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Learning Perl, Second Edition by Randal L. Schwartz, Tom Christiansen

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5.1. What Is a Hash?

A hash[1] is like the array that we discussed earlier, in that it is a collection of scalar data, with individual elements selected by some index value. Unlike a list array, the index values of a hash are not small nonnegative integers, but instead are arbitrary scalars. These scalars (called keys) are used later to retrieve the values from the array.

[1] In older documentation, hashes were called "associative arrays," but we got tired of a seven-syllable word for such a common item, so we replaced it with a much nicer one-syllable word.

The elements of a hash have no particular order. Consider them instead like a deck of filing cards. The top half of each card is the key, and the bottom half is the value. Each time you put a value into the hash, a new card is created. Later, when you want to modify the value, you give the key, and Perl finds the right card. So, really, the order of the cards is immaterial. In fact, Perl stores the cards (the key-value pairs) in a special internal order that makes it easy to find a specific card, so Perl doesn't have to look through all the pairs to find the right one. You cannot control this order, so don't try.[2]

[2] Actually, modules like IxHash and DB_file do provide some ordering, but at the cost of a non-trivial performance penalty.

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