The CYCLE and EXIT Statements

The CYCLE and EXIT statements specify that the remaining statements in the current iteration of a particular active (enclosing) DO loop are to be skipped.

CYCLE specifies that these statements are skipped, but the END DO statement that marks the end of the DO loop be executed--that is, the next iteration, if any, is to be started. If the statement marking the end of the DO loop is not END DO--in other words, if the loop is not a block DO--the CYCLE statement does not execute that statement, but does start the next iteration (if any).

EXIT specifies that the loop specified by the DO construct is terminated.

The DO loop affected by CYCLE and EXIT is the innermost enclosing DO loop when the following forms are used:

     CYCLE
     EXIT
     

Otherwise, the following forms specify the construct name of the pertinent DO loop:

     CYCLE construct-name
     EXIT construct-name
     

CYCLE and EXIT can be viewed as glorified GO TO statements. However, they cannot be easily thought of as GO TO statements in obscure cases involving FORTRAN 77 loops. For example:

           DO 10 I = 1, 5
           DO 10 J = 1, 5
              IF (J .EQ. 5) EXIT
           DO 10 K = 1, 5
              IF (K .EQ. 3) CYCLE
     10    PRINT *, 'I=', I, ' J=', J, ' K=', K
     20    CONTINUE
     

In particular, neither the EXIT nor CYCLE statements above are equivalent to a GO TO statement to either label 10 or 20.

To understand the effect of CYCLE and EXIT in the above fragment, it is helpful to first translate it to its equivalent using only block DO loops:

           DO I = 1, 5
              DO J = 1, 5
                 IF (J .EQ. 5) EXIT
                 DO K = 1, 5
                    IF (K .EQ. 3) CYCLE
     10             PRINT *, 'I=', I, ' J=', J, ' K=', K
                 END DO
              END DO
           END DO
     20    CONTINUE
     

Adding new labels allows translation of CYCLE and EXIT to GO TO so they may be more easily understood by programmers accustomed to FORTRAN coding:

           DO I = 1, 5
              DO J = 1, 5
                 IF (J .EQ. 5) GOTO 18
                 DO K = 1, 5
                    IF (K .EQ. 3) GO TO 12
     10             PRINT *, 'I=', I, ' J=', J, ' K=', K
     12          END DO
              END DO
     18    END DO
     20    CONTINUE
     

Thus, the CYCLE statement in the innermost loop skips over the PRINT statement as it begins the next iteration of the loop, while the EXIT statement in the middle loop ends that loop but not the outermost loop.