C Program to Convert Binary Number to Decimal and vice-versa

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

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

Program to convert binary to decimal

#include 
#include 
int convert(long long n);
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;
}

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: 110110111
110110111 in binary = 439

Program to convert decimal to binary

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

long long convert(int n) {
    long long bin = 0;
    int rem, i = 1, step = 1;
    while (n != 0) {
        rem = n % 2;
        printf("Step %d: %d/2, Remainder = %d, Quotient = %d\n", step++, n, rem, n / 2);
        n /= 2;
        bin += rem * i;
        i *= 10;
    }
    return bin;
}

Output

Enter a decimal number: 19
Step 1: 19/2, Remainder = 1, Quotient = 9
Step 2: 9/2, Remainder = 1, Quotient = 4
Step 3: 4/2, Remainder = 0, Quotient = 2
Step 4: 2/2, Remainder = 0, Quotient = 1
Step 5: 1/2, Remainder = 1, Quotient = 0
19 in decimal = 10011 in binary