hexa-decimal to decimal conversion (using implicit type casting)

I think there's some problem in my vs code I am new to this coding stuff even after writing the correct code it gives me wrong results in almost every second code I write i get uncertain results Plz guys help me with this , plz check running this code in your machine....

#include <iostream>
using namespace std;
int main()
{

    char a[30];
    cout << "enter the hexadecimal";
    cin >> a;
    int i = 0, c, digit, decimal = 0, p = 1;
    while (a[i] != '\0') {
        i++;
    }
    for (int j = i; j >= 0; j--) {
        c = a[j];
        if (c >= 48 && c <= 57) {
            digit = c - 48;
        }
        else if (c >= 97 && c <= 112) {
            digit = c - 87;
        }
        decimal += digit * p;
        p *= 8;
    }
    cout << "\ndecimal is " << decimal;
    return 0;
}

while entering hexa decimal plz only enter small alphabets i have not taken capital letters into consideration

for cheking hexadecimal to decimal use this site https://www.rapidtables.com/convert/number/hex-to-decimal.html?x=146