The number of digits in a number entered by the user

[PlusPower]

Hi guys,

I'm new to c++ and I've got a question. I want to make a c++ program that displays the number of digits in a number entered by the user and I want to make sure he doesn't enter alphabetical character.(For this I use the isdigit() function).
I know I can use strlen to find out the length of a string. But is something like this for numbers? I mean a function or something that returns the number of digits in a number?
I would appreciate any reply that can help me out.
Thanks in advance!

[hamsterman]

If you use isdigit on something, then you have a string and can use strlen. If you do have an int N, you could

unsigned int copy = N;
int count = N == 0 ? 1 : 0;
while( copy > 0 ) copy/=10, count++;

(works for non negative integers)
or you could
int count = N == 0 ? 1 : log10(N)+1;(same restriction)
or you could

std::stringstream s;
s << N;
int count = s.str().length();

(works for anything)

[Duthomhas]

This is a homework question, so forget the string stuff.

If the user did not enter a number like he should have, then cin will be in a fail state. You need to do something like the following:

unsigned int n;
while (true)
  {
  // tell the user what to do
  cout << "Please enter a non-negative integer: ";

  // get the number
  // if everything is ok, then we're done
  if (cin >> n) break;

  // fix cin to work again
  cin.clear();
  // get rid of the incorrect input (not a number)
  cin.ignore( 1000, '\n' );
  // complain about it to the user
  cout << "Hey, that was not correct!\n";
  }

cout << "Good job! You entered the number " << n << "\n";

To get the number of digits, count how many times you can divide the number by ten before it becomes zero, and add one.

@hamsterman
Your algorithm indicates that my number "0" has zero digits. (Count should always start at one. :-)

[PlusPower]

Thank you both for your posts!
@Duoas My program works. And why should I forget the string stuff ?Before trying this I made a program that checks whether the user had entered a number or not with isdigit() function and I thought the function would help me with this too.
@hamsterman The first method you used is allright for me but the others...I didn't get it. If you want, will you please explain me so I can understand?
I want to use all the methods well and by the way, this isn't a homework.