manual string reverse

**phoneix_hallows** · 08-27-2009

enlighten me with my mistake

Code:

#include<stdio.h>

int str_rev(char *s1)
{
  char str2[50];
  char *s2=str2;
  int i=0,len=0;
  while(*(s1+i) != '\0')
     i++;
  i--;
  while(i!=0)
     {
        *(s2+len)=*(s1+i);
        i--;
        len++;
     }
// *(s2+len)='\0';
//  printf("\nreversed string == > %s",s2);
  while(*(s2+i) != '\0')
     {
        *(s1+i)=*(s2+i);
     }
  *(s1+i)='\0';
  printf("\nreversed string 2 ==> %s\n",s1);
  return 0;
}

int main()
{
 char str[20];
 char *s1 = str;
 printf("\nEnter string\n");
 scanf("%s",s1);
 str_rev(s1);
 return 0;
}

**sean** · 08-27-2009

It's a lot easier to track down a problem when we know what the problem is - are you getting compiler errors? runtime errors? And if so, what do the errors say and where do they occur? Is the program running but just outputting incorrect output? And if so, what are your test inputs / outputs?

**phoneix_hallows** · 08-27-2009

i'm getting an incorrect output.... no errors... sorry for not specifying these things in my first post...

Code:

./a.out

Enter string
abcd



dfkj
dsaf
asf
nkl
^Z
[1]+  Stopped

**phoneix_hallows** · 08-27-2009

all the characters in the output screen are what i typed

**dwks** · 08-27-2009

Code:

 char str[20];
 char *s1 = str;
 printf("\nEnter string\n");
 scanf("%s",s1);
 str_rev(s1);

You don't have to do that, you know. You can use the name of an array by itself to treat it as a pointer (to the first element of the array). In fact, "array" is just like "&array[0]".

Code:

char str[20];
scanf("%s", str);

Also, fgets() is better suited to user input than scanf("%s") . . . SourceForge.net: Scanf woes - cpwiki

Finally, there are easier ways to reverse a string. Consider this pseudo-code:

Code:

set start to start of string
set end to point to the end of the string
while start < end:
    swap *start and *end
    increment start, decrement end

**phoneix_hallows** · 08-27-2009

thanks for ur time mate

**tabstop** · 08-27-2009

As to what could be going wrong *(s2+i) != '\0' doesn't ever have to be true, since you never terminate the s2 string....

**dwks** · 08-27-2009

Well, not to mention that i is never changed in this loop, so it's probably infinite.

Code:

  while(*(s2+i) != '\0')
     {
        *(s1+i)=*(s2+i);
     }

**phoneix_hallows** · 08-27-2009

still.... i'm not able to find my error.....

***incremented the value of i in the while loop**** that was very silly from my side

**dwks** · 08-27-2009

If you'd said at the beginning "it hangs when I run it," then we'd have known immediately to look for an infinite loop and helped you that much faster. So why don't you tell us what it does now . . . ?

Incidentally, this loop

Code:

  while(i!=0)
     {
        *(s2+len)=*(s1+i);
        i--;
        len++;
     }

won't process *(s1 + 0). So the first element of the string probably won't be copied.

**phoneix_hallows** · 08-27-2009

the pseudo code helped me.... thanks!!

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