Want to Truncate A Number... NOT round it

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

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Re: Want to Truncate A Number... NOT round it by liz (Monsignor) on Feb 11, 2004 at 20:29 UTC
Maybe something like: `my $n=34.57889; my $na=substr($n,0,index($n,'.') + 1 + 3); print "$n $na\n"; __END__ 34.57889 34.578` [download] Note the "3" is for the number of digits after the decimal "." you want to display. Liz	[reply] [d/l]
Re: Re: Want to Truncate A Number... NOT round it by Anonymous Monk on Feb 11, 2004 at 20:37 UTC
Most Righteous! Exactly what I was looking for ! Thanks!	[reply]
Re^2: Want to Truncate A Number... NOT round it by Siddhartha_90 (Initiate) on May 30, 2014 at 02:17 UTC
Just wanted to let you know that I found this really useful. 10 years late alright!	[reply]
Re: Want to Truncate A Number... NOT round it by halley (Prior) on Feb 11, 2004 at 20:28 UTC
The `int()` routine will truncate to an integer value, "toward zero." So `int( $x * 100 ) / 100` will truncate to the penny. `my $n = 34.57889; my $na = int($n * 1000) / 1000; print "$n $na\n"; __OUTPUT__ 34.57889 34.578` [download] A slower but interesting alternative approach... if you're dealing with long strings of digits, is to render the number to MORE digits than required, then strip the undesired digits. `my $n = 34.57889; my $na = sprintf("%3.4f", $n); $na =~ s/(?<=\.\d\d\d)\d*$//; print "$n $na\n"; __OUTPUT__ 34.57889 34.578` [download] Take care when you're talking about "truncate", "floor", "ceiling" and "rounding" when you deal with negative numbers... make sure you're defining your problem correctly, too. -- `[ e d @ h a l l e y . c c ]`	[reply] [d/l] [select]
Re: Re: Want to Truncate A Number... NOT round it by liz (Monsignor) on Feb 11, 2004 at 20:39 UTC
Wouldn't `$na =~ s/(?<=\.\d\d\d)\d$//;` [download] be done better and clearer as: `chop $na;` [download] ? Liz Update*: Since the 4 digits are guaranteed by the sprintf, there should be no problem in using `chop()`.	[reply] [d/l] [select]
Re^3: Want to Truncate A Number... NOT round it by Nkuvu (Priest) on Feb 12, 2004 at 00:33 UTC
Only if you're guaranteed to have exactly one digit after the desired three. Update in response to someone else's update: Aliens must have inserted that sprintf when I wasn't looking.	[reply]
Re: Want to Truncate A Number... NOT round it by metadoktor (Hermit) on Feb 11, 2004 at 20:34 UTC
`#!/usr/local/bin/perl -w use strict; my $n = 34.57889; my $d = 4; # Num of right hand digits to grab my ($lhs,$rhs) = split /\./,sprintf("%f",$n),2; $n = join '.',$lhs,substr($rhs,0,$d); print "$n\n";` [download] metadoktor "The doktor is in."	[reply] [d/l]
Re: Want to Truncate A Number... NOT round it by CountZero (Bishop) on Feb 11, 2004 at 20:53 UTC
TMTOWTDI: `my $n=34.57889; $n=~/(-?\d+\.\d{3})/; print "$n - $1\n";` [download] Update: Added -? to the regex to handle negative numbers (thanks halley) CountZero "If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law	[reply] [d/l]
Re: Re: Want to Truncate A Number... NOT round it by halley (Prior) on Feb 11, 2004 at 21:15 UTC
Breaks on negative numbers. -- `[ e d @ h a l l e y . c c ]`	[reply]
Re: Want to Truncate A Number... NOT round it by jeffa (Bishop) on Feb 11, 2004 at 21:20 UTC
More TIMTOWTDI ... reverse makes for cleaner looking code ;) `$n = reverse substr(reverse($n), 2);` [download] But that's only good if all your numbers have the same number of right hand side digits ... jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re: Want to Truncate A Number... NOT round it by kvale (Monsignor) on Feb 11, 2004 at 20:30 UTC
I would try this: `$n = int(1000*$n)/1000;` [download] -Mark	[reply] [d/l]