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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp
Subject: Re: Another newbie question: if-elseif-elseif-elseif..etc.
Date: Mon, 2 Sep 2024 10:38:55 -0000 (UTC)
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Peter Seibel wrote:

> > In the C version of my program I have:
> >
> > for (i=0; i<nchar; i=i+1) {
> >     if (tmp[i]>=0 && tmp[i]<=31) {
> >       dat[i]=tmp[i]+128;}
> >     else if (tmp[i]>=32 && tmp[i]<=63) {
> >       dat[i]=tmp[i];}
> >     else if (tmp[i]>=64 && tmp[i]<=95) {
> >       dat[i]=tmp[i]-64;}
> >     else if (tmp[i]>=128 && tmp[i]<=159) {
> >       dat[i]=tmp[i]+64;}
> >     else if (tmp[i]>=160 && tmp[i]<=191) {
> >       dat[i]=tmp[i]-64;}
> >     else if (tmp[i]>=192 && tmp[i]<=223) {
> >       dat[i]=tmp[i]-128;
> >     }
> >   }
> >
> > I'd like to achieve the same thing in Common Lisp (Clisp). But I'm
> > confused by the fact that there doesn't seem to be an elseif
> > construct. I see there is an if-then-else though.
> >
> > Part of my problem is likely due to the fact that I'm slavishly
> > translating the C form into Lisp, when I should really be thinking
> > in different terms. That is, I probably need to do this in an
> > entirely new way - one, unfortunately, I cannot see at the moment.
> >
> > Can anyone give me an idea of how this type of thing should be done in
> > Lisp?
> 
> Try COND.
> 
>   (loop for i from 0 below nchar
>         for e across tmp do
>     (cond
>       ((>= 0 e 31)    (setf (aref dat i) (+ e 128)))
>       ((>= 32 e 63)   (setf (aref dat i) e))
>       ((>= 64 e 95)   (setf (aref dat i) (- e 64)))
>       ((>= 128 e 159) (setf (aref dat i) (+ e 64)))
>       ((>= 160 e 191) (setf (aref dat i) (- e 64)))
>       ((>= 192 e 223) (setf (aref dat i) (- e 128)))))
> 
> Or even:
> 
>   (loop for i from 0 below nchar
>         for e across tmp do
>     (setf (aref dat i)
>       (+ e (cond
>             ((>= 0 e 31)    128)
>             ((>= 32 e 63)   0)
>             ((>= 64 e 95)   -64)
>             ((>= 128 e 159) 64)
>             ((>= 160 e 191) -64)
>             ((>= 192 e 223) -128)

Kenny Tilton wrote:

> >   (loop for i from 0 below nchar
> >         for e across tmp do
> >     (setf (aref dat i)
> >       (+ e (cond
> >             ((>= 0 e 31)    128)
> >             ((>= 32 e 63)   0)
> >             ((>= 64 e 95)   -64)
> >             ((>= 128 e 159) 64)
> >             ((>= 160 e 191) -64)
> >             ((>= 192 e 223) -128)
> 
> Nice. But the cond is not exhaustive and would return nil for e from 96
> to 127 or above 223, so perhaps a final clause is desirable:

Two CL gurus thought that that nonsense made sense.
But does it?  I can't think of a number e that satisfies
(>= 0 e 31).  [In other words, 0 >= e and e >= 31.]
Let's see if we can find one.

(do ((e -1 (+ 1 e)))
  ((> e 32))
  (format #t "~d: ~a " e
      (cond
        ((>= 0 e 31)  'Yes)
        (#t '_))))

-1: _ 0: _ 1: _ 2: _ 3: _ 4: _ 5: _ 6: _ 7: _ 8: _ 9: _ 10: _
11: _ 12: _ 13: _ 14: _ 15: _ 16: _ 17: _ 18: _ 19: _ 20: _
21: _ 22: _ 23: _ 24: _ 25: _ 26: _ 27: _ 28: _ 29: _ 30: _
31: _ 32: _

No, it seems that there is no such number.  Perhaps it should be
(<= 0 e 31).

(do ((e -1 (+ 1 e)))
  ((> e 32))
  (format #t "~d: ~a " e
      (cond
        ((<= 0 e 31)  'Yes)
        (#t '_))))

-1: _ 0: Yes 1: Yes 2: Yes 3: Yes 4: Yes 5: Yes 6: Yes 7: Yes
8: Yes 9: Yes 10: Yes 11: Yes 12: Yes 13: Yes 14: Yes 15: Yes
16: Yes 17: Yes 18: Yes 19: Yes 20: Yes 21: Yes 22: Yes 23: Yes
24: Yes 25: Yes 26: Yes 27: Yes 28: Yes 29: Yes 30: Yes 31: Yes 32: _

If two "CL gurus" agree on something, it's probably wrong.