Re: [colorforth] if then else

[Nick Maroudas <alice@xxxxxxxxxxxxxxxxxxxxxxx>]

Quoting Jason Kemp <jason.kemp@xxxxxxxxxxxxxxxx>:

>... " Good evening,
>... " 
>... " I can’t find an ‘else’ so how does one construct
>... " an if-then-else? Should 
>... " it really be done in two words:
>... " 
>... " : word1 if ... ; then ... ;
>... " : word2 stuff-before word1 stuff-after ;
>... " 
>... " Thanks,
>... " Jason
>... " 
>... " 
Nick here:  I always get confused by logic so I made up
a simple example.  Word1 does nothing but observe
whether n was 1 or else 2, then (whatever the number)
word2 adds 10: is this if-else-then? 

 ( n) : word1 dup ( nn) -1 

"If" jumps to the next "then" while " ; " returns to
the
calling word (word2).  One can concatenate " ; " for
instance word1 has 3 alternatives - the final one drops
any numbers other than 1 or 2, and replaces them by 3,
so the only results from word2 are 11, 12 and 13.

  I think one can also omit intermediate " ; " (but
not the final return ;) to make this crude one-word
form of if-else-then

  ( n)  : word1 dup ( nn) -1 + drop ( n) 0if ( n,f)
drop
1 ( no ";" ) then ( n/1) dup ( nn/11) -2 + drop 2 then
(
still no ";") 3 + ; 

   Results from this word1 are 0>>3 1>>4 2>>5  n>>n+3. 
The recognized n 1 and 2 can be manipulated - but,
without their own individual ";" to call each one back
home they will be further processed while passing
along the chain of ifs to the final common return ";".

Caritas,
Nick

   

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