Open Watcom FORTRAN 77 Language Reference

9 Program Structure Control Statements
9.1 Introduction
The use of structured programming statements has been found to encourage better programming and design
practices among beginners, and aids the more experienced programmer in writing error-free programs.
The format of these statements and their blocks is illustrated below. Following this, the use and meaning of
each statement is described and illustrated with examples. In each of these illustrations, the blocks are
denoted by statement(s) and are delimited by control statements.
In the descriptions, logical-expression can also be an integer expression, in which case the result
of the integer expression is compared for inequality to the integer value 0.
Example:
IF( LEN - 1 )THEN
In the preceding example, the expression LEN - 1 is interpreted as LEN - 1 .NE. 0.
9.2 IF - ELSE - END IF
The ELSE portion of this construct is optional, thus there are two possible formats.
(a) IF( logical-expression )THEN
statement(s)
END IF
(b) IF( logical-expression )THEN
statement(s)
ELSE
statement(s)
END IF
[: block-label]
[: block-label]
This construct is an enhancement of the FORTRAN logical IF statement. If the value of the parenthesized
logical expression is true in (a), the block of statements following the IF statement is executed, after which
control passes to the statement following the END IF statement; otherwise, control will pass directly to the
statement following the END IF statement. When the ELSE statement is used and the logical expression
is true, the block of statements between the IF and the ELSE statements is executed and then control
passes to the statement following the END IF statement; otherwise the block of statements following
ELSE statement is executed and then control passes to the statement following the END IF statement.
An optional block label may be specified with the IF statement (see the CYCLE, EXIT or QUIT
statement for more information).
IF - ELSE - END IF 193