C program to find ones complement of a binary number

Write a C program to read binary number from user and find one's complement of binary number using loop. How to find 1's complement of a binary number in C programming. Logic to find ones complement of binary number in C program.



Input binary number: 01000011


Ones complement: 10111100

Required knowledge

Basic C programming, If else, For loop, String

What is Ones complement?

Ones complement of a binary number is defined as value obtained by inverting all binary bits. It is the result of swapping all 1's to 0's and all 0's to 1's.

Ones complement of binary value

Logic to find ones complement

Below is the step by step logic to find ones complement of binary number.

  1. Input a binary string from user. Store it in some variable say binary.
  2. Run a loop from 1 to length of binary string, incrementing 1 in each iteration. The loop structure should look like for(i=0; i<SIZE; i++) (where SIZE is the length of binary digit).
  3. Inside the loop there can be two cases -
    Case I: Print 0 for each 1 binary bit i.e. if(binary[i] == '1').
    Case II: Print 1 for each 0 binary bit i.e. if(binary[i] == '0').

Program to find ones complement

 * C program to find 1's complement of a binary number

#include <stdio.h>

#define SIZE 8

int main()
    char binary[SIZE + 1], onesComp[SIZE + 1];
    int i, error=0;

    printf("Enter %d bit binary value: ", SIZE);
    // Read binary string from user

     * Store all inverted bits of binary value to onesComp
    for(i=0; i<SIZE; i++)
        if(binary[i] == '1')
            onesComp[i] = '0';
        else if(binary[i] == '0')
            onesComp[i] = '1';
            printf("Invalid Input");
            error = 1;

            // Exits from loop
    // Marks the end of onesComp string
    onesComp[SIZE] = '\0';

    if(error == 0)
        printf("Original binary = %s\n", binary);
        printf("Ones complement = %s", onesComp);

    return 0;


Enter any 8 bit binary value: 00001111
Original binary = 00001111
Ones complement = 11110000

Happy coding 😉

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