2’s complement for a given string using XOR

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Given a binary string, task is to convert this string in to two’s complement with the help of XOR operator.

Examples:

Input : 00000101
Output :11111011

Input : 10010
Output : 01110

We have discussed an approach in previous post to find 2’s complement

For 2’s complement, we first find one’s complement. We traverse the one’s complement starting from LSB (least significant bit), and look for 0. We flip all 1’s (change to 0) until we find a 0. Finally, we flip the found 0.

We traverse from the last bot and keep ignoring all 0s until we find a 1. We ignore first 1 also. Then we toggle all bits by doing XOR with 1.

C++

// C++ program to find 2's complement using XOR.
#include <bits/stdc++.h>
using namespace std;
 
string TwoscomplementbyXOR(string str)
{
    int n = str.length();
 
    // A flag used to find if a 1 bit is seen
    // or not.
    bool check_bit = 0;
 
    for (int i = n - 1; i >= 0; i--) {
        if (str[i] == '0' && check_bit == 0) {
            continue;
        }
        else {
 
            // xor operator is used to flip the  
            if (check_bit == 1)
                str[i] = (str[i] - '0') ^ 1 + '0'; 
 
            // bits after converting in to
            // ASCII values
            check_bit = 1;
        }
    }
 
    // if there is no 1 in the string so just add 
    // 1 in starting of string and return
    if (check_bit == 0) 
        return "1" + str; 
    else
        return str;
}
 
// Driver code
int main()
{
    string str = "101";
    cout << TwoscomplementbyXOR(str);
    return 0;
}

Java

// Java program to find 2's complement using XOR.
import java.util.*; 
 
class GFG{
 
static void TwoscomplementbyXOR(String str)
{
    int n = str.length();
     
    // A flag used to find if a 1 bit is seen
    // or not.
    boolean check_bit = false;
 
    for(int i = n - 1; i >= 0; i--)
    {
        if (str.charAt(i) == '0' && 
            check_bit == false)
        {
            continue;
        }
        else
        {
             
            // xor operator is used to flip the  
            if (check_bit == true)
            {
                if (str.charAt(i) == '0')
                    str = str.substring(0, i) + '1' +
                          str.substring(i + 1);
                else
                     str = str.substring(0, i) + '0' +
                           str.substring(i + 1);
            }
             
            // bits after converting in to
            // ASCII values
            check_bit = true;
        }
    }
     
    // If there is no 1 in the string so just add 
    // 1 in starting of string and return
    if (check_bit == false)
    {
        System.out.println("1" + str); 
    } 
    else
        System.out.println(str);
}
 
// Driver code
public static void main(String[] args) 
{ 
    String str = "101";
     
    TwoscomplementbyXOR(str);
}
}
 
// This code is contributed by amreshkumar3

Python3

# Python program to find 2's complement using XOR.
def TwoscomplementbyXOR(str):
    n = len(str)
 
    # A flag used to find if a 1 bit is seen
    # or not.
    check_bit = 0
    i = n - 1
    s = list(str)
    while (i >= 0):
        if (s[i] == '0' and check_bit == 0):
            continue
        else:
 
            # xor operator is used to flip the
            if (check_bit == 1):
                s[i] = chr((ord(s[i]) - 48) ^ 1 + 48)
 
            # bits after converting in to
            # ASCII values
            check_bit = 1
        i -= 1
 
    # if there is no 1 in the string so just add
    # 1 in starting of string and return
    str = "".join(s)
    if (check_bit == 0):
        return "1" + str
    else:
        return str
 
# Driver code
str = "101"
print(TwoscomplementbyXOR(str))
 
# This code is contributed by subhammahato348.

C#

// C# program to find 2's complement using XOR.
using System;
class GFG {
 
  static void TwoscomplementbyXOR(string str)
  {
    int n = str.Length;
 
    // A flag used to find if a 1 bit is seen
    // or not.
    bool check_bit = false;
    for(int i = n - 1; i >= 0; i--)
    {
      if (str[i] == '0' && 
          check_bit == false)
      {
        continue;
      }
      else
      {
 
        // xor operator is used to flip the  
        if (check_bit == true)
        {
          if (str[i] == '0')
            str = str.Substring(0, i) + '1' +
            str.Substring(i + 1);
          else
            str = str.Substring(0, i) + '0' +
            str.Substring(i + 1);
        }
 
        // bits after converting in to
        // ASCII values
        check_bit = true;
      }
    }
 
    // If there is no 1 in the string so just add 
    // 1 in starting of string and return
    if (check_bit == false)
    {
      Console.WriteLine("1" + str); 
    } 
    else
      Console.WriteLine(str);
  }
 
  // Driver code
  static void Main() {
    string str = "101";
 
    TwoscomplementbyXOR(str);
  }
}
 
// This code is contributed by divyeshrabadiya07.

PHP

<?php
// PHP program to find 2's complement 
// using XOR. 
 
function TwoscomplementbyXOR($str) 
{ 
    $n =strlen($str); 
 
    // A flag used to find if a 1 
    // bit is seen or not. 
    $check_bit = 0; 
 
    for ($i = $n - 1; $i >= 0; $i--)
    { 
        if ($str[$i] == '0' && 
            $check_bit == 0) 
        { 
            continue; 
        } 
        else
        { 
 
            // xor operator is used
            // to flip the 
            if ($check_bit == 1) 
                $str[$i] = ($str[$i] - '0') ^ 1 + '0'; 
 
            // bits after converting in to 
            // ASCII values 
            $check_bit = 1; 
        } 
    } 
 
    // if there is no 1 in the string 
    // so just add 1 in starting of 
    // string and return 
    if ($check_bit == 0) 
        return "1" + $str; 
    else
        return $str; 
} 
 
// Driver code 
$str = "101"; 
echo TwoscomplementbyXOR($str); 
     
// This code is contributed by akt_mit
?>

Javascript

<script>
// Javascript program to find 2's complement 
// using XOR. 
 
function TwoscomplementbyXOR(str) {
    let n = str.length;
 
    // A flag used to find if a 1 
    // bit is seen or not. 
    let check_bit = 0;
 
    for (let i = n - 1; i >= 0; i--) {
        if (str[i] == '0' && check_bit == 0) {
            continue;
        }
        else {
 
            // xor operator is used
            // to flip the 
            if (check_bit == 1) {
                if (str.charAt(i) == '0')
                    str = str.substr(0, i) + '1' + str.substr(i + 1);
                else
                    str = str.substr(0, i) + '0' + str.substr(i + 1);
            }
 
            // bits after converting in to 
            // ASCII values 
            check_bit = 1;
        }
    }
 
    // if there is no 1 in the string 
    // so just add 1 in starting of 
    // string and return 
    if (check_bit == 0)
        return "1" + str;
    else
        return str;
}
 
// Driver code 
let str = "101";
document.write(TwoscomplementbyXOR(str));
 
// This code is contributed by _saurabh_jaiswal
</script>

Output:

Time complexity: O(n) where n is the length of the given string
Auxiliary space: O(1)

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