String Reverse Using pointers

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I have written a code to reverse the string . It is not working . Is my logic right? How to make it work . I wish to make this one work . I have written using some other logic and it is working . I wish to know how to fix this logic.

C#

#include "stdafx.h"


int _tmain(int argc, _TCHAR* argv[])
{
    char *a="This is Anfield"; //Char string
    char *b;  // Reverse string to be copied here
    int count=0; // Count the length of string
    while(*a!='\0')
    {
        count++;
        a++;
    }
    int len=count+1; // Length of string
    int i,j;
    i=j=0;
    for(i=count;i>=0;i--)
    {
        *(a+count)=*(b+j); //Tail of string will be copied to beginning
        j++;
    }
    printf("\n%s",*b);// Print Reversed String
    return 0;
}

There is some run time error in "char *b" Line..

Posted 23-Nov-12 19:15pm

steven8Gerrard

Updated 15-Aug-14 1:44am

Kim Togo

v2

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Comments

Richard MacCutchan 24-Nov-12 4:41am

You can use the strlen() function to find the length of the string. Apart from that, read Jochen's suggestions.

Philippe Mori 16-Aug-14 9:33am

You can use a debugger and trace the code line per line. If would then be easy to figure that a point to the end of the string after the first loop so adding count to it point outside of the allocated space (and item immediatly after) and thus is illegal.

Then b is not initialized so string would be written to random memory location which is really bad. The behavior of that is undefined. Here it is very easy to figure out half of the problem since the problem occurs at first use. Inspecting b with a debugger and you immediatly knows that it point to garbage.

By the way, that kind of code is good to learn how pointers works but in practice, you should use string class or at least standard C string functions.

5 solutions

Solution 1

Hi Steven,

Please update your code as below

C#

int _tmain(int argc, _TCHAR* argv[])
{
    char *a="This is Anfield"; //Char string
    char *b;  // Reverse string to be copied here
    int count=0; // Count the length of string
    while(*a!='\0')
    {
        count++;
        a++;
    }
    int len=count+1; // Length of string
    int i,j;
    i=j=0;
    for(i=count;i>=0;i--)
    {
        *(b+j) = *(a+i); //Tail of string will be copied to beginning
        j++;
    }
    printf("\n%s",*b);// Print Reversed String
    return 0;
}

Hope this will help you.

Posted 23-Nov-12 19:22pm

Mohd. Mukhtar

Updated 23-Nov-12 19:25pm

v2

Comments

steven8Gerrard 24-Nov-12 3:03am

STill getting error . Uninitialised Local variable used

Solution 5

C++

int _tmain(int argc, _TCHAR* argv[])
{
    char *a="This is c Language"; //Char string
    char *b;  // Reverse string to be copied here
    int count=0; // Count the length of string
    while(*a!='\0')
    {
        count++;
        a++;
    }
    int i;
    b =(char*) malloc(count);//Allocate memory for reverse string
    for(i=0;i<count;i++)>
    {
        *(b+i) = *(a-i-1); //Tail of string will be copied to beginning
    }
    *(b+i)='\0';//Add termination of string
    printf("\n%s" ,b);// Print Reversed String
    free(b);

    return 0;
}

Posted 14-Aug-14 23:48pm

Member 11014617

Updated 5-Oct-14 6:45am

v3

Comments

Richard MacCutchan 16-Aug-14 6:48am

Please don't waste time posting answers to old questions. It is most unlikely that the questioner still has any interest in the problem.

Member 11014617 5-Oct-14 12:46pm

I am new to this community . I will care from next times.

Philippe Mori 16-Aug-14 9:45am

You have forgotten to free allocated memory...

Member 11014617 5-Oct-14 12:47pm

Corrected ..

CPallini 5-Oct-14 16:03pm

1) You don't check malloc return value.
2) In
*(b+i)='\0';//Add termination of string
You are assigning b[count]. That is buffer overflow.

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Andy Bantly · Accepted Answer · 2012-11-26T02:18:00

This must be a homework assignment because in real life you wouldn't do it this way. You would use the locale aware type, like TCHAR and the function _tcsrev. In a multi-byte project this is equivalent to _strrev and in a Unicode project it becomes _wcsrev.

This or other solutions are not safe from buffer overrun attacks. Better to use a std::string/wstring or std::vector<tchar> as the container for the string data.

ex:

TCHAR p[] = _T("This is a string");
_tcsrev(p);

CPallini · Accepted Answer · 2012-11-23T23:53:00

Solution 3

If pointers you want, then pointers you'll have:
(the following C code reverses the string in-place)

C

#include <stdio.h>
int main()
{
  char s[] = "This is Anfield";
  char * p, *q;

  p = q = s;
  while (*++q);
  while (*q) q++; // more robust, see Member 9586495 comment.
  q--;
  while (q>p)
  {
    char c = *p;
    *p++ = *q;
    *q-- = c;

  }
  printf("%s\n", s);
  return 0;
}

Posted 23-Nov-12 23:53pm

CPallini

Updated 25-Nov-12 23:28pm

v4

Comments

Nelek 24-Nov-12 5:59am

That's evil :laugh: +5

CPallini 24-Nov-12 6:14am

It is not my fault, he asked for pointers!
:-)

nv3 24-Nov-12 6:26am

You could have copied the original string to a buffer and then reverse it in place. ;-)

CPallini 24-Nov-12 7:00am

Yes. If he doesn't want to modify the original string then a preventive call to strdup is enough.

sja63 26-Nov-12 3:58am

Solution 3 works not correct with char s[] = "";
My proposal is to replace the line "while (*++q);" by "while (*q)q++;"
Best regards

CPallini 26-Nov-12 4:18am

How did you test it (it works on my system)?

sja63 26-Nov-12 5:17am

Hi,
I changed only the code line to 'char s[] = "";' and I got
nonsens as print output on my windows 7 system.

CPallini 26-Nov-12 5:29am

OK, now I got it, you are right, I've updated my solution. Thank you.

Philippe Mori 16-Aug-14 9:22am

See my other comment. Although updated code would works, q would point before s if the string was empty thus either decrementing should be moved or you should check for empty string first.

missak boyajian 26-Nov-12 9:27am

Hey. Can someone please explain this:
//while (*++q);
while (*q) q++;
q--;

Philippe Mori 16-Aug-14 9:19am

I think that q--; should be done Inside loop only. Thus second line in loop block should be *p++ - *--q; and third line should be *q = c;.

Otherwise, you make q point before the array. I would think that it is undefined behavior by the standard but it would always works in practice as an address of 0 would never be allocated by C language.

Jochen Arndt · Accepted Answer · 2012-11-23T21:52:00

Your code contains multiple errors:

You are copying data to unalloacted memory (*b)
The variable b is not initialized
The trailing NULL char for b is not set
The variable len is never used
You are passing the wrong parameter (*b) to printf

You must allocate memory for the destination string, add a NULL byte after coyping the reversed data to it, and finally free the memory:

C++

b = new char[len];
// Reverse code here
b[count] = 0;
printf("\n%s", b);
delete [] b;

String Reverse Using pointers

5 solutions

Solution 4

Solution 3

Solution 2

Solution 1

Solution 5

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