How to extract the first digit of a 16 digit number? - c++

Question

I have this code right here that compiles just fine getting the first digit of a 10 digit number.
I am wondering how to get my code to get the first digit of a 16 digit number?
I have tried changing 10 to 16 in the while loop and my program does not count the first digit. Here is my code down below:
#include <iostream>
using namespace std;
int main()
{
int number;
cout << "Enter a number : " << endl;
cin >> number;
// cout << "Last digit is : " << number % 10 << endl;
while(number >= 10)
{
number = number/10;
}
cout<< "First digit is : "<< number << endl;
return 0;
}

Answer 1

int has at least 16 bits. The exact size is implementation defined. Even with 32 bits the maximum value is 2147483647. You cannot store a 16 digit number in an int.
You can store numbers with more digits than any integer type can hold in a std::string. Because just reading some string from the user and printing the first character would be too much cheating, at least you should check that the user actually did enter a number:
#include <iostream>
#include <limits>
#include <cctype>
int main() {
std::cout << "max int : " << std::numeric_limits<int>::max() << "\n";
std::string number;
std::cin >> number;
for (const auto& c : number) {
if (std::isdigit(c) == 0) {
std::cout << c << " is not a valid digit !";
return 1;
}
}
if (number.size() > 0 && number[0] != '0') std::cout << number[0];
else std::cout << "invalid input";
}
Live Demo

Answer 2

Learning how to use <string> will prove very, VERY useful for you:
#include <iostream>
#include <string> //extremely useful
using namespace std;
int main()
{
string aux;
string number;
cout<<"Enter a number: "<<endl;
cin>>aux;
number=aux.substr(0,1); //create a substring of aux, starting from the first position (first argument) and getting only one character (second argument)
cout<<"First digit is "<<number<<endl;
return 0;
}
if you need the variable as int, use stoi():
int num=stoi(number);
if you want to be sure the input has 10 or 16 digits, use size():
int size=aux.size();
if (size!=10) //or 16, whatever you want
cout<<"invalid number"<<endl;

Answer 3

Changing 10 with 16 won't work at all, the problem is the upper limits of an int which is usually 2147483647, inputing values above this will not work, meaning the digit will always be 2.
What you need to do is to use a larger type. You can safely use long long int which can take 19 digits.
long long number;
It's max value is guaranteed to be 9223372036854775807 at least.
Ideally, if possible, you should extract the user input as a string, it would be a much easier task to then extract the first character as exemplified by largest_prime_is_463035818.