How to quickly determine how many digits in an int?

Question

I know I can do:

n = floor(log10(i)) + 1;

Or I can do a quick loop:

while(i) {
  n++;
  i/=10;
}

Is there any better way than a complicated math operation, or a loop to achieve the goal? For example: if i = 1234, then n = 4.

Base 10 operations and "quickly" tend not to go hand in hand. What do you need the length for and why does it need to be fast? — Carl Norum, Oct 09 '13 at 22:27
Have you actually determined that this part of your code needs to be faster? — chris, Oct 09 '13 at 22:27
what about `i <= 0` ? note that in this case log10(i) is not defined. Similarly, your loop code doesn't work for i = 0 — Arunas, Oct 09 '13 at 22:36
Better than "an overly complicated math operation"?! What do you think the "number of digits" *is* other than some "complicated math operation"? It's the logarithm of the number after all... — Kerrek SB, Oct 09 '13 at 22:55
@texasbruce - that sort of assumption should be stated in the question. However, you really should look into the duplicate question as has been noted by others, as the solutions are very elegant and fast in most cases. — Arunas, Oct 09 '13 at 23:14

score 1 · Answer 1 · answered Oct 09 '13 at 22:27

1

The shortest way I know of (not computationally, just in terms of typing) is to call snprintf(3):

int n = snprintf(NULL, 0, "%d", i);

answered Oct 09 '13 at 22:27

Carl Norum

2

But this is not even shorter than version proposed by OP himself. – sasha.sochka Oct 09 '13 at 22:29
It doesn't do any floating point and doesn't require linking `libm`. Decisions, decisions. – Carl Norum Oct 09 '13 at 22:29
What if the value of `i` is negative? – Captain Obvlious Oct 09 '13 at 22:51
Then you'll get one more than the number of digits. – Carl Norum Oct 09 '13 at 23:05

score 0 · Answer 2 · answered Oct 09 '13 at 22:27

0

convert it to a string (itoa) and count the number of characters? (might be not the best performance-wise though)

answered Oct 09 '13 at 22:27

Ashalynd

2 Answers2