# 2's complement in C++

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## 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:**

- Take a binary number input as a string of length
`size`

. - Initialize an integer
`fail`

to 0. - Start a loop from
`i`

= 0 to`i`

<`size`

.- If
`binary[i]`

is equal to 1, set`one[i]`

to 0. - Else, if
`binary[i]`

is equal to 0, set`one[i]`

to 1. - Else, print an error message and set
`fail`

to 1. Break out of the loop.

- If
- Place a null character at the end of the string
`one`

. - If
`fail`

is equal to 0, print the one's complement. - Start a loop from
`i`

=`size`

- 1 to`i`

>= 0.- If
`one[i]`

is equal to 1 and`carry`

is equal to 1, set`two[i]`

to 0. - Else, if
`one[i]`

is equal to 0 and`carry`

is equal to 1, set`two[i]`

to 1 and set`carry`

to 0. - Else, set
`two[i]`

to`one[i]`

.

- If
- Place a null character at the end of the string
`two`

. - 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
```