Blushing at the facts

My first thought was, 'Silly guy, why is he writing C code? Doesn't he know the Perl(tm) way?'.

Then I thought, sprintf() is expensive .... how fast is it to run the integer math?

use strict;
use warnings;
use diagnostics;
 
use Benchmark qw( :all );;
 
my ( $number, $num );
 
$number = 1100.64686081907;
 
cmpthese( -1, {
                sprintf => sub { $num = sprintf "%.2f", $number; },
                round   => sub { $num = int( $number * 100 + .5 ) / 10
+0; },
              }
        );

# produced the results ...

            Rate sprintf   round
sprintf 123242/s      --    -86%
round   851644/s    591%      --
[download]

So integer math rounding is 6 times as fast as using sprintf.

Of course, if you're doing less than 50,000, you may not notice the time difference, especially if you blink.

--
TTTATCGGTCGTTATATAGATGTTTGCA

Comment on Blushing at the facts Download Code

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Re: Blushing at the facts by Abigail-II (Bishop) on Mar 25, 2004 at 16:47 UTC
There is a subtle, but important difference between `$var1 = sprintf "%.2f" => $number;` [download] and `$var2 = int ($number * 100 + .5) / 100;` [download] `$var1` is a `PV`, that is, the interval variable has a string value, but not a numeric value, while `$var2` is an `NV`, that is, it has a numeric value, but not a string value. This means that `$var1` will have two digits after the decimal point when printed - sprintf has garanteed that. But that's not necessarely true for `$var2`. Since you cannot represent `1/100` exactly in binary, you left the possibility open that if you stringify the number, you end up with more than 2 characters after the decimal point. Now, it may not happen in a thousand testcases, but can you guarantee it will never happen? See `perldoc -q decimal`.	[reply] [d/l] [select]

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Re: Blushing at the facts
by Abigail-II (Bishop) on Mar 25, 2004 at 16:47 UTC

    $var1 = sprintf "%.2f" => $number;
[download]

    $var2 = int ($number * 100 + .5) / 100;
[download]

$var1

PV

$var2

NV

$var1

$var2

1/100

See perldoc -q decimal.

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