Manual Reference Pages - if (7)

NAME

if(7f) - [FORTRAN:EXECUTION CONTROL] selects a block based on a sequence of logical expressions.

Synopsis
Description
Examples

SYNOPSIS


[if_construct_name:] IF ((scalar-logical-expr)) THEN
block
ELSEIF ((scalar-logical-expr)) THEN [if_construct_name]
block
ELSE [if_construct_name]
block
ENDIF [if_construct_name]

        or

IF ((scalar-logical-expression)) action-statement

DESCRIPTION

The IF construct selects for execution at most one of its constituent blocks. The selection is based on a sequence of logical expressions.
If an if-construct-name is specified, both the IF and ENDIF must use that same name. If an ELSE or ELSEIF uses an if-construct-name it must be the same as the one specified on the corresponding IF/ENDIF.
Execution of an IF construct
At most one of the blocks in the IF construct is executed. If there is an ELSE statement in the construct, exactly one of the blocks in the construct is executed. The scalar logical expressions are evaluated in the order of their appearance in the construct until a true value is found or an ELSE statement or ENDIF statement is encountered. If a true value or an ELSE statement is found, the block immediately following is executed and this completes the execution of the construct. The scalar logical expressions in any remaining ELSEIF statements of the IF construct are not evaluated. If none of the evaluated expressions is true and there is no ELSE statement, the execution of the construct is completed without the execution of any block within the construct.
It is permissible to branch to an ENDIF statement only from within its IF construct. Execution of an ENDIF statement has no effect.
Execution of an IF statement
The IF statement controls the execution of a single action statement based on a single logical expression.
The action-stmt in the if-stmt shall not be an end-function-stmt, end-mp-subprogram-stmt, end-program-stmt, end-subroutine-stmt, or if-stmt.
Execution of an IF statement causes evaluation of the scalar logical expression. If the value of the expression is true, the action statement is executed. If the value is false, the action statement is not executed and execution continues.
The execution of a function reference in the scalar logical expression may affect entities in the action statement.
An example of an IF statement is:
       IF (A > 0.0) A = LOG (A)

EXAMPLES

Sample IF constructs:

     if (cvar == ’RESET’) then
        i = 0; j = 0; k = 0

endif

     OUTER: if (case.eq.0)then
        PROOF_DONE: if (PROP) then
           write (3, ’(’’QED’’)’)
           exit OUTER
        else
           PROP = nextprop
        endif PROOF_DONE
        write(*,*)’END OF PROOF_DONE’
     else OUTER
             write(*,*)’else outer’
     endif OUTER

     if (a > 0) then
        b = c/a
        if (b > 0) then
           d = 1.0
        end if
     elseif (c > 0) then
        b = a/c
        d = -1.0
     else
        b = abs (max (a, c))
        d = 0
     endif

if (7)

March 18, 2019

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Manual Reference Pages - if (7)

NAME

CONTENTS

SYNOPSIS

DESCRIPTION

EXAMPLES