Re: C function for returning number of digits?

Ingo Menger wrote: 
Richard Bos schrieb:


Keith Thompson <kst-u@xxxxxxx> wrote:


"Luke Wu" <LookSkywalker@xxxxxxxxx> writes:

Ohh my.... didn't think it would be a two liner.......thank you
#include<math.h>

int numdigits(int n)
   return log10(n) + 1;


I don't think a floating-point solution is best here.  A loop using
integer arithmetic is likely to be faster and more accurate.  For that
matter, a binary search on a lookup table holding powers of 10 is
likely to be even quicker.

*g* Never knock the simple solution. You're quite right, of course 


How about:
  int length;
  char digits[100];   /* should be big enough even for 128 bit longs */
  sprintf(digits, "%d", n);
  length = strlen(digits) - (n<0 ? 1 : 0);

Boxing this code in a function and/or handling special cases (has 0 1
digit or none?) is left as exercise for the OP.


Is sprintf followed by a strlen actually any faster than log10?
I'm dubious about it on modern hardware. Plus the additional test
for sign.

How about a simple integer loop (destroys n, so make a copy
if you need to keep it)

  int length;
  while(n)
  {
    length++;
    n /= 10;
  }

but it still might be slower depending on the availability
of a hardware log instruction vs. integer division speed.

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