Example Of SELECT CASE Construct

A simple example, (the corresponding IF structure is given as a comment,)

      SELECT CASE (num)

       CASE (6,9,99,66)
     ! IF(num==9.OR. .. .OR.char==66) THEN
        PRINT*, "Woof woof"

       CASE (10:65,67:98)
     ! ELSEIF((num.GE.10.AND.LE.65) .OR. ...
        PRINT*, "Bow wow"

       CASE (100:)
     ! ELSEIF (num.GE.100) THEN
        PRINT*, "Bark"

       CASE DEFAULT
     ! ELSE
        PRINT*, "Meow"

      END SELECT     
     ! ENDIF

An IF .. ENDIF construct could be used but a SELECT CASE is neater and more efficient.

Important points are,

• the  case-expr in this case is num.
• the first  case-selector , (6,9,99,66) means ``if num is equal to either 6, 9, 66 or 99 then'',
• the second  case-selector , (10:65,67:98) means ``if num is between 10 and 65 (inclusive) or 67 and 98 (inclusive) then'',
• (100:) specifies the range of greater than or equal to one.
• if a case branch has been executed then when the next  case-selector is encountered control jumps to the END SELECT statement.
• if a particular case expression is not satisfied then the next one is tested.

A more complex exmaple is given below, this also demonstrates how SELECT CASE constructs may be named.

     ...
    outa: SELECT CASE (n)
     CASE (:-1) outa
      M = -1
     CASE (1:) outa
      DO i = 1, n
       inna: SELECT CASE (line(i:i))
        CASE ('@','&','*','$')
         PRINT*, "At EOL"
        CASE ('a':'z','A':'Z')
         PRINT*, "Alphabetic"
        CASE DEFAULT
         PRINT*, "CHAR OK"
       END SELECT inna
      END DO
     CASE DEFAULT outa
      PRINT*, "N is zero"
     END SELECT outa

Analysis:

• the first SELECT CASE statement is named outa and so is the corresponding END SELECT statement.
• the  case-expr in this case is n this is evaluated first and its value stored somewhere.
• the first  case-selector , (:-1) means ``if n is less than or equal to -1'' so if this is true then this branch is executed -- when the next  case-selector is encountered, control jumps to the END SELECT statement.
• if the above case expression is not satisfied then the next one is tested. (1:) specifies the range of greater than or equal to one. If n satisfies this then the DO loop containing the nested CASE construct is entered. If it does not then the DEFAULT action is carried out. In this case this branch corresponds to the value 0, the only value not covered by the other branches.
• the inner case structure demonstrates a scalar CHARACTER  case-expr which is matched to a list of possible values or, indeed, a list of possible ranges. If the character substring line(i:i) matches a value from either the list or list of ranges then the appropriate branch is executed. If it is not matched then the DEFAULT branch is executed. Note a CHARACTER substring cannot be written as line(i)
• this inner case structure is executed n times, (due to the loop,) and then control passes back to the outer case structure. Since the executable statements of the case branch have now been completed the construct is exited.