C program to convert Binary to Octal number system

Example

Input

Input binary number: 00110111

Output

Octal number: 67

Logic to convert Binary to Octal number system

To make things simple and sound, I have divided the logic in two easy steps.

1. Group all binary bits to 3 digits starting from right side.
2. Write corresponding octal value for each grouped binary value.

Binary to octal conversion table

Binary	Octal
000	0
001	1
010	2
011	3
100	4
101	5
110	6
111	7

Step by step logic to convert binary to octal number system.

1. Input binary number from user. Store it in a variable say binary.
2. Initialize a variable to store converted octal say octal = 0.
3. Find the last three digits of binary say digit = num % 1000.
4. Find the octal equivalent (using binary to octal table) of the three binary digits found above.
5. Add octal value of binary found in above step to octal, by increasing the place value.
6. Remove the last three digits of the binary number. Since they are processed, say binary = binary / 1000.
7. Increase the place value of octal by using place = place * 10.
8. Repeat step 3 to 7 till binary > 0.

/**
 * C program to convert binary to octal number system
 */

#include <stdio.h>

int main()
{
    int octalConstant[] = {0, 1, 10, 11, 100, 101, 110, 111};

    long long binary, octal, tempBinary;
    int digit, place, i;

    octal = 0;
    place= 1;
    
    /* Input binary number from user */
    printf("Enter any binary number: ");
    scanf("%lld", &binary);

    /* Copy original binary value to temp variable */
    tempBinary = binary;
    

    while(tempBinary != 0)
    {
        /* Extract last three digit of binary */
        digit = tempBinary % 1000;

        /* Find octal equivalent of 3 digit binary */
        for(i=0; i<8; i++)
        {
            if(octalConstant[i] == digit)
            {
                /*
                 * Increase the place value of octal
                 * and add the previous octal value
                 */
                octal = (i * place) + octal;
                break;
            }
        }

        /* Remove the last three digit of binary */
        tempBinary /= 1000;

        /* Increase the place value */
        place *= 10; 
    }

    printf("Original binary number = %lld\n", binary);
    printf("Octal number = %lld", octal);

    return 0;
}

/*
Output :

Enter any binary number: 11001111
Original binary number = 11001111
Octal number = 317
*/