C++ Exercises: Find two's complement of a binary number

Last update on August 14 2025 12:55:30 (UTC/GMT +8 hours)

65. Two's Complement of a Binary Number

Write a program in C++ to find the two's complement of a binary number.

Sample Solution:-

C++ Code :

#include <iostream> // Including the input/output stream library
#define SZ 8 // Defining a constant named SZ with a value of 8

using namespace std; // Using the standard namespace

int main() // Main function where the execution of the program starts
{
    // Declare variables and character arrays to store binary values and their complements
    char bn[SZ + 1], onComp[SZ + 1], twComp[SZ + 1];
    int i, carr = 1; // Declare variables for iteration and carry initialization
    int er = 0; // Initialize an error flag

    // Display messages asking for input
    cout << "\n\n Find two's complement of a binary value:\n";
    cout << "----------------------------------------------\n";
    cout << " Input a " << SZ << " bit binary value: ";
    cin >> bn; // Read a binary value from the user

    // Loop to find the one's complement of the entered binary value
    for (i = 0; i < SZ; i++) 
    {
        // Check each bit of the input binary number and create its one's complement
        if (bn[i] == '1') 
        {
            onComp[i] = '0';
        }
        else if (bn[i] == '0') 
        {
            onComp[i] = '1';
        }
        else 
        {
            cout << "Invalid Input. Input the value of assign bits." << endl; // Display an error message
            er = 1; // Set error flag to 1
            break; // Exit the loop
        }
    }
    onComp[SZ] = '\0'; // Assign null terminator at the end of the one's complement array

    // Loop to find the two's complement of the one's complement
    for (i = SZ - 1; i >= 0; i--) 
    {
        // Check each bit of the one's complement to calculate the two's complement
        if (onComp[i] == '1' && carr == 1) 
        {
            twComp[i] = '0';
        }
        else if (onComp[i] == '0' && carr == 1) 
        {
            twComp[i] = '1';
            carr = 0;
        }
        else 
        {
            twComp[i] = onComp[i];
        }
    }
    twComp[SZ] = '\0'; // Assign null terminator at the end of the two's complement array

    // If no error occurred during input processing, display the original binary value, one's complement, and two's complement
    if (er == 0) 
    {
        cout << " The original binary = " << bn << endl;
        cout << " After ones complement the value = " << onComp << endl;
        cout << " After twos complement the value = " << twComp << endl;
    }
}

Sample Output:

 Find two's complement of a binary value:                              
----------------------------------------------                         
 Input a 8 bit binary value: 01101110                                  
 The original binary = 01101110                                        
 After ones complement the value = 10010001                            
 After twos complement the value = 10010010

Flowchart:

For more Practice: Solve these Related Problems:

Write a C++ program to calculate the two's complement of an 8-bit binary number by first finding its one's complement then adding 1.
Write a C++ program that reads an 8-bit binary string and outputs its two's complement using bitwise operations.
Write a C++ program to compute the two's complement of a binary number by iterating through the string and applying arithmetic addition.
Write a C++ program that converts an 8-bit binary input to its two's complement by flipping bits and incrementing the result.

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