Reversing string using character array

Question

I wrote a program to reverse string where the string is a character array. The program is :

#include<iostream>
void rev(char *str)
{
  char *end;
  end = str;// end will now point to the same address of str
  // now we need to incremet the end pointer upto it encounter null
  if(str)// this checks if the string is present or not
  {
    while(*end)
      ++end;
    --end;// set one character back, since last character is null
    // swap characters from start of string with the end of the string
    // until the pointers meet in middle.
    while(str < end)
    {
      char temp = *str;
      *str++ = *end;
      *end-- = temp;
    }
  }
  std::cout<<"\n The reversed string is : \n";
  std::cout<<str<<std::endl;
}
int main()
{
  char str[500];
  std::cout<<"\n Enter the string : ";
  std::cin.getline(str, sizeof(str));
  rev(str);
  return 0;
}

One output instance is :

Enter the string : asdfgh

 The reversed string is : 
dsa

I want to solve this problem inplace and using character array.

There are many solutions available on internet. I have seen them. But I want to know where I'm going wrong with this implementation. If there is some extra increment in str. I want to know how to correct it.

Well, you keep incrementing str pointer, and at the end of rev() it points into the middle of the array... — Joker_vD
– Joker_vD, Commented Mar 4, 2016 at 19:20
Try some separation of concerns. A function named rev should reverse. Not dance, not play guitar, not bring you coffee and a croissant, not print anything. Reverse. — n. m. could be an AI
– n. m. could be an AI, Commented Mar 4, 2016 at 19:28
We programmers have reduced attention span, that's why we prefer short functions that do one thing. Not so many things to keep track of == good thing. If you insist on doing two things in the same function, you gotta keep track of stuff. For example, when you print str, are you sure it's the whole string? Have you kept track of what you've used str for? — n. m. could be an AI
– n. m. could be an AI, Commented Mar 4, 2016 at 19:41

erip · Accepted Answer · 2016-03-04 19:19:32Z

5

I think you're making the problem too confusing.

void reverse(char* s) {
  // return if s is nullptr
  if(!s) return;

  size_t len = strlen(s);

  // swap ith from front with ith from back
  for(size_t i = 0; i < len/2; ++i) {
    char temp = s[i];
    s[i] = s[len - i - 1];
    s[len - i - 1] = temp;
  }
}

answered Mar 4, 2016 at 19:19

erip

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8 Comments

Ayushi bhardwaj Over a year ago

Sorry, this is not a helpful answer. Internet is full of such answers.

erip Over a year ago

@Ayushibhardwaj You didn't ask the right question. :)

Ami Tavory Over a year ago

Nice. I suppose you could have used std::swap, though, no?

Galik Over a year ago

The OP is not asking to copy someone else's solution, the question is where the OP's code is going wrong.

Galik Over a year ago

Okay I can see that. But I think it was implied that they were asking why their code was not working. The revision clears that up though.

|

surajs1n · Accepted Answer · 2016-03-04 19:40:33Z

4

Well, the error is that you kept on increasing the str pointer and after that decreasing the end pointer, which finally met somewhere at the middle of the string. And right after, you printed out the string which starts somewhere from the middle in reverse order. So, I would suggest to do as follows:

#include<iostream>
void rev(char *str) {
  char *end;
  end = str;

  if(str) {
    while(*end)
        ++end;
    --end;

    while(str < end) {
      char temp = *str;
      *str++ = *end;
      *end-- = temp;
    }
  }
  /* Don't print the string here itself */
}

int main() {
  char str[500];
  std::cout<<"\n Enter the string : ";
  std::cin.getline(str, sizeof(str));
  rev(str);

  /* Print your string here */
  std::cout<<"\n The reversed string is : \n";
  std::cout<<str<<std::endl;
  return 0;
}

OUTPUT:

 Enter the string : qwertyuiop

 The reversed string is : 
poiuytrewq

edited Mar 4, 2016 at 19:40

answered Mar 4, 2016 at 19:32

surajs1n

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2 Comments

Galik Over a year ago

@erip No, this code works because, by moving the printing outside the function, it prints out a copy of the pointer that is used to reverse the string.

Galik Over a year ago

This appears to be the only correct answer to the actual question being asked.

Ami Tavory · Accepted Answer · 2016-03-04 19:31:30Z

3

Well, since this is tagged c++, you should note that you can do this as

void rev(char* str)
{
    std::reverse(str, str + strlen(str));
}

Looking at the documentation for std::reverse you can deduce that it does it in place.

answered Mar 4, 2016 at 19:31

Ami Tavory

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Comments

Han Arantes · Accepted Answer · 2016-03-04 19:19:42Z

0

Try this:

#include<bits/stdc++.h>

int main()
{
    char str[500], revstr[500];
    int i,j,len;
    std::cout<<"\n Enter the string : ";
    std::cin.getline(str, sizeof(str));

    //reverse str
    len = strlen(str);
    for(i=len-1, j=0; i>=0; i--, j++){
        revstr[j]=len[i];
    }
    revstr[j] = '\0'; //marks end of string

    std::cout<<"\n The reversed string is : \n";
    std::cout<<revstr<<std::endl;

  return 0;
}

answered Mar 4, 2016 at 19:19

Han Arantes

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2 Comments

Ayushi bhardwaj Over a year ago

sorry, but i didn't asked for this.

erip Over a year ago

This solution doesn't fulfill the in-place requirement.

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