C program to convert Hexadecimal to Octal number system

Example

Input

Input hexadecimal: 1A

Output

Octal number: 32

Logic to convert hexadecimal to octal

There isn’t any easy and direct conversion algorithm from hexadecimal to octal number system. Below is the step by step descriptive logic to convert hexadecimal to decimal number system.

1. Convert the given hex number to binary.
2. Group the converted binary in groups of 3 bits.
3. Write the corresponding octal value of for each grouped binary number digit.

Binary to Octal conversion table

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

/**
 * C program to convert Hexadecimal to Octal number system
 */

#include <stdio.h>

int main()
{
    char hex[17];
    long long octal, bin, place;
    int i = 0, rem, val;

    /* Input hexadecimal number from user */
    printf("Enter any hexadecimal number: ");
    gets(hex);

    octal = 0ll;
    bin = 0ll;
    place = 0ll;

    /* Hexadecimal to binary conversion */
    for(i=0; hex[i]!='\0'; i++)
    {
        bin = bin * place;

        switch(hex[i])
        {
            case '0':
                bin += 0;
                break;
            case '1':
                bin += 1;
                break;
            case '2':
                bin += 10;
                break;
            case '3':
                bin += 11;
                break;
            case '4':
                bin += 100;
                break;
            case '5':
                bin += 101;
                break;
            case '6':
                bin += 110;
                break;
            case '7':
                bin += 111;
                break;
            case '8':
                bin += 1000;
                break;
            case '9':
                bin += 1001;
                break;
            case 'a':
            case 'A':
                bin += 1010;
                break;
            case 'b':
            case 'B':
                bin += 1011;
                break;
            case 'c':
            case 'C':
                bin += 1100;
                break;
            case 'd':
            case 'D':
                bin += 1101;
                break;
            case 'e':
            case 'E':
                bin += 1110;
                break;
            case 'f':
            case 'F':
                bin += 1111;
                break;
            default:
                printf("Invalid hexadecimal input.");
        }

        place = 10000;
    }

    place = 1;

    /* Binary to octal conversion */
    while(bin > 0)
    {
        rem = bin % 1000;

        switch(rem)
        {
            case 0:
                val = 0;
                break;
            case 1:
                val = 1;
                break;
            case 10:
                val = 2;
                break;
            case 11:
                val = 3;
                break;
            case 100:
                val = 4;
                break;
            case 101:
                val = 5;
                break;
            case 110:
                val = 6;
                break;
            case 111:
                val = 7;
                break;
        }

        octal = (val * place) + octal;
        bin /= 1000;

        place *= 10;
    }

    printf("Hexadecimal number = %s\n", hex);
    printf("Octal number = %lld", octal);

    return 0;
}

/*
Output :

Enter any hexadecimal number: fff
Hexadecimal number = fff
Octal number = 7777
*/