how to reverse a string without using standard library

jamesbon · 09-22-2010, 06:55 AM

I am making a program to reverse a string

Code:

#include<stdio.h>
int main ()
{
int i,j;
char *p,*s,*g;
 p="abcdefgt";
 i=0;j=0;
 s=NULL;

        while (p[i] != '\0')
        {
        printf("%d  %c\n ",i,p[i]);
        i++;
        }
        i--;

       while (i>=0){

         s[j]=p[i];
         j++;
        i--;
        printf("\n j=%d i=%d",j,i);
        }
  printf(" %s\n",s);
}

but the above code is giving me a segmentation fault on line

Code:

         s[j]=p[i];

Why is that happening?

JohnGraham · 09-22-2010, 07:15 AM

Quote:

Originally Posted by jamesbon

Code:

char *p,*s,*g;
...
s=NULL;
...
s[j]=p[i];
}

but the above code is giving me a segmentation fault on line

Code:

s[j]=p[i];

Why is that happening?

You're dereferencing a NULL pointer - malloc() some memory or make s into an array of adequate size.

jamesbon · 09-22-2010, 08:27 AM

The program has worked the way you suggested.But can you give me a link to what you said.

JohnGraham · 09-22-2010, 08:56 AM

Quote:

Originally Posted by jamesbon

The program has worked the way you suggested.But can you give me a link to what you said.

I don't know what you mean - if you want info just search the internet or any C book's index for "dynamic memory allocation" or "arrays".

theNbomr · 09-22-2010, 10:29 AM

You have a pointer to a char 's', and what char array does it point to? In C, if you haven't allocated memory, and specifically assigned the pointer to point to that memory, then de-referencing the pointer (assigning to the place to which it points) will cause a write to someplace random, violating memory protection, and resulting in the segmentation fault that you see. Probably the number one C novice conceptual hurdle.

--- rod.

jamesbon · 09-22-2010, 01:05 PM

theNombr thanks I got your point.