Control statements

In principle, a net program contains three control structure:

• sequence,
• selection and
• iteration.

These program statements for FORTRAN are discussed below. Other constructions and FORTRAN control statements should not be applied.

• Sequence.
  This is a series of statements and/or references to a process elsewhere (via CALL). Exit a subroutine only via END.
• Selection.
  There are two constructions allowed:
  • IF - THEN - ELSE
  • SELECT CASE
  The first construction enables a choice to be made between two options and is programmed as follows:

            IF (condition) THEN
               statements
            ELSE
               andere statements
            ENDIF
  

  The second construction is programmed as follows:

            SELECT CASE (variable)
               CASE (value1)
                   statements
               CASE (value2)
                   statements
               ...
               CASE default
                   other statements
            END SELECT
  

  An alternative for the latter construction is the construction IF - THEN - ELSEIF - ELSE. A major disadvantage of this construction is that all alternatives need to be tested, even if it takes until the end of the list to find that an option in fact applies.
• Iteration.
  There are various iteration options, but only the following three are allowed:
  • DO - ENDDO
  • DO WHILE
  • REPEAT UNTIL
  The first construction is a counter-controlled loop:

             DO i = istart, iend, istep
                statements
             ENDDO
  

  If necessary, use the CYCLE and EXIT statements to immediately start the next iteration step of the loop or to exit the loop, respectively.
  
  
  Although, FORTRAN 90 does recognize the DO WHILE statement, it is recommended to simulate this construction by using the EXIT statement:

             DO
                IF (.NOT.condition) EXIT
                statements
             ENDDO
  

  instead of

             DO WHILE (condition)
                statements
             ENDDO
  

  The third construction is programmed as follows:

             DO
                statements
                IF (condition) EXIT
             ENDDO