Perl has several ways to round a floating number, each has its own specific purpose, here are some of them.

## Int()

a=int(2.65); # a has value 2.

as you can see int() merely truncates toward 0.

## POSIX ceil() and floor()

use POSIX;

$ceil = ceil(3.5); # 4

$floor = floor(3.5); # 3

## sprintf() and printf()

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

## Math round()

Math::Round supplies functions that will round numbers in different ways. The functions round and nearest are exported by default; others are available as described below. "use ... qw(:all)" exports all functions.

use Math::Round;

my $rounded = round( $float );

Some of functions:

### round LIST

Rounds the number(s) to the nearest integer. In scalar context, returns a single value; in list context, returns a list of values. Numbers that are halfway between two integers are rounded "to infinity"; i.e., positive values are rounded up (e.g., 2.5 becomes 3) and negative values down (e.g., -2.5 becomes -3).

Starting in Perl 5.22, the POSIX module by default exports all functions, including one named "round". If you use both POSIX and this module, exercise due caution.

### round_even LIST

Rounds the number(s) to the nearest integer. In scalar context, returns a single value; in list context, returns a list of values. Numbers that are halfway between two integers are rounded to the nearest even number; e.g., 2.5 becomes 2, 3.5 becomes 4, and -2.5 becomes -2.

### round_odd LIST

Rounds the number(s) to the nearest integer. In scalar context, returns a single value; in list context, returns a list of values. Numbers that are halfway between two integers are rounded to the nearest odd number; e.g., 3.5 becomes 3, 4.5 becomes 5, and -3.5 becomes -3.

...

## Other way

my $rounded = int($float + $float/abs($float*2));

With this calculation -1.4 is rounded to -1, and -1.6 to -2.

## Note:

In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2.

Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.

To see why, notice how you'll still have an issue on half-way-point alternation:

for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7

0.8 0.8 0.9 0.9 1.0 1.0

Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32 bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.

Comments powered by CComment