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Intelligent Matching in Perl

Is 10 greater than 8?

As a number, yes it is ... but if you're comparing character strings and you would for a book's index, then the character "1" comes before the character "8", so in this context the 10 is earlier than (i.e. less than) 8.

Is 14.0 the same as 14?

As a numeric value, yes it is. As a string of characters, no, it isn't.

Language authors have had the conundrum of how to make comparisons such as these since computer programming started. Does "equal" mean "identical", or does it mean "having the same value as", or even "occupying the same memory location as" or just "looks like". When he wrote Perl, Larry Wall provided a range of comparisons so that you - the programmer - can get exactly what you want in each circumstance.

==Numeric Equality (same value)also != < <= > and >=
eqString Equality (same characters)also ne lt le gt and ge
=~Pattern match (looks like a pattern)also !~

As from Perl 5.10, the ~~ intelligent match operator provides a single syntax through which the most appropriate of these will be chosen in each circumstance, bearing in mind the types of operands passed in. So:

use feature ':5.10';
$first = "Hello";
$second = "World";
say ("numerically the same") if ($first == $second);
say ("stringily the same") if ($first eq $second);
say ("look the same") if ($first =~ $second);
say ("intelligently the same") if ($first ~~ $second);

reports just:
  -bash-4.0$ perl im10
  numerically the same

as the natural way to compare two strings is the eq comparator.

But the ~~ (intelligent match) operator goes further - it provides all sort of other natural comparisons (there's an authoritative list [here]) that can save you the need for loops and other obfuscations of your code. For example - if you compare two lists, Perl's ~~ operator will (intelligently) compare each item in the first list with the item at the same position in the second list, and will return you a true value if and only if both lists have the same length, and each item is equal to its equivalent item:

use feature ':5.10';
@keywords = ("Melksham","Hotel","Community","Open Source","Training","Courses");
@morekeys = ("Melksham","Hotel","Community","Open Source","Courses","Training");
say ("The lists are identical (1)") if (@keywords ~~ @morekeys);
@kw1 = sort(@keywords);
@kw2 = sort(@morekeys);
say ("The lists are identical (2)") if (@kw1 ~~ @kw2);

And that reports
  -bash-4.0$ perl im2
  The lists are identical (2)

On the basis that the two initial lists weren't the same - the contained the same things in a different order. But once sorted, they become the same in every element.

As a final example, what happens when we compare a list to a hash?
  say ("They match") if (@keywords ~~ %wiltshire);

The hash is said to match the list if any of the keys of the hash is equal to any member of the list. So what we're saying (in a simple comparison) is "do you have data for any of [these] in the hash table?".

So this program:

use feature ':5.10';
@keywords = ("Melksham","Hotel","Community","Open Source","Training","Courses");
%wiltshire = (
  Salisbury => "Cathedral City in South of County",
  Stonehenge => "Ancient Tourist Attraction",
  Pewsey => "Small Town in a Vale");
say ("They match (1)") if (@keywords ~~ %wiltshire);
$wiltshire{"Melksham"} = "Friendly town in the west of the county";
say ("They match (2)") if (@keywords ~~ %wiltshire);

does not match the first time, but does the second.
  -bash-4.0$ perl im3
  They match (2)

Writing your own modules - and using them from the CPAN too.We cover matching on all of our fundamental Perl Courses - and we revise and revisit the subject during advanced courses too. The "Intelligent Matching" is currently [June 2010] covered and demonstrated, but does not form a core part of later examples due to the limited adoption of Perl 5.10 and 5.12 up to this point / the need for most author's code to work with older releases of Perl. See [here] for a discussion of the issues that we have considered in making this (current) decision. This is a changing situation - on private courses, we'll be guided by the customer's needs, and over coming months / years, coverage of intelligent matching, given, say, etc will ease into our courses.

Picture - delegates on a recent private Perl course which I gave in Germany

Complete source code for the examples in this article may be found on our site - you are welcome to cut and paste them to try them out, but please note that they're demonstrations only and we cannot take responsibility for the consequences of their use in your production code. We do retain copyright, and we'll get rather upset with you if you copy loads of our examples and start giving unauthorized courses yourself based on our materials.

[source] - Intelligent match on scalars
[source] - Comparing Complete Lists
[source] - Checking for keys in a hash

(written 2010-06-18, updated 2010-06-20)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
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