Perl Cookbook, 2nd Edition

Chapter 5. Hashes

Doing linear scans over an associative array is like trying to club someone to death with a loaded Uzi.

—Larry Wall

Introduction

People and parts of computer programs interact in all sorts of ways. Single scalar variables are like hermits, living a solitary existence whose only meaning comes from within the individual. Arrays are like cults, where multitudes marshal themselves under the name of a charismatic leader. In the middle lies the comfortable, intimate ground of the one-to-one relationship that is the hash. (Older documentation for Perl often called hashes associative arrays, but that’s a mouthful. Other languages that support similar constructs sometimes use different terms for them; you may hear about hash tables, tables, dictionaries, mappings, or even alists, depending on the language.)

Unfortunately, this isn’t a relationship of equals. The relationship encoded in a hash is that of the genitive case or the possessive, like the word “of " in English, or like “’s”. We could encode that the boss of Nat is Tim. Hashes only give convenient ways to access values for Nat’s boss; you can’t ask whose boss Tim is. Finding the answer to that question is a recipe in this chapter.

Fortunately, hashes have their own special benefits, just like relationships. Hashes are a built-in data type in Perl. Their use reduces many complex algorithms to simple variable accesses. They are also fast and convenient to build indices and quick lookup tables.

Only use the % when referring ...