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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Software landmines (loops)
Date: 1998/09/17
Message-ID: <m3d88v6a15.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <6sbuod$fra$1@hirame.wwa.com> <35f51e53.48044143@
 <6t6l4n$rep@jbm.nada.kth.se>
NNTP-Posting-Date: Wed, 16 Sep 1998 21:02:19 PDT
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-09-17T00:00:00+00:00
List-Id: <comp.lang.ada>

jbm@jbm.nada.kth.se (Jonas M�ls�) writes:

> 	Also, I have never encountered a real situation
> 	where I have needed an extra flag in the predicate.
> 	One can actually do fine without them. As an example,
> 	I hereby present to you three variations of str_index.
> 
> 	The first one strictly exits at the top. The other two
> 	in the middle. The first one is much easier to reason 
> 	about, and to understan, IMHO. Anyway, introducing an
> 	extra flag in the predicate is very often a sign of
> 	"patching the loop". Done right, one does not need it.
> 
> 
> 
> /** str_index
>  *
>  * str_index returns a pointer to the first occurrence of
>  * the given character in the given string.
>  * If the given character does not occur in the given
>  * string, str_index returns NULL.
>  * 
>  * If the given pointer is NULL, the result is undefined.
>  * 
>  */
> 
> 

Here's another idea:

char *
str_index (
    char   *s,			/* string to search           */
    char    c)			/* character to search for    */
{
    for (;;)
        if (*s == c)
           return s;
        
        else if (*s == '\0')
           return NULL;

        else
           s++;
}

I don't know if this is typical C code, however.  I suspect one would
move some of the work up into the for loop declaration:

   for (; *s != '\0'; s++)
      if (*s == c) 
         return s;

   return NULL;


An Ada programmer has an easier go of it:

function Get_Position (S : String; C : Character) return Natural is
begin
   for Index in S'Range loop
      if S(Index) = C then
         return Index;
      end if;
   end loop;
  
   return 0;
end Get_Position;