Chapter 5. Hashes

Hashes and arrays are the two basic "aggregate" data types supported by most modern programming lagnguages. The basic interface of a hash is similar to that of an array. The difference is that while an array stores items according to a numeric index, the index of a hash can be any object at all.

Arrays and strings have been built into programming languages for decades, but built-in hashes are a relatively recent development. Now that they're around, it's hard to live without them: they're at least as useful as arrays.