My computer doesn't like the number 3? (Forget it! It's rounding!!!)

BrowserUk has asked for the wisdom of the Perl Monks concerning the following question:

In the following runs of the test code below, when

$K = 1e2, $L == 2 and the random keys produced have length 2.
$K = 1e4, $L == 4 and the keys produced have length 4.
But when $K = 1e3, $L == 3 as expected, but the random keys have length 2?

Maybe I'm having a "thick" day today, but for the life of me, I cannot see why?

C:\test>junk0 -K=1e2
K:1e2 L:2
bd => 21537
ux => 27554
su => 32707
db => 10843
fe => 3691
aj => 23335
co => 12919
br => 21357
xw => 15950
hd => 24808

C:\test>junk0 -K=1e3
K:1e3 L:3
ne => 32586
qz => 4931
sp => 30318
tr => 9694
gl => 21489
hn => 6456
xx => 24771
sf => 3369
ro => 27152
pv => 1438

C:\test>junk0 -K=1e4
K:1e4 L:4
alof => 16425
tjtd => 16200
pief => 25920
nlly => 18027
oezy => 19290
eyzv => 5510
ztxl => 11976
yqbz => 8356
hoym => 6193
exid => 29312
[download]

The code (run as perl -s scriptname -K=nnn):

#! perl -slw
use strict;

sub rndStr{ join'', @_[ map{ rand @_ } 1 .. shift ] }

sub log10{ log( $_[0] ) / log( 10 ) }

our $K ||= 1e3;
our $L ||= log10( $K );

print "K:$K L:$L";

my %hash;

$hash{ $_ } = int rand 32767 for map{ rndStr( $L, 'a'..'z' ) } 1 .. $K
+;

print "$_ => $hash{ $_ }" for ( keys %hash )[ 0 .. 9 ];
[download]

'Unthick' me please :)

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

Comment on My computer doesn't like the number 3? (Forget it! It's rounding!!!) Select or Download Code

Replies are listed 'Best First'.
Re: My computer doesn't like the number 3? (Forget it! It's rounding!!!) by FunkyMonk (Bishop) on Apr 08, 2008 at 17:59 UTC
It's floating point rounding errors - $L is just a smidgin under 3, but appears to be exactly 2 & 4 for 102 & 104. `our $K \|\|= 1e3; our $L \|\|=log10( $K ); print $L-3; # -4.44089209850063e-16` [download]	[reply] [d/l] [select]
Re: My computer doesn't like the number 3? (Forget it! It's rounding!!!) by ikegami (Patriarch) on Apr 09, 2008 at 00:56 UTC
A little bit of light for those who didn't follow: `sub log10{ log( $_[0] ) / log( 10 ) } printf("%.16e\n", log10($_)) for 1e2, 1e3, 1e4;` [download] `2.0000000000000000e+000 2.9999999999999996e+000 4.0000000000000000e+000` [download]	[reply] [d/l] [select]