strange behavior with perl v 5.8.8

Steve Holden steve@holdenweb.com
Thu Sep 27 15:54:00 GMT 2007


Andrew Louie wrote:
> Hello,
> 
> I've run into a strange adding problem with perl:
> 
> when incrementing by 0.1 i get a strange behavior where at some
> arbitrary  number, it will append 0.9999999999 to the end of the
> number.
> 
> test case:
> ------------------------------------------------------------------------------------
> my $start = 0;
> my $interval = 0.1;
> my $end = 10;
> 
> do{ print "start: $start\n"; $start += $interval;}until($start >= $end);
> ------------------------------------------------------------------------------------
> Results:
> ...
> start: 4.5
> start: 4.6
> start: 4.7
> start: 4.8
> start: 4.9
> start: 5
> start: 5.1
> start: 5.2
> start: 5.3
> start: 5.4
> start: 5.5
> start: 5.6
> start: 5.7
> start: 5.8
> start: 5.9
> start: 5.99999999999999
> start: 6.09999999999999
> start: 6.19999999999999
> start: 6.29999999999999
> start: 6.39999999999999
> start: 6.49999999999999
> start: 6.59999999999999
> start: 6.69999999999999
> start: 6.79999999999999
> start: 6.89999999999999
> ------------------------------------------------------------------------------------
> 
> why all of suddun i get 5.999999999?
> 
Because binary floating-point numbers can't represent decimal values 
like 0.1 exactly.

> Is this a perl problem?
> 
No, it's not a Cygwin problem either. It's a floating-point problem. We 
have to deal with such queries all the time in the Python community:

http://www.python.org/doc/faq/general/#why-am-i-getting-strange-results-with-simple-arithmetic-operations

[...]

The correct way out of this is to use integer variables to control the 
loop and calculate the start value each time around.

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC/Ltd           http://www.holdenweb.com
Skype: holdenweb      http://del.icio.us/steve.holden

Sorry, the dog ate my .sigline


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