/ 
Number-Tolerant-1.44
River stage one • 2 direct dependents • 6 total dependents
/ Number::Tolerant

NAME

Number::Tolerant -- tolerance ranges for inexact numbers

VERSION

version 1.42

$Id: /my/cs/projects/tolerant/trunk/lib/Number/Tolerant.pm 18116 2006-01-27T02:41:47.507682Z rjbs  $

SYNOPSIS

use Number::Tolerant;

my $range  = tolerance(10 => to => 12);
my $random = 10 + rand(2);

die "I shouldn't die" unless $random == $range;

print "This line will always print.\n";

DESCRIPTION

Number::Tolerant creates a number-like object whose value refers to a range of possible values, each equally acceptable. It overloads comparison operations to reflect this.

I use this module to simplify the comparison of measurement results to specified tolerances.

reject $product unless $measurement == $specification;

METHODS

Instantiation

Number::Tolerance->new( ... )

tolerance( ... )

There is a new method on the Number::Tolerant class, but it also exports a simple function, tolerance, which will return an object of the Number::Tolerant class. Both use the same syntax:

my $range = tolerance( $x => $method => $y);

The meaning of $x and $y are dependant on the value of $method, which describes the nature of the tolerance. Tolerances can be defined in five ways, at present:

 method              range
-------------------+------------------
 plus_or_minus     | x ± y
 plus_or_minus_pct | x ± (y% of x)
 or_more           | x to Inf
 or_less           | x to -Inf
 more_than         | x to Inf, not x
 less_than         | x to -Inf, not x
 to                | x to y
 infinite          | -Inf to Inf
 offset            | (x + y1) to (x + y2)

For or_less and or_more, $y is ignored if passed. For infinite, neither $x nor $y is used; "infinite" should be the sole argument. The first two arguments can be reversed for more_than and less_than, to be more English-like.

Offset tolerances are slightly unusual. Here is an example:

my $offset_tolerance = tolerance(10 => offset => (-3, 5));
# stringifies to: 10 (-3 +5)

An offset is very much like a plus_or_minus tolerance, but its center value is not necessarily the midpoint between its extremes. This is significant for comparisons and numifications of the tolerance. Given the following two tolerances:

my $pm_dice = tolerance(10.5 => plus_or_minus => 7.5);
my $os_dice = tolerance(11 => offset => (-8, 7));

The first will sort as numerically less than the second.

from_string($stringification)

A new tolerance can be instantiated from the stringification of an old tolerance. For example:

my $range = Number::Tolerant->from_string("10 to 12");

die "Everything's OK!" if 11 == $range; # program dies of joy

This will not yet parse stringified unions, but that will be implemented in the future. (I just don't need it yet.)

stringify_as($type)

This method does nothing! Someday, it will stringify the given tolerance as a different type, if possible. "10 +/- 1" will stringify_as('plus_or_minus_pct') to "10 +/- 10%" for example.

numify

This returns the numeric form of a tolerance. If a tolerance has both a minimum and a maximum, and they are the same, then that is the numification. Otherwise, numify returns undef.

Overloading

Tolerances overload a few operations, mostly comparisons.

boolean

Tolerances are always true.

numify

Most tolerances numify to undef; see "numify".

stringify

A tolerance stringifies to a short description of itself, generally something like "m < x < n"

infinite  - "any number"
to        - "m <= x <= n"
or_more   - "m <= x"
or_less   - "x <= n"
more_than - "m < x"
less_than - "x < n"
plus_or_minus     - "x +/- y"
plus_or_minus_pct - "x +/- y%"
equality

A number is equal to a tolerance if it is neither less than nor greater than it. (See below).

comparison

A number is greater than a tolerance if it is greater than its maximum value.

A number is less than a tolerance if it is less than its minimum value.

No number is greater than an "or_more" tolerance or less than an "or_less" tolerance.

"...or equal to" comparisons include the min/max values in the permissible range, as common sense suggests.

tolerance intersection

A tolerance & a tolerance or number is the intersection of the two ranges. Intersections allow you to quickly narrow down a set of tolerances to the most stringent intersection of values.

tolerance(5 => to => 6) & tolerance(5.5 => to => 6.5);
# this yields: tolerance(5.5 => to => 6)

If the given values have no intersection, () is returned.

An intersection with a normal number will yield that number, if it is within the tolerance.

tolerance union

A tolerance | a tolerance or number is the union of the two. Unions allow multiple tolerances, whether they intersect or not, to be treated as one. See Number::Tolerant::Union for more information.

EXTENDING

This feature is slighly experimental, but it's here. Custom tolerance types can be created by adding entries to the hash returned by the _tolerance_type method. Keys are package names, and values are ignored. (This registration interface is all but sure to be rewritten in the near future.)

The packages should contain classes that subclass Number::Tolerant, providing at least these methods:

construct  - returns the reference to be blessed into the tolerance object
parse      - used by from_string; returns the object that represents the string
             or undef, if the string doesn't represent this kind of tolerance
valid_args - passed args from ->new() or tolerance(); if they indicate this 
             type of tolerance, this sub returns args to be passed to
             construct

The Number::Tolerant constructor looks through the list of packages for one whose valid_args likes the arguments passed to the constructor. That package's construct is used to build the guts of the object. (This is a simplification; some other logic is applied, including passing literal numbers through unblessed by default.)

TODO

  • Extend from_string to cover unions.

  • Extend from_string to include Number::Range-type specifications.

  • Allow translation into forms not originally used:

    $range = tolerance(9 => to => 17); 
$range->convert_to('plus_minus');
$range->stringify_as('plus_minus_pct');

    stringify_as can be faked, for a few tolerance types, with something like this:

    Number::Tolerance->_tolerance_type->{'destination_type'}->stringify($range);

    Besides being ugly, it's a side-effect that isn't tested or guaranteed to work very often.

  • Break the basic types into their own modules and use Module::Pluggable.

SEE ALSO

The module Number::Range provides another way to deal with ranges of numbers. The major differences are: N::R is set-like, not range-like; N::R does not overload any operators. Number::Tolerant will not (like N::R) attempt to parse a textual range specification like "1..2,5,7..10" unless specifically instructed to. (The valid formats for strings passed to from_string does not match Number::Range exactly. See TODO.)

The Number::Range code:

$range = Number::Range->new("10..15","20..25");

Is equivalent to the Number::Tolerant code:

$range = Number::Tolerant::Union->new(10..15,20..25);

...while the following code expresses an actual range:

$range = tolerance(10 => to => 15) | tolerance(20 => to => 25);

THANKS

Thanks to Yuval Kogman and #perl-qa for helping find the bizarre bug that drove the minimum required perl up to 5.8

AUTHOR

Ricardo SIGNES, <rjbs@cpan.org>

COPYRIGHT

(C) 2004-2006, Ricardo SIGNES. Number::Tolerant is available under the same terms as Perl itself.

1 POD Error

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