How many digits are in a number?

[The Snowman]

This might sound like a simple question, but I can't think of a reasonable answer. Suppose you make a program where a user must input an ID number, and the ID number must be exactly seven digits, how would you write a function that could count the amount of digits in a number input by the user? The only way I've been able to think of is to make an if statement that says if(user_number < 999,999 || user_number > 10,000,000), but that seems unreasonable. I can't think of a way to do it with an array, because you wouldn't ask the user to input an ID one digit at a time, so the numbers couldn't be stored individually in an array. Is there some type of mathematical expression that can solve how many digits are in a number?

[mbozzi]

floor(log(abs(n))/log(radix)) + 1
Works for integers, at least.

I would go with your first solution. It's dead simple.

[SamuelAdams]

Your way seems simplest to me.
another way would be count the 0-9's
another way would be check the string size, but that will not catch alpha chars.

[48T7M4Gy]

Your way seems simplest to me too. Add a few other checks as below and decimal numbers, leading zeroes and non integer characters get captured as errors.

#include <iostream>

int main()
{
    int ID = 0;
    
    while ( std::cout << "Please enter  ID: " and (!(std::cin >> ID) or ID > 10000000 or ID < 999999 ))
    {
        std::cout << "Wrong!\n";
        std::cin.clear();
        std::cin.ignore(1000, '\n');
    }
    std::cout << ID << '\n';
    return 0;
}

[jonnin]

Yea, what they said... if you handle the garbage and don't allow leading zeros, the comparison is the easy, simple answer.