# IF Directive Construct

General Compiler Directive: A conditional compilation construct that begins with an IF or IF DEFINED directive. IF tests whether a logical expression is .TRUE. or .FALSE.. IF DEFINED tests whether a symbol has been defined.

## Syntax

!DIR$ IF (expr) -or- !DIR$ IF DEFINED (name)

block

[!DIR$ ELSEIF (expr)

block] ...

[!DIR$ ELSE

block]

!DIR$ ENDIF

expr | Is a logical expression that evaluates to .TRUE. or .FALSE..
name | Is the name of a symbol to be tested for definition.
block | Are executable statements that are compiled (or not) depending on the value of logical expressions in the IF directive construct.

The IF and IF DEFINED directive constructs end with an ENDIF directive and can contain one or more ELSEIF directives and at most one ELSE directive. If the logical condition within a directive evaluates to .TRUE. at compilation, and all preceding conditions in the IF construct evaluate to .FALSE., then the statements contained in the directive block are compiled.

A name can be defined with a DEFINE directive, and can optionally be assigned an integer value. If the symbol has been defined, with or without being assigned a value, IF DEFINED (name) evaluates to .TRUE.; otherwise, it evaluates to .FALSE..

If the logical condition in the IF or IF DEFINED directive is .TRUE., statements within the IF or IF DEFINED block are compiled. If the condition is .FALSE., control transfers to the next ELSEIF or ELSE directive, if any.

If the logical expression in an ELSEIF directive is .TRUE., statements within the ELSEIF block are compiled. If the expression is .FALSE., control transfers to the next ELSEIF or ELSE directive, if any.

If control reaches an ELSE directive because all previous logical conditions in the IF construct evaluated to .FALSE., the statements in an ELSE block are compiled unconditionally.

You can use any Fortran logical or relational operator or symbol in the logical expression of the directive, including: .LT., <, .GT., >, .EQ., ==, .LE., <=, .GE., >=, .NE., /=, .EQV., .NEQV., .NOT., .AND., .OR., and .XOR.. The logical expression can be as complex as you like, but the whole directive must fit on one line.

## Example

    !  When the following code is compiled and run,
    !  the output is:
    !  Or this compiled if all preceding conditions .FALSE.
    !
    !DIR$ DEFINE flag=3
    !DIR$ IF (flag .LT. 2)
       WRITE (*,*) "This is compiled if flag less than 2."
    !DIR$ ELSEIF (flag >= 8)
       WRITE (*,*) "Or this compiled if flag greater than &
                    or equal to 8."
    !DIR$ ELSE
       WRITE (*,*) "Or this compiled if all preceding &
                    conditions .FALSE."
    !DIR$ ENDIF
    END

Parent topic: H to I

## See Also

DEFINE and UNDEFINE
IF Construct
General Compiler Directives
Syntax Rules for Compiler Directives