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In this example, you will learn to convert binary numbers to octal 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:

• C Functions
• C User-defined functions

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Program to Convert Binary to Octal

In this program, we will first convert a binary number to decimal. Then, the decimal number is converted to octal.

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

int convert(long long bin) {
    int oct = 0, dec = 0, i = 0;

    // converting binary to decimal
    while (bin != 0) {
        dec += (bin % 10) * pow(2, i);
        ++i;
        bin /= 10;
    }
    i = 1;

    // converting to decimal to octal
    while (dec != 0) {
        oct += (dec % 8) * i;
        dec /= 8;
        i *= 10;
    }
    return oct;
}

Output

Enter a binary number: 101001
101001 in binary = 51 in octal

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Program to Convert Octal to Binary

In this program, an octal number is converted to decimal at first. Then, the decimal number is converted to binary number.

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

long long convert(int oct) {
    int dec = 0, i = 0;
    long long bin = 0;

    // converting octal to decimal
    while (oct != 0) {
        dec += (oct % 10) * pow(8, i);
        ++i;
        oct /= 10;
    }
    i = 1;

   // converting decimal to binary
    while (dec != 0) {
        bin += (dec % 2) * i;
        dec /= 2;
        i *= 10;
    }
    return bin;
}

Output

Enter an octal number: 67
67 in octal = 110111 in binary