Re: [perl #24092] sprintf problem

Alex (via RT) wrote:
> I try to round 1.045 on my Perl (v5.6.1) but output is very strange:
> perl -e "print sprintf('%.2f', '1.045')"
> 1.04
> perl -e "print sprintf('%.2f', '1.0451')"
> 1.05
> Is it the bug?

Nope, it's just the way that sprintf() rounds when the digit after the precision 
limit is exactly 5.  In this case, sprintf always rounds towards zero:

$ perl -e "print sprintf('%.2f', '-1.045')"
-1.04

I'll grant that this is suprising given the way most kids are taught to round in 
school.  But there are several ways to define rounding, the most "accurate" 
being "banker's rounding" or "even rounding" where

	1.045 => 1.04
and 	1.055 => 1.06

which turns out to be (over large numbers of calculations) less biased.

Here's a similar thing in C:

#include <stdio.h>

int main()
{
     printf("%.2f\n",1.045);
     printf("%.2f\n",1.0451);
     printf("%.2f\n",1.055);
     printf("%.2f\n",1.0551);
}

which outputs:

1.04
1.05
1.05
1.06


HTH

John

-- 

John Peacock
Director of Information Research and Technology
Rowman & Littlefield Publishing Group
4720 Boston Way
Lanham, MD 20706
301-459-3366 x.5010
fax 301-429-5747

• [perl #24092] sprintf problem by Alex
• Re: [perl #24092] sprintf problem by Nick Ing-Simmons
• Re: [perl #24092] sprintf problem by John Peacock
• Re: [perl #24092] sprintf problem by John Peacock