Comments on: Round a Float to # of Significant Digits Following The Decimal Point http://jasonmock.com/2007/02/22/round-a-float-to-of-significant-digits/?utm_source=rss&utm_medium=rss&utm_campaign=round-a-float-to-of-significant-digits [insert something witty here] Tue, 24 Jan 2012 02:45:58 +0000 hourly 1 By: Jason Mock http://jasonmock.com/2007/02/22/round-a-float-to-of-significant-digits/#comment-52600 Jason Mock Sat, 10 May 2008 02:41:40 +0000 http://jasonmock.com/wordpress/2007/02/22/round-a-float-to-of-significant-digits/#comment-52600 Actually, your comment made me realize that I didn't properly describe what it was that I had created. In my specific case, I didn't care about the number of significant digits, I only cared about the number digits following the decimal point. So, I may even re-title the post since it is obviously incorrect. Thank you for pointing that out and not letting me lead others astray. Actually, your comment made me realize that I didn’t properly describe what it was that I had created. In my specific case, I didn’t care about the number of significant digits, I only cared about the number digits following the decimal point. So, I may even re-title the post since it is obviously incorrect. Thank you for pointing that out and not letting me lead others astray.

]]> By: David A. Pimentel http://jasonmock.com/2007/02/22/round-a-float-to-of-significant-digits/#comment-52596 David A. Pimentel Wed, 07 May 2008 21:03:59 +0000 http://jasonmock.com/wordpress/2007/02/22/round-a-float-to-of-significant-digits/#comment-52596 Actually, this is not quite correct assuming you want significant figures instead of decimal places. Your function rounds to a spcified number of places after the decimal point. A definition of significant figures (or digits) can be found at http://en.wikipedia.org/wiki/Significant_figures. To retain only a specified number of significant figures, the function should be: <code>inline double fround(double n, unsigned d) {   unsigned p;   p = d - (unsigned)log10(n);   return floor(n * pow(10., p) + .5) / pow(10., p); }</code> Given fround(1703.12345689,6), the result will be 1703.12, whereas, your original function will return 1703.123157. Here is a simple test to verify the results: <code>#include <stdio.h> #include <math.h> inline double fround1(double n, unsigned d) {   return floor(n * pow(10., d) + .5) / pow(10., d); } inline double fround(double n, unsigned d) {   unsigned p;   p = d - (unsigned)log10(n);   return floor(n * pow(10., p) + .5) / pow(10., p); } int main() {   printf("old fround: %21.8fn", fround1(1703.12345689,6));   printf("new fround: %21.8fn", fround (1703.12345689,6)); }</code> Cheers. Actually, this is not quite correct assuming you want significant figures instead of decimal places. Your function rounds to a spcified number of places after the decimal point. A definition of significant figures (or digits) can be found at http://en.wikipedia.org/wiki/Significant_figures. To retain only a specified number of significant figures, the function should be:

inline double fround(double n, unsigned d)
{
  unsigned p;
  p = d - (unsigned)log10(n);
  return floor(n * pow(10., p) + .5) / pow(10., p);
}

Given fround(1703.12345689,6), the result will be 1703.12, whereas, your original function will return 1703.123157.

Here is a simple test to verify the results:

#include <stdio.h>
#include <math.h>

inline double fround1(double n, unsigned d)
{
  return floor(n * pow(10., d) + .5) / pow(10., d);
}

inline double fround(double n, unsigned d)
{
  unsigned p;
  p = d - (unsigned)log10(n);
  return floor(n * pow(10., p) + .5) / pow(10., p);
}

int main() {
  printf("old fround: %21.8fn", fround1(1703.12345689,6));
  printf("new fround: %21.8fn", fround (1703.12345689,6));
}

Cheers.

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