Re^3: Calculating base difference of numbers

[almut]

In the examples above I'm looking to see the base point movements of +1,+4 and -5.

I'm not really sure I understand, but maybe something like this?

#!/usr/bin/perl

use strict;
use warnings;

print base_point_movement($_),"\n" for (
          1.5553 - 1.5552,
          0.9984 - 0.998,
          100.25 - 100.3,
      );

sub base_point_movement {
    my $diff = shift;
    (sprintf "%+e", $diff) =~ /([+-]\d)\./;
    return $1;
}

Output:

+1
+4
-5

Update: as has been pointed out, this only works for +/-9 — which is not too surprising, as it just extracts the before-comma digit of the difference in exponential floating-point representation  (not knowing what a "base point" is defined as, I figured it might suffice...).

Anyhow, here's another variant, kind of extending the idea to what might have been meant — though honestly, I don't have the foggiest whether that's what the OP had in mind.  (Of course, this will also run into problems when the number of significant digits exceeds floating-point precision.)

sub base_point_movement {
    my $bpm = sprintf "%+e", shift;
    $bpm =~ s/0*e.*$//;
    $bpm =~ tr/.//d;
    return $bpm;
}

print base_point_movement($_),"\n" for (
          1.5553    - 1.5552,      # +1
          1.5553    - 1.55,        # +53
          1.56      - 1.55,        # +1
          155.53    - 155.52,      # +1
          15553     - 15552,       # +1
          1555300   - 1555200,     # +1
          1555300   - 155520,      # +139978
          1555.300  - 1555.20,     # +1
          1555300   - 1555201,     # +99
          1.5553    - 1.555201,    # +99
          1.5553    - 1.55521,     # +9
          1.5553    - 1.555301,    # -1
          1.5553    - 1.55530101,  # -101
          1.5553    - 1.5553101,   # -101
          15553     - 15552.9999,  # +1
          15553.001 - 15552.999,   # +2
          15553.001 - 15552.9999,  # +11
          15553.1   - 15552.99,    # +11
          # etc. (you get the idea :)
      );

[tachyon-II]

Sadly this will fail for any movement of greater than 9 points. This problem is insoluble without a definition of what constitutes a point. Consider the upper number pair and some possible variations that could exist with trailing zero truncation:

1.5553 - 1.5552 # 1 point
1.5553 - 1.55   # 53 points (your sub will fail with this)
1.56   - 1.55   # 1 point? 100 points? (insoluble from data alone)

You either have to decide what is a point, or guess and accept that there will be errors:

sub points {
    my ($was,$is,$exp) = @_;

    unless ($exp) {
        $was =~ m/\.(\d+)$/;
        my $x = $1 ? length($1) : 0;
        $is =~ m/\.(\d+)$/;
        my $y = $1 ? length($1) : 0;
        $exp = $x > $y ? $x : $y;
    }
    print "($exp) $was => $is\t";
    $exp = 10**$exp;
    my $dif = $is*$exp - $was*$exp;
    # add correction factor to allow int to round correctly
    # also ensures that FP "error" such as 4.999999 ends up as +5
    $dif += $dif < 0 ? -0.5 : +0.5;
    return sprintf "%+d", $dif;
}
print points(1.5553,1.5552), $/;
print points(1.55,1.5553 ),  $/; 
print points(1.55,1.56),     $/;
print points(1.55,1.56,4),   $/;  
print points(0.9984,0.998),  $/;

__DATA__
(4) 1.5553 => 1.5552    -1
(4) 1.55 => 1.5553        +53
(2) 1.55 => 1.56        +1
(4) 1.55 => 1.56        +100
(4) 0.9984 => 0.998        -4

[FunkyMonk]

This fails when the difference is 10 or more base points. Try it with test data of 1.5563 - 1.5552

[Anonymous Monk]

That's it almut!

Thanks....I looked at sprintf but couldn't figure it out....still much Perl goodness to learn from these monks!