Re: cannot understand strrev implementation

From: Robert Stankowic (pcdoktor_at_netway.at)
Date: 11/09/03 

Date: Sun, 9 Nov 2003 07:43:54 +0100

"tuchka" <iazva@hotmail.com> schrieb im Newsbeitrag
news:5fe034e4.0311082147.1cb78ec6@posting.google.com...
> Hi, guys!
> I am very new here and just started to learn C. I have previous java
> exp. however.
> I'm abs. stuck on pointers and i'm unable comprehend algorithm of
> simple program that reverses chars in string no matter how long I'm
> staring at it.

OK, here is an attempt to explain how it works

Be aware, that given
char *something = string;
++something; yields the value of something + 1 and increments something,
while
something++; yields the original value of something and increments something

--something; yields the value of something - 1 and decrements something,
while
something--; yields the original value of something and decrements something

#include <stdlib.h>
#include <stdio.h>

char *my_strrev(char *string)
{
   char *original = string;
   char *forward = string;
   char temp;

   /*string points to the first character ('h' in our example)*/
   while(*string)/* *string yields the content of the location where string
points*/
   {
      string++;
      /*this makes string point to the next character,
      and the loop is repeated until the terminating '\0' is found
      ^ shows where string points during the loop
      'h' 'e' 'l' 'l' 'o' '\0'
       ^
      'h' 'e' 'l' 'l' 'o' '\0'
           ^
      'h' 'e' 'l' 'l' 'o' '\0'
               ^
      'h' 'e' 'l' 'l' 'o' '\0'
                   ^
      'h' 'e' 'l' 'l' 'o' '\0'
                       ^
      'h' 'e' 'l' 'l' 'o' '\0'
                            ^
                            *string is now 0,
      which terminates the loop
      */
   }
   /*string now points to the terminating '\0'
   (immidiately after the 'o' in "hello"), and forward still points to
   the beginning of the string (the 'h')*/
   while(forward < string)/*this loop decrements string and increments
forward
                          until the pointers meet in the middle of the
                          original string*/
   {
      /*now our pointers point as shown below
      'h' 'e' 'l' 'l' 'o' '\0'*/
       ^ ^
     temp=*(--string);
      *string=*forward;
      *forward++=temp;
      /*the statements above swap the contents of where (string - 1)
      and forward point. the result, step by step is:
      pass 1
      --string points to the 'o' and decrements string:
      'h' 'e' 'l' 'l' 'o' '\0'
       ^ ^
      *(--string) is 'o', *forward is 'h'
         temp becomes 'o', *string becomes 'h', *forward becomes 'o'
         string is now "oellh"
      pass 2
      'o' 'e' 'l' 'l' 'h' '\0'
           ^ ^
      *(--string) is 'l', *forward is 'e'
         temp becomes 'l', *string becomes 'e', *forward becomes 'l'
         string is now "olleh"
      now we have
      'o' 'l' 'l' 'e' 'h' '\0'
               ^^
      and string as well as forward point to the same location - we are
done*/
   }
   return (original);
}

int main(void)
{
   char string[] = "hello";

   puts(my_strrev(string));
   return EXIT_SUCCESS;
}

HTH
Robert

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