3.3. Arrays

An array is a sequence of values that allows values to be accessed by their position, or index, in the sequence. In Ruby, the first value in an array has index 0. The size and length methods return the number of elements in an array. The last element of the array is at index size-1. Negative index values count from the end of the array, so the last element of an array can also be accessed with an index of –1. The second-to-last has an index of –2, and so on. If you attempt to read an element beyond the end of an array (with an index >= size) or before the beginning of an array (with an index < -size), Ruby simply returns nil and does not throw an exception.

Ruby’s arrays are untyped and mutable. The elements of an array need not all be of the same class, and they can be changed at any time. Furthermore, arrays are dynamically resizeable; you can append elements to them and they grow as needed. If you assign a value to an element beyond the end of the array, the array is automatically extended with nil elements. (It is an error, however, to assign a value to an element before the beginning of an array.)