in reply to Re: How to calculate the sum of columns to be equal to 100?
in thread How to calculate the sum of columns to be equal to 100?

One floating-point technique that is sometimes used is called banker’s rounding,” which rounds even-ending numbers one way, odd-ending numbers the other. (I don’t rightly recall if Perl implements it.)

Yes, Perl implements it, because it uses C libraries which implement the rounding recommended by IEEE.

The idea is the following. Suppose, to take a simple case, that you want to round to the unit numbers which have only one decimal digit. Any number where the decimal digit is less than 5 will be rounded down and any number where the decimal digit is larger than 5 is rounded up. But what do you do if the decimal part is exactly 5? Say, for example, how do you round 3.5? The most usual method rounds such a number up. But bankers claim that this introduces a bias towards rounding up: out of ten possible decimal digits, one will not be rounded (0), 4 will be rounded down (1, 2, 3 and 4) and 5 will be rounded up (5 to 9). This can make a difference if you add a long series of numbers. So they decided that the rounding of the 5 decimal digit will be rounded up or down, depending on whether the previous digit is odd or even.

This is what you can see in the somewhat strange output of a Perl one-liner below: 

$ perl -e 'print "raw\t\trounded\n"; printf "%f\t%.0f\n", $_+0.5,  $_+
+0.5 for 0..10; '
raw             rounded
0.500000        0
1.500000        2
2.500000        2
3.500000        4
4.500000        4
5.500000        6
6.500000        6
7.500000        8
8.500000        8
9.500000        10
10.500000       10

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I actually once had to write a special rounding module just because my client considered the above to be simply wrong and wanted 2.5 to be rounded to 3.

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Re^3: How to calculate the sum of columns to be equal to 100?
by Athanasius (Archbishop) on Aug 15, 2013 at 15:33 UTC

    Thanks Laurent_R, this is useful information I hadn’t come across before. Is it documented anywhere? I don’t see anything in sprintf or the Camel Book, and the latest C Standard doesn’t seem to cover it either?

    I actually once had to write a special rounding module just because my client considered the above to be simply wrong and wanted 2.5 to be rounded to 3.

    Simply adding a minimum field width to the format seems to do the trick:

     1:03 >perl -wE "printf qq[%9f\t%.0f\t%2.0f\n], ($_ + 0.5) x 3 for 0 .
+. 10;"
 0.500000       0        1
 1.500000       2        2
 2.500000       2        3
 3.500000       4        4
 4.500000       4        5
 5.500000       6        6
 6.500000       6        7
 7.500000       8        8
 8.500000       8        9
 9.500000       10      10
10.500000       10      11

 1:03 >perl -v

This is perl 5, version 18, subversion 0 (v5.18.0) built for MSWin32-x
+86-multi-thread-64int

    [download]

    It should perhaps also be pointed out that where the internal representation of a floating point number is inexact, all bets are off:

     1:29 >perl -wE "printf qq[%21.18f\t%.1f\t%4.1f\n], ($_ + 0.05) x 3 fo
+r 0 .. 10;"
 0.050000000000000003   0.1      0.1
 1.050000000000000000   1.1      1.1
 2.049999999999999800   2.0      2.0
 3.049999999999999800   3.0      3.0
 4.049999999999999800   4.0      4.0
 5.049999999999999800   5.0      5.0
 6.049999999999999800   6.0      6.0
 7.049999999999999800   7.0      7.0
 8.050000000000000700   8.1      8.1
 9.050000000000000700   9.1      9.1
10.050000000000001000   10.1    10.1

 1:29 >

    [download]

    Cheers,

    Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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      I read it originally in the documentation of the standard C library. You can certainly find the IEEE standard on this.

      Strangely, your "trick" with the format does not work for me on 5.14.2: 

      $ perl -wE 'printf qq[%9f\t%.0f\t%2.0f\n], ($_ + 0.5) x 3 for 0 .. 10;
+'
 0.500000       0        0
 1.500000       2        2
 2.500000       2        2
 3.500000       4        4
 4.500000       4        4
 5.500000       6        6
 6.500000       6        6
 7.500000       8        8
 8.500000       8        8
 9.500000       10      10
10.500000       10      10

      [download]
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        The trick works for me on DWIM Perl 5.14.2:

        11:55 >perl -wE "printf qq[%9f\t%.0f\t%2.0f\n], ($_ + 0.5) x 3 for 0 .
+. 10;"
 0.500000       0        1
 1.500000       2        2
 2.500000       2        3
 3.500000       4        4
 4.500000       4        5
 5.500000       6        6
 6.500000       6        7
 7.500000       8        8
 8.500000       8        9
 9.500000       10      10
10.500000       10      11

11:55 >perl -v

This is perl 5, version 14, subversion 2 (v5.14.2) built for MSWin32-x
+86-multi-thread

        [download]

        Likewise on my 3 Strawberry Perls:

        • This is perl, v5.10.1 (*) built for MSWin32-x86-multi-thread
        • This is perl 5, version 16, subversion 0 (v5.16.0) built for MSWin32-x86-multi-thread-64int
        • This is perl 5, version 18, subversion 0 (v5.18.0) built for MSWin32-x86-multi-thread-64int

        I’m running under Windows Vista 32-bit. So it seems that Perl’s printf behaves differently under Windows. No mention of this in perlport or perlwin32, though. Strange.

        Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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