Re: strange arithmetic results

One workaround is to *impose* precision: here, you want equality up to the first two decimal places, so you could multiply your numbers by 100 and take the int() value of them to compare. Here's a subroutine:

sub cmp_dollar {
   my ($c, $d) = map { int( $_ * 100 ) } @_;
   $c == $d;
}
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This oughta deal with the trailing \n issue, too, as that will be ignored by the automagic string-number conversion.

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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Re: Re: strange arithmetic results by tommyw (Hermit) on Sep 25, 2001 at 17:24 UTC
Actually, that's not the way to deal with it: you want to get your data values converted into cents (and error correct at this point) very early, and then work in cents (or tenths of cents) throughout, rather than holding them in dollars and multiplying by 100 at odd moments to do comparisons. The reason this is a problem is that, as with the fraction 1/3 in decimal, 1/10, or 1/100 does not have a finite representation in binary. So the end of the value gets chopped off. Then, when it's multiplied back up, that chopped piece is not reinstated. (Exactly the same way that 1/3 * 3 on a calculator can give you the answer 0.99999) So if you've got two amounts which differ by very small fractions of a cent, and multiply them both by 100, you multiply the error by 100. That might be enough to make `int` return different answers. `sub dollars_to_cents { map { int ($_*100+0.5) } @_; } $dollars=<STDIN> $cents=dollars_to_cents($dollars)` [download] would be better way of dealing with the conversion. Change the multiplier in the function, if you wish more accuracy (so 1000 if you wish to deal in tenths of a cent). The addition of 0.5 brings the answer to the closest cent, to allow for the possible rounding error. It doesn't need changing	[reply] [d/l] [select]

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Re: Re: strange arithmetic results
by tommyw (Hermit) on Sep 25, 2001 at 17:24 UTC

Actually, that's not the way to deal with it: you want to get your data values converted into cents (and error correct at this point) very early, and then work in cents (or tenths of cents) throughout, rather than holding them in dollars and multiplying by 100 at odd moments to do comparisons.

The reason this is a problem is that, as with the fraction 1/3 in decimal, 1/10, or 1/100 does not have a finite representation in binary. So the end of the value gets chopped off. Then, when it's multiplied back up, that chopped piece is not reinstated. (Exactly the same way that 1/3 * 3 on a calculator can give you the answer 0.99999)

So if you've got two amounts which differ by very small fractions of a cent, and multiply them both by 100, you multiply the error by 100. That might be enough to make int return different answers.

sub dollars_to_cents { map { int ($_*100+0.5) } @_; }
$dollars=<STDIN>
$cents=dollars_to_cents($dollars)
[download]

Change the multiplier in the function, if you wish more accuracy (so 1000 if you wish to deal in tenths of a cent). The addition of 0.5 brings the answer to the closest cent, to allow for the possible rounding error. It doesn't need changing

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