範圍 Range

通常以下面的常值語法建立一個範圍（Range）。 It is typically constructed with a range literal, consisting of two or three dots:

• x..y: 用兩個點來表示閉區間，包含 x 及 y 以及中間的所有值（在數學上我們用 [x, y] 表示）
• x...y: 用三個點來表示半開區間，包含 x 到 y 之間的中間的所有值，但並不包含 y（在數學上我們用 [x, y) 表示）

(0..5).to_a  # => [0, 1, 2, 3, 4, 5]
(0...5).to_a # => [0, 1, 2, 3, 4]

NOTE: Range literals are often wrapped in parentheses, for example if it is meant to be used as the receiver of a call. 0..5.to_a without parentheses would be semantically equivalent to 0..(5.to_a) because method calls and other operators have higher precedence than the range literal.

一個簡單好記的祕訣就是：y 會被多餘的點（.）推出去這個範圍，所以他就被排除在外了。

The literal x..y is semantically equivalent to the explicit constructor Range.new(x, y) and x...y to Range.new(x, y, true).

The begin and end values do not necessarily need to be of the same type: true..1 is a valid range, although pretty useless Enumerable methods won't work with incompatible types. They need at least to be comparable.

Ranges with nil as begin are called begin-less and nil as end are called end-less ranges. In the literal notation, nil can be omitted: x.. is an end-less range starting from x, and ..x is an begin-less range ending at x.

numbers = [1, 10, 3, 4, 5, 8]
numbers.select(6..) # => [10, 8]
numbers.select(..6) # => [1, 3, 4, 5]

numbers[2..] = [3, 4, 5, 8]
numbers[..2] = [1, 10, 3]

A range that is both begin-less and end-less is valid and can be expressed as .. or ... but it's typically not very useful.