Manual Reference Pages - if (7fortran)

NAME

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

SYNOPSIS

Syntax:

         [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]

         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

If there is an ELSE statement in the construct it acts as a default if all the preceding conditionals on the IF or ELSEIF where false, ensuring 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 an ELSE statement is found, the block immediately following is executed and this completes the execution of the construct. That is, an ELSE should follow any ELSEIF statements. 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.

STANDALONE IF

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. That is, if values are changed by the functions used in the logical expressions the selected block will use those values. It is generally a bad idea to use functions changing the values, but what would you expect this to produce?
       Calling a function with side-effects on **i**;
            program change
            i=1
            if(increment(i).gt.10)then
               write(*,*)’IF’,i
            elseif(increment(i).ge.20)then
               write(*,*)’ELSEIF’,i
            else
               write(*,*)’ELSE’,i
            endif
            contains
            function increment(i)
               write(*,*)’INC’,i
               increment=i*5
               i=i+3
               write(*,*)’INC’,i
            end function increment
            end program change
Result:
         > INC           1
         > INC           4
         > INC           4
         > INC           7
         > ELSEIF           7
An example of an IF statement is:
           IF (A > 0.0) A = LOG (A)

EXAMPLES

Sample IF constructs:

        program demo_if
        implicit none
        character(len=:),allocatable :: cvar
        logical :: PROP=.false.
        real :: a, b, c, d
        integer :: case=0
        integer :: i, j, k
        logical :: nextprop=.true.
         !
         ! basic IF
         !
         cvar=’NO’
         if (cvar == ’RESET’) then
            i = 0; j = 0; k = 0
         endif
         !
         ! labeled and nested IF constructs
         !
         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-elseif-endif
         !
         if (a > 0) then
            b = c/a
            if (b > 0) then
               d = 1.0
            endif
         elseif (c > 0) then
            b = a/c
            d = -1.0
         else
            b = abs (max (a, c))
            d = 0
         endif
         !
        end program demo_if

o	do - construct
o	if - selects a block based on a sequence of logical expressions.
o	cycle - construct
o	exit - statement
o	associate - associate construct
o	block - construct
o	goto - jump to target line
o	select - select a block based on the value of an expression (a case)
o	case - select a block based on the value of an expression (a case)
o	endselect - select a block based on the value of an expression (a case)

Manual Reference Pages - if (7fortran)

NAME

SYNOPSIS

DESCRIPTION

EXECUTION OF AN IF CONSTRUCT

STANDALONE IF

EXAMPLES

SEE ALSO