# 2's complement in C++

## Program to find two’s complement of a binary number in C++

Problem –  Binary Number as a string, we have to print its 2’s complement.

2’s complement of a binary number is another binary number obtained by adding 1 to one’s complement.

Example:

Input: 101010

One’s complement the number: 010110

### Algorithm:

1. Take a binary number input as a string of length `size`.
2. Initialize an integer `fail` to 0.
3. Start a loop from `i` = 0 to `i` < `size`.
1. If `binary[i]` is equal to 1, set `one[i]` to 0.
2. Else, if `binary[i]` is equal to 0, set `one[i]` to 1.
3. Else, print an error message and set `fail` to 1. Break out of the loop.
4. Place a null character at the end of the string `one`.
5. If `fail` is equal to 0, print the one's complement.
6. Start a loop from `i` = `size` - 1 to `i` >= 0.
1. If `one[i]` is equal to 1 and `carry` is equal to 1, set `two[i]` to 0.
2. Else, if `one[i]` is equal to 0 and `carry` is equal to 1, set `two[i]` to 1 and set `carry` to 0.
3. Else, set `two[i]` to `one[i]`.
7. Place a null character at the end of the string `two`.
8. If `fail` is equal to 0, print the two's complement.

Code:

``````// Program to find two's complement of a binary number
#include <iostream>

#define size 6
using namespace std;

int main() {
char binary[size + 1], one[size + 1], two[size + 1];
int i, carry = 1, fail = 0;
cout << "Input a " << size << " bit binary number: ";
cin >> binary;
for (i = 0; i < size; i++) {
if (binary[i] == '1') {
one[i] = '0';
} else if (binary[i] == '0') {
one[i] = '1';
} else {
cout << "Error! Input the number of assigned bits." << endl;
fail = 1;
break;
}
}
one[size] = '\0';

for (i = size - 1; i >= 0; i--) {
if (one[i] == '1' && carry == 1) {
two[i] = '0';
} else if (one[i] == '0' && carry == 1) {
two[i] = '1';
carry = 0;
} else {
two[i] = one[i];
}
}
two[size] = '\0';
if (fail == 0) {
cout << "The original binary = " << binary << endl;
cout << "After ones complement the value = " << one << endl;
cout << "After twos complement the value = " << two << endl;
}
}

``````

Output

``````Input a 6 bit binary number: 101101
The original binary = 101101
After ones complement the value = 010010
After twos complement the value = 010011

``````