Open Watcom FORTRAN 77 Language Reference

Language Reference
9.4 DO - END DO
DO init-expr,end-value[,inc-value] [: block-label]
statement(s)
END DO
This extension to FORTRAN 77 allows the creation of DO-loops without the introduction of statement
numbers. An optional block label may be specified (see the CYCLE, EXIT or QUIT statement for more
information). The END DO statement is used to indicate the end of the range of its corresponding DO
statement. A statement number may not be specified in the corresponding DO statement. Nested DO-loops
of this form require separate END DO statements to terminate the range of the corresponding DO statement.
Since a statement number may appear on the END DO statement, the number may be used to terminate
outer DO-loops. This is not a recommended practice (a CONTINUE statement or a structured DO statement
should be used). A transfer of control from within the DO-loop to a statement number on the END DO
statement is treated in the same manner as if the word CONTINUE had been used instead of END DO.
Some examples follow.
Example:
DO I = 1, 3
DO J = 1, 5
PRINT *, MATRIX( I, J )
END DO
END DO
The above is equivalent to the following example which uses statement numbers.
Example:
DO 10 I = 1, 3
DO 20 J = 1, 5
PRINT *, MATRIX( I, J )
20 CONTINUE
10 CONTINUE
The next example demonstrates the use of a GO TO statement to control execution of all or part of a
DO-loop.
Example:
DO I = 1, 3
DO J = 1, 5
PRINT *, ’INNER LOOP - J=’, J
IF( J .LE. 3 )GO TO 20
PRINT *, ’J > 3’
20 END DO
PRINT *, ’OUTER LOOP - J=’, J
END DO
A result of this example is that the character string J > 3 is printed 6 times (i.e., twice for each iteration
of the outer loop). Of course there is a much better way of coding this algorithm using the IF-END IF
construct. The example is included to illustrate the behaviour of transfers of control to an END DO
statement. The following example is an equivalent algorithm to the one above but the intent is much
clearer.
196 DO - END DO