C Program to Convert Decimal to Binary (Using Bitwise Operator, Array, Function, and Recursion)

Introduction

This article presents various ways to write a C program to convert decimal to binary, covering five popular approaches:

• Using bitwise operator

• Without using array

• Using function

• Using array

• Using recursion

These examples help in understanding binary number conversion and common programming techniques such as loops, functions, bitwise operations, and recursion.

All the programs follow a clean, beginner-friendly, and well-commented C programming structure.

 

1. Write a C Program to Convert Decimal to Binary Number System Using Bitwise Operator

// C program to convert decimal to binary using bitwise operator
#include <stdio.h>

// The main function of the program
int main()
{
    // Step-1 Declare variables
    int num, i;

    // Step-2 Input a decimal number from the user
    printf("Enter a decimal number: ");
    scanf("%d", &num);

    // Step-3 Display binary representation using bitwise operations
    printf("Binary representation: ");
    for(i = 31; i >= 0; i--) {
        // Extract i-th bit from the left
        int bit = (num >> i) & 1;
        printf("%d", bit);
    }

    printf("\n");
    return 0;
}

Run

Output

 

Explanation

In this program:

• We input a decimal number using scanf().

• We use a for loop that iterates from bit position 31 down to 0 (total 32 bits).

• Inside the loop:

  • (num >> i) & 1 extracts the i-th bit of the number.

• This allows us to directly print each bit from the most significant to least significant bit.

• This technique works efficiently for displaying a full 32-bit binary number.

 

2. C Program to Convert Decimal to Binary Without Using Array

// C program to convert decimal to binary without using array
#include <stdio.h>

// The main function of the program
int main()
{
    // Step-1 Declare variables
    int num;
    long long binary = 0;
    int remainder, place = 1;

    // Step-2 Input a decimal number from the user
    printf("Enter a decimal number: ");
    scanf("%d", &num);

    // Step-3 Convert decimal to binary manually using loop
    while(num != 0) {
        remainder = num % 2;
        binary += remainder * place;
        num /= 2;
        place *= 10;
    }

    // Step-4 Output the binary number
    printf("Binary representation: %lld\n", binary);

    return 0;
}

Run

Output

 

Explanation

In this version:

• We declare a long long variable to store the binary result.

• Instead of using arrays or strings, we build the binary number using integer math.

• Each binary digit is calculated using num % 2, and added at the correct place value using multiplication.

• The place variable starts at 1 and increases tenfold each time to simulate binary positions in decimal digits.

• This version is simple and doesn't use any array, hence it’s memory-efficient for small numbers.

 

3. C Program to Convert Decimal to Binary Using Function

// C program to convert decimal to binary using a function
#include <stdio.h>

// Step-1 Function to convert decimal to binary
long long convertToBinary(int num)
{
    long long binary = 0;
    int place = 1;

    // Convert decimal to binary using loop
    while(num != 0) {
        binary += (num % 2) * place;
        num /= 2;
        place *= 10;
    }

    return binary;
}

// The main function of the program
int main()
{
    // Step-2 Declare variables
    int num;
    long long binary;

    // Step-3 Input a decimal number from the user
    printf("Enter a decimal number: ");
    scanf("%d", &num);

    // Step-4 Call the function and store the result
    binary = convertToBinary(num);

    // Step-5 Output the result
    printf("Binary representation: %lld\n", binary);

    return 0;
}

Run

Output

 

Explanation

Here:

• We define a separate function called convertToBinary() that performs the conversion.

• The logic inside the function is identical to the non-array version:

  • It calculates the binary number and returns it.

• The main function handles input and output, while the conversion logic is modular.

• This approach is great for reusability and clean code separation.

 

4. C Program to Convert Decimal to Binary Using Array

// C program to convert decimal to binary using array
#include <stdio.h>

// The main function of the program
int main()
{
    // Step-1 Declare variables
    int num, binary[32], i = 0;

    // Step-2 Input a decimal number from the user
    printf("Enter a decimal number: ");
    scanf("%d", &num);

    // Step-3 Convert decimal to binary and store in array
    while(num > 0) {
        binary[i] = num % 2;
        num /= 2;
        i++;
    }

    // Step-4 Print the binary number in reverse order
    printf("Binary representation: ");
    for(int j = i - 1; j >= 0; j--) {
        printf("%d", binary[j]);
    }

    printf("\n");
    return 0;
}

Run

Explanation

In this method:

• We use an integer array binary[] to store binary digits (0s and 1s).

• The conversion happens by taking num % 2 and storing the remainder in the array.

• After conversion, we print the array in reverse order since the least significant digit is stored first.

• This approach is ideal for learning how data can be stored and reversed using arrays.

 

5. Write a C Program to Convert Decimal to Binary Using Recursion

// C program to convert decimal to binary using recursion
#include <stdio.h>

// Step-1 Recursive function to convert and print binary
void convertToBinary(int num)
{
    if(num == 0)
        return;

    // Recursive call
    convertToBinary(num / 2);

    // Print remainder
    printf("%d", num % 2);
}

// The main function of the program
int main()
{
    // Step-2 Declare variable
    int num;

    // Step-3 Input a decimal number from the user
    printf("Enter a decimal number: ");
    scanf("%d", &num);

    // Step-4 Output binary using recursive function
    printf("Binary representation: ");
    if(num == 0)
        printf("0");
    else
        convertToBinary(num);

    printf("\n");
    return 0;
}

Run

Explanation

This program:

• Defines a recursive function that divides the number by 2 until it becomes 0.

• During the recursive unwinding phase, it prints the remainder num % 2.

• This results in the binary number being printed in the correct order.

• It’s a classic recursion problem that also demonstrates how function calls stack up and resolve.

 

Conclusion

In this article, we explored five different methods to convert a decimal number to binary in C, covering various approaches like:

• Bitwise operator

• Without using array

• Using function

• Using array

• Using recursion