perldoc -q round gives us this out of perlfaq4 =). It shows you were on one of the right tracks with printf().

Does Perl have a round() function? What about ceil() and floor()? Trig functions?

   printf("%.3f", 3.1415926535);       # prints 3.142
[download]

   use POSIX;
   $ceil   = ceil(3.5);                # 4
   $floor  = floor(3.5);               # 3
[download]

--
Allolex

sprintf "%.0f", $n

sprintf "%d", $n

It seems strange to me that POSIX (a standard module) implements ceil() and floor() and zillions of (mathematical and other) functions, doesn't seem to include a round() function.

It seems strange to me that POSIX (a standard module) implements ceil() and floor() and zillions of (mathematical and other) functions, doesn't seem to include a round() function.

There are very many different forms of rounding.

use warnings;
use Math::Round;

print round(7.4999999), $/;   # yields 7
print round(7.5000000), $/;   # yields 8

print nearest(5,7.4), $/;     # yields 5
print nearest(5,7.6), $/;     # yields 10
[download]

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

The floating-point parts of s?printf() are not implemented directly in perl itself, but handed off to the system library's sprintf() function. As a result, you are at the mercy of any bugs (in rounding or anything else) in the particular version of the system library you (or your customer) are using.

This is the only part of s?printf() handed off in that way in modern perls. In general, I tend to avoid use of floating-point numbers as far as possible in any application for which precise results are important - often you can model the values you need with integers or rationals, and the Math::Big* family of modules can also help out.

Hugo

sub round { int( $_[0] + ( $_[0] < 0 ? -.5 : .5 ) ); }
[download]

Cheers,Ben.

But that breaks on negative numbers. Best to stick with the FAQ answers rather than making up your own. {sigh}

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

I supposed you could get the int() method to work on negatives by using abs() and some arithmetic with it.

if ( $value < 0 ) {
   $rounded = ( int( abs($value) + 0.5) );
   $rounded = $rounded - ( $rounded * 2 );
} else {
$rounded = int($value + 0.5)
}
[download]

or something...

Seems more complicated than sprintf() =)

--
Allolex

Update: Added closing bracket. (2003-05-15 19:27:26 CEST)

Note that sprintf will round values exactly halfway between two numbers (0.5 + int $X) up for odd and down for even numbers to avoid a bias toward rounding up.

Makeshifts last the longest.

I've heard that proposed many times, but I think it's a fallacy. Should you alternate between rounding N.0 up and down also? N.5 needs to be rounded up, and -N.5 needs to be rounded down.

For any integer N, for all reals in the range [ N, N+1 ), that is, including N but not quite including N+1, then N+0.5 is in the upper half of the range closest to N+1, while anything less is in the lower half of the range closest to N.

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[ e d @ h a l l e y . c c ]

benn already offered that solution and merlyn pointed out why it is not optimal. Read more carefully next time.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

/me was too embarassed to point it out :)