5.8.1 Selection

     flag
     IF
       code
     ENDIF

If flag is non-zero (as far as IF etc. are concerned, a cell with any bit set represents truth) code is executed.

     flag
     IF
       code1
     ELSE
       code2
     ENDIF

If flag is true, code1 is executed, otherwise code2 is executed.

You can use THEN instead of ENDIF. Indeed, THEN is standard, and ENDIF is not, although it is quite popular. We recommend using ENDIF, because it is less confusing for people who also know other languages (and is not prone to reinforcing negative prejudices against Forth in these people). Adding ENDIF to a system that only supplies THEN is simple:

     : ENDIF   POSTPONE then ; immediate

[According to Webster's New Encyclopedic Dictionary, then (adv.) has the following meanings:

... 2b: following next after in order ... 3d: as a necessary consequence (if you were there, then you saw them).

Gforth also provides the words ?DUP-IF and ?DUP-0=-IF, so you can avoid using ?dup. Using these alternatives is also more efficient than using ?dup. Definitions in ANS Forth for ENDIF, ?DUP-IF and ?DUP-0=-IF are provided in compat/control.fs.

     n
     CASE
       n1 OF code1 ENDOF
       n2 OF code2 ENDOF
       ...
       ( n ) default-code ( n )
     ENDCASE ( )

Executes the first codei, where the ni is equal to n. If no ni matches, the optional default-code is executed. The optional default case can be added by simply writing the code after the last ENDOF. It may use n, which is on top of the stack, but must not consume it. The value n is consumed by this construction (either by a OF that matches, or by the ENDCASE, if no OF matches).

Programming style note: To keep the code understandable, you should ensure that you change the stack in the same way (wrt. number and types of stack items consumed and pushed) on all paths through a selection construct.