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Re: Print Number With Implied Decimal Point
by tobyink (Canon) on Apr 18, 2012 at 14:17 UTC
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use 5.010;
use Math::BigFloat;
my $number = "999999999999999999999900";
my $decimal_places = 2;
Math::BigFloat->precision(-$decimal_places);
say Math::BigFloat->new($number)/(10**$decimal_places);
__END__
9999999999999999999999.00
substr will be faster though.
perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
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use Benchmark qw(cmpthese);
use Math::BigFloat;
Math::BigFloat->precision(-2);
my $number = "999999999999999999999900";
cmpthese -1, {
substr => sub { my $num=$number; substr($num,-2,0) = '.'; },
BigFloat => sub { my $num=$number; Math::BigFloat->new($num)/100;
+},
};
__END__
Rate BigFloat substr
BigFloat 3794/s -- -100%
substr 1668189/s 43874% --
And that doesn't even involve accessing or printing out the constructed BigFloat again. | [reply]
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Re: Print Number With Implied Decimal Point
by choroba (Cardinal) on Apr 18, 2012 at 14:23 UTC
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I am not sure whether this answers your question or whether the benchmark is correct, but I hope it can help you:
#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';
use bigint;
use Benchmark qw/cmpthese/;
my @chars = ('0' .. '9', 'a' .. 'f');
my @list = map {join q(), map $chars[rand @chars], 1 .. 32} 1 .. 100;
say for @list;
$_ = hex $_ for @list;
say for @list;
cmpthese(0, {
substr => sub {
my @l = @list;
substr $_, -2, 0, '.' for @l;
},
regex => sub {
my @l = @list;
s/(..)$/.$1/ for @l;
}
});
__END__
Rate substr regex
substr 327/s -- -14%
regex 380/s 16% --
Update: See the replies for more correct benchmarks and better solutions. Thanks, kennethk and Eliya. | [reply]
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use bigint applied globally severely distorts the results.
As both of your test cases work on strings anyway, you'd get a more useful comparison when restricting the scope of bigint to where it's required.
With your original version, I get on my machine:
Rate substr regex
substr 481/s -- -14%
regex 558/s 16% --
while with the restricted scope of bigint, I get
#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';
use Benchmark qw/cmpthese/;
my @list;
{
use bigint;
my @chars = ('0' .. '9', 'a' .. 'f');
@list = map {join q(), map $chars[rand @chars], 1 .. 32} 1 .. 100;
say for @list;
$_ = (hex $_)."" for @list;
say for @list;
}
cmpthese(0, {
substr => sub {
my @l = @list;
substr $_, -2, 0, '.' for @l;
},
regex => sub {
my @l = @list;
s/(..)$/.$1/ for @l;
}
});
__END__
Rate regex substr
regex 5672/s -- -80%
substr 28469/s 402% --
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Limiting the bigint scope makes a HUGE difference in my tests. 2,100 rows per second vs 700 per sec. Thanks for the tip!
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These numbers look too good to be true:
$_ = "9876543";
s/(?=..)$/./;
say;
__END__
9876543
You cannot actual replace a zero-width assertion.
And before benchmarking, you should always check whether you're producing the correct results. Bogus solutions noone cares for, even if they're fast. | [reply]
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Re: Print Number With Implied Decimal Point
by JavaFan (Canon) on Apr 18, 2012 at 14:24 UTC
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substr tedious? substr $number, -2, 0, "." does the trick, I don't think that's very tedious. A little faster may be:
print substr($number, 0, -2), ".", substr($number, -2);
as that doesn't have to write back a value. But you'll have to benchmark that yourself. | [reply]
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Re: Print Number With Implied Decimal Point
by Anonymous Monk on Apr 18, 2012 at 15:18 UTC
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use bigint;
my $n = hex(unpack('H*', "\x00\x00\x00\x00\x00\x00\xd3\xc2\x1b\xce\xcc
+\xed\xa0\xff\xff\x9c"));
substr $n, -2, 0, '.';
What's tedious about this? | [reply]
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Re: Print Number With Implied Decimal Point
by j355ga (Initiate) on Apr 19, 2012 at 13:52 UTC
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Thanks to all for the detailed analysis. Looks like substr is the best approach after all! | [reply] |
