The indentation scheme shown in the examples above is not mandatory but the practice of indenting each block by a few characters relative to the rest of the program is strongly recommended. It makes the structure of the block immediately apparent and reduces the risk of failing to match each IF with an END IF. An indenting scheme is especially useful when IF-blocks are nested within others. For example:
IF(POWER .GT. LIMIT) THEN
IF(.NOT. WARNED) THEN
CALL SET('WARNING')
WARNED = .TRUE.
ELSE
CALL SET('ALARM')
END IF
END IF
The limited width of the statement field can be a problem when
IF-blocks are nested to a very great depth: but this tends to mean that
the program unit is getting too complicated and that it will usually be
beneficial to divide it into subroutines. If you accidentally omit an
END IF statement the compiler will flag the error but will not know
where you forgot to put it. In such cases the compiler may get confused
and generate a large number of other error messages.
When an IF-block which is executed frequently contains a large number of ELSE IF statements it will be slightly more efficient to put the most-likely conditions near the top of the list as when they occur the tests lower down in the list will not need to be executed.