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

I found a odd question when i wanna round a float number
with Perl buildin function "sprintf" my code is 
use warnings;
use strict;

my $price1   = sprintf("%.1f",100.550);
my $price2   = sprintf("%.0f",100.50);
print "$price1\n";  # price1  has rounded
print "$price2\n";  # but price2 hasn't , Why?

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Of course, i can use the manual round function like some referrence: 
sub round {
  my $f = @_;
  return int($f + .5);
}

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But the question is Why $price1 has been rounded and price2 has not been rounded?

Replies are listed 'Best First'.
Re: A bug of sprintf or not?
by ikegami (Patriarch) on Oct 20, 2008 at 10:20 UTC
    Anonymous Monk is mistaken. This isn't a precision issue. It's banker's rounding. 
    printf("%.0f\n", 100.5);  # 100
printf("%.0f\n", 101.5);  # 102
printf("%.0f\n", 102.5);  # 102
printf("%.0f\n", 103.5);  # 104
printf("%.0f\n", 104.5);  # 104
printf("%.0f\n", 105.5);  # 106

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    Note that all those numbers (but not 100.55) can be represented precisely in floats (0.5 = 2-1).

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Re: A bug of sprintf or not?
by Anonymous Monk on Oct 20, 2008 at 10:02 UTC
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