IF Construct

The block-IF is more flexible than the single statement IF since there can be a number of alternative mutually exclusive branches guarded by (different) predicates. The control flow is a lot more structured than if a single statement IF plus GOTO statements were used. The scenario is that the predicates in the IF or ELSEIF lines are tested in turn, the first one which evaluates as true transfers control to the appropriate inner block of code; when this has been completed control passes to the ENDIF statement and thence out of the block. If none of the predicates are true then the  else-block (if present) is executed.

Syntax,

 
[   name   :]IF(   logical-expression   )THEN

		    then-block   

[ ELSEIF(   logical-expression   )THEN [   name   ]

		    elseif-block   

 ... ]

[ ELSE [   name   ]

		    else-block    ]

END IF [   name   ]

The first branch to have a .TRUE. valued  logical-expression is the one that is executed. If none are found then the  else-block , if present, is executed.

For example,

    IF (x .GT. 3) THEN
     CALL SUB1
    ELSEIF (x .EQ. 3) THEN
     CALL SUB2
    ELSE
     CALL SUB3
    ENDIF

(A further IF construct may appear in the  then-block , the  else-block or the  elseif-block . This is now a nested IF structure.)

Statements in either the  then-block , the  else-block or the  elseif-block may be labelled but jumps to such labelled statements are permitted only from within the block containing them. Entry into a block-IF construct is allowed only via the initial IF statement. Transfer out of either the  then-block , the  else-block or the  elseif-block is permitted but only to a statement entirely outside of the whole block defined by the IF...END IF statements. A transfer within the same block-IF between any of the blocks is not permitted.

Certain types of statement, eg, END SUBROUTINE, END FUNCTION or END PROGRAM, statement are not allowed in the  then-block , the  else-block or the  elseif-block .