6.9.1 Selection


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


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).

Forth’s THEN has the meaning 2b, whereas THEN in Pascal and many other programming languages has the meaning 3d.]

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 Standard Forth for ENDIF, ?DUP-IF and ?DUP-0=-IF are provided in compat/control.fs.

  x1 OF code1 ENDOF
  x2 OF code2 ENDOF
  ( x ) default-code ( x )

Executes the first codei, where the xi is equal to x. If no xi 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 x, which is on top of the stack, but must not consume it. The value x is consumed by this construction (either by an OF that matches, or by the ENDCASE, if no OF matches). Example:

: num-name ( n -- c-addr u )
   0 of s" zero " endof
   1 of s" one "  endof
   2 of s" two "  endof
   \ default case:
   s" other number" 
   rot \ get n on top so ENDCASE can drop it
 endcase ;

You can also use (the non-standard) ?of to use case as a general selection structure for more than two alternatives. ?Of takes a flag. Example:

: sgn ( n1 -- n2 )
    \ sign function
	dup 0< ?of drop -1 endof
	dup 0> ?of drop 1 endof
	dup \ n1=0 -> n2=0; dup an item, to be consumed by ENDCASE
    endcase ;

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 structure.