PRINT *, ”Done.”

PAUSE statement

The PAUSE statement causes a temporary break in program execution.

Syntax

PAUSE [ pause-code ]

where pause-code is a character constant or a list of up to 5 digits.

Execution logic

If the standard input device is other than a terminal, the message is:

To resume execution, execute a kill -15 pid 
command.

pid is the unique process identification number of the suspended program. The kill command can be issued at any terminal at which the user is logged in.

Example

PAUSE 999

STOP statement

The STOP statement terminates program execution.

Syntax

STOP [ stop-code ]

where stop-code is a character constant, a named constant, or a list of up to 5 digits.

Execution logic

Program terminates execution. If stop-code is specified, the following is written to standard output:

STOP stop-code

Example

STOP ”Program has stopped executing.”