Octal to Hexadecimal in C

#include <stdio.h>
#include <stdlib.h>

int main() {
    unsigned long octalNum;

    // Input an octal number from the user
    printf("Enter an octal number: ");
    scanf("%lo", &octalNum);

    // Convert octal to decimal
    unsigned long decimalNum = 0, base = 1;
    while (octalNum > 0) {
        int digit = octalNum % 10;
        decimalNum += digit * base;
        base *= 8; // Octal is base-8
        octalNum /= 10;
    }

    // Convert decimal to hexadecimal
    char hex[20]; // Assuming enough space for the hexadecimal representation
    int index = 0;
    while (decimalNum > 0) {
        int remainder = decimalNum % 16;
        if (remainder < 10) {
            hex[index++] = remainder + '0';
        } else {
            hex[index++] = remainder - 10 + 'A';
        }
        decimalNum /= 16;
    }
    hex[index] = '\0'; // Null-terminate the hexadecimal string

    // Reverse the hexadecimal string
    int start = 0, end = index - 1;
    while (start < end) {
        char temp = hex[start];
        hex[start] = hex[end];
        hex[end] = temp;
        start++;
        end--;
    }

    printf("Hexadecimal equivalent: 0x%s\n", hex);

    return 0;
}