decimal calculation

[bestfa]

I coded a script to print from 0.01 to 7.99.

    $j=0;
    for($i=0;$i<799;$i++)
    {
        $j=$j+0.01;
        print $WF1 "$j\t";
    }

Strangely, I got results like

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.44 0.45 0.46 0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.57 0.58 0.59 0.6 0.61 0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79 0.8 0.810000000000001 0.820000000000001 0.830000000000001 0.840000000000001 0.850000000000001 0.860000000000001 0.870000000000001 0.880000000000001 0.890000000000001 0.900000000000001 0.910000000000001 0.920000000000001 0.930000000000001 0.940000000000001 0.950000000000001 0.960000000000001 0.970000000000001 0.980000000000001 0.990000000000001 1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.1 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.2 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.3 1.31 1.32 1.33 1.34 1.35 1.36 1.37 1.38 1.39 1.4 1.41 1.42 1.43 1.44 1.45 1.46 1.47 1.48 1.49 1.5 1.51 1.52 1.53 1.54 1.55 1.56 1.57 1.58

Why did I get 0.810000000000001? Thank you for many replies. I should study them when I am free.

[Ratazong]

Please have a look at the FAQ. And if you really want to know, look at the excellent (long) article linked there: David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic".

HTH, Rata

[bestfa]

Thank you for your reply. I should study it when I am free. Thanks again.

[Marshall]

possible alternate code using printf() to round off the imprecision with a power of 10 in fractional base 2 math.

#!usr/bin/perl
use strict;
use warnings;

my $j=0;
for (1..800) #loop 800 times
{
    $j+= 0.01;
    printf ("%0.2f\t", $j); #2 decimal places
}
__END__

Output:

0.01    0.02    0.03    0.04    0.05    0.06 ......
7.95    7.96    7.97    7.98    7.99    8.00    
Process completed successfully

[Athanasius]

Hello bestfa,

In the spirit of TMTOWTDI, here’s an approach that avoids the difficulties of floating point numbers by using only integers:

#! perl
use strict;
use warnings;

for my $i (1 .. 799)
{
    $i =  sprintf "%03d", $i;
    $i =~ s{(\d)(\d{2})}{$1.$2};
    print "$i\t";
}

Output:

19:22 >perl 1570_SoPW.pl
0.01    0.02    0.03    0.04    0.05    0.06    0.07    0.08    0.09  
+  0.10
0.11    0.12    0.13    0.14    0.15    0.16    0.17    0.18    0.19  
+  0.20
...
7.81    7.82    7.83    7.84    7.85    7.86    7.87    7.88    7.89  
+  7.90
7.91    7.92    7.93    7.94    7.95    7.96    7.97    7.98    7.99
19:22 >

Hope that helps,

[syphilis]

See:

perldoc -q decimal
perldoc perlnumber

Cheers,
Rob

[KurtSchwind]

I see you already have some nice answers to your question.

It appears that you are working with monetary units. This is something I work with often. The answer is nearly ALWAYS to work in 'cents' so that you can keep everything in an integer format and then just modify your presentation layer to add the decimal place where you want it. I'm sure that the FAQ linked will say something similar.