C Program to Convert Binary Number to Decimal and vice-versa | C Programs for Beginners

In this example, you will learn to convert binary numbers to decimal and vice-versa manually by creating a user-defined function.

To understand this example, you should have the knowledge of the following C programming topics:

·         C Functions

·         C User-defined functions

Example 1: C Program to Convert Binary Number to Decimal

// convert binary to decimal

#include <stdio.h>
#include <math.h>

// function prototype
int convert(long long);

int main() {
  long long n;
  printf("Enter a binary number: ");
  scanf("%lld", &n);
  printf("%lld in binary = %d in decimal", n, convert(n));
  return 0;
}

// function definition
int convert(long long n) {
  int dec = 0, i = 0, rem;

  while (n!=0) {
    rem = n % 10;
    n /= 10;
    dec += rem * pow(2, i);
    ++i;
  }

  return dec;
}

Output

Enter a binary number: 1101

1101 in binary = 13 in decimal

In the program, we have included the header file math.h to perform mathematical operations in the program.

We ask the user to enter a binary number and pass it to the convert() function to convert it decimal.

Suppose n = 1101. Let's see how the while loop in the convert() function works.

n != 0	rem = n % 10	n /= 10	i	dec += rem * pow(2, i)
1101 != 0	1101 % 10 = 1	1101 / 10 = 110	0	0 + 1 * pow (2, 0) = 1
110 != 0	110 % 10 = 0	110 / 10 = 11	1	1 + 0 * pow (2, 1) = 1
10 != 0	11 % 10 = 1	11 /10 = 1	2	1 + 1 * pow (2, 2) = 5
1 != 0	1 % 10 = 1	1 / 10 = 0	3	5 + 1 * pow (2, 3) = 13
0 != 0	-	-	-	Loop terminates

So, 1101 in binary is 13 in decimal.

Now, let's see how we can change the decimal number into a binary number.

Example 2: C Program to convert decimal number to binary

// convert decimal to binary

#include <stdio.h>
#include <math.h>

long long convert(int);

int main() {
  int n, bin;
  printf("Enter a decimal number: ");
  scanf("%d", &n);
  bin = convert(n);
  printf("%d in decimal =  %lld in binary", n, bin);
  return 0;
}

long long convert(int n) {
  long long bin = 0;
  int rem, i = 1;

  while (n!=0) {
    rem = n % 2;
    n /= 2;
    bin += rem * i;
    i *= 10;
  }

  return bin;
}

Output

Enter a decimal number: 13

13 in decimal = 1101 in binary

Suppose n = 13. Let's see how the while loop in the convert() function works.

n != 0	rem = n % 2	n /= 2	i	bin += rem * i	i * = 10
13 != 0	13 % 2 = 1	13 / 2 = 6	1	0 + 1 * 1 = 1	1 * 10 = 10
6 != 0	6 % 2 = 0	6 / 2 = 3	10	1 + 0 * 10 = 1	10 * 10 = 100
3 != 0	3 % 2 = 1	3 / 2 = 1	100	1 + 1 * 100 = 101	100 * 10 = 1000
1 != 0	1 % 2 = 1	1 / 2 = 0	1000	101 + 1 * 1000 = 1101	1000 * 10 = 10000
0 != 0	-	-		-	Loop terminates

Thus, 13 in decimal is 1101 in binary.

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