unexpected result using '>='

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

The following code enters the 'unless' block even though the two sides seem equal. could someone possibly explain why this is and suggest how I should express this?

#!/usr/bin/perl -w
use strict;

print "Content-type: text/html\n\n";

my $in = 119.8;
my $in_view = $in * 2;

my $count = 0;
my $period = 599;
while ($count < 5){
  unless  ($period >= $in_view) {
    print "$period ::$in_view<br>";
    $in = $period;    
   } 

   $count += 1;
   $period = $period - $in;
  
}
[download]

Comment on unexpected result using '>=' Download Code

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Re: unexpected result using '>=' by GrandFather (Saint) on Aug 16, 2006 at 23:01 UTC
Adding `print "\n", $in_view - $period, "\n";` [download] after your current print line tells a story: `Content-type: text/html 239.6 ::239.6<br> 2.8421709430404e-014 0 ::239.6<br> 239.6` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^2: unexpected result using '>=' by jonnyfolk (Vicar) on Aug 16, 2006 at 23:18 UTC
I see, thanks How can I force the calculation to calculate to 2 decimal places only - I've tried sprintf but the result is still in discrepancy.	[reply]
Re^3: unexpected result using '>=' by ysth (Canon) on Aug 16, 2006 at 23:47 UTC
You mean something like `$period - $in_view > .01` instead of `$period >= $in_view`? By the way, it's just a matter of time till you burn yourself by misinterpreting the sense of an `unless`. Some people recommend never using `unless`; I think it's ok so long as the expression is a single simple variable.	[reply] [d/l] [select]
Re^4: unexpected result using '>=' by GrandFather (Saint) on Aug 16, 2006 at 23:51 UTC
Re^3: unexpected result using '>=' by GrandFather (Saint) on Aug 16, 2006 at 23:27 UTC
You could use `ge` rather than `>=` which stringises the values. Your original code then prints: `Content-type: text/html 119.8 ::239.6<br>` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^4: unexpected result using '>=' by Thelonius (Priest) on Aug 17, 2006 at 16:31 UTC
Re^4: unexpected result using '>=' by jonnyfolk (Vicar) on Aug 16, 2006 at 23:33 UTC
Re: unexpected result using '>=' by ysth (Canon) on Aug 16, 2006 at 23:42 UTC
Because we use base 10, not base 2, floating point numbers have a non-integral number of digits of precision. When showing them as strings, the next lower number of digits of precision is used, and what you see is slightly rounded. So $period and $in_view can be not quite equal and still stringify to the same thing. This is generally regarded as a good thing, since people don't expect `print .1` to produce "0.10000000000000001" instead of ".1".	[reply] [d/l]