On 9/1/10 Wed Sep 1, 2010 1:46 PM, "Jim" <j...@lowcarbfriends.com>
scribbled:
> Can anyone comment how they handle floating point precision situations
> like this?
>
> Here is a basic toy program:
> #!/usr/bin/perl
>
> use strict;
>
> my $total = 0;
> my $a = 21835.30;
> my $b = -21715.90;
> my $c = 9652.20;
> my $d = -9771.60;
>
> print ("total = $total\n");
> $total += $a;
> print ("total = $total\n");
> $total += $b;
> print ("total = $total\n");
> $total += $c;
> print ("total = $total\n");
> $total += $d;
> print ("total = $total\n");
> $total = sprintf("%.2f", $total);
> print ("total = $total\n");
>
>
> And here is the output:
> total = 0
> total = 21835.3
> total = 119.399999999998
> total = 9771.6
> total = -1.81898940354586e-12
> total = -0.00
>
> Note the lack of precision adding these simple numbers which do not have
> a large number of digits to the right of the decimal.
>
> If the line:
> $total = sprintf("%.2f", $total);
> would produce 0.00 rather than -0.00 I could live with this.
>
> But the only solution I have to get the final answer to properly be 0.00
> is to format
> $total = sprintf("%.2f", $total);
> after each increment is added to $total
>
> or maybe put a string comparison at the end checking for "-0.00" and
> changing it to "0.00" if it matches.
>
> I don't like either of these solutions.
>
> How do others deal with this?
I don't worry about it. I do the calculations and print out the result with
printf to whatever precision I need and what I feel is warranted by the
accuracy of the input data. Double-precision 64-bit floating-point has
ALWAYS been accurate enough for my needs.
If I happen to print out some intermediate results, I don't worry about such
"errors" as you have shown, since I know they are normal for computer-based
arithmetic. In real life, an arithmetic evaluation never results in a value
that will print as "-0.00". And if it did, it wouldn't bother me. It is
still zero to two places.
One good approach: plot your data rather than printing it, if possible. It
is easier to see patterns, averages, outliers, etc. in a plot than it is in
a list of numbers.
--
To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
