Difference between '..' (double-dot) and '...' (triple-dot) in range generation?
The documentation for Range† says this:
Ranges constructed using
..
run from the beginning to the end inclusively. Those created using...
exclude the end value.
So a..b
is like a <= x <= b
, whereas a...b
is like a <= x < b
.
Note that, while to_a
on a Range of integers gives a collection of integers, a Range is not a set of values, but simply a pair of start/end values:
(1..5).include?(5) #=> true
(1...5).include?(5) #=> false
(1..4).include?(4.1) #=> false
(1...5).include?(4.1) #=> true
(1..4).to_a == (1...5).to_a #=> true
(1..4) == (1...5) #=> false
†The docs used to not include this, instead requiring reading the Pickaxe’s section on Ranges. Thanks to @MarkAmery (see below) for noting this update.
That is correct.
1.9.3p0 :005 > (1...10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
1.9.3p0 :006 > (1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
The triple-dot syntax is less common, but is nicer than (1..10-1).to_a
a...b
excludes the end value, while a..b
includes the end value.
When working with integers, a...b
behaves as a..b-1
.
>> (-1...3).to_a
=> [-1, 0, 1, 2]
>> (-1..2).to_a
=> [-1, 0, 1, 2]
>> (-1..2).to_a == (-1...3).to_a
=> true
But really the ranges differ on a real number line.
>> (-1..2) == (-1...3)
=> false
You can see this when incrementing in fractional steps.
>> (-1..2).step(0.5).to_a
=> [-1.0, -0.5, 0.0, 0.5, 1.0, 1.5, 2.0]
>> (-1...3).step(0.5).to_a
=> [-1.0, -0.5, 0.0, 0.5, 1.0, 1.5, 2.0, 2.5]
The API docs now describe this behaviour:
Ranges constructed using
..
run from the beginning to the end inclusively. Those created using...
exclude the end value.-- http://ruby-doc.org/core-2.1.3/Range.html
In other words:
2.1.3 :001 > ('a'...'d').to_a
=> ["a", "b", "c"]
2.1.3 :002 > ('a'..'d').to_a
=> ["a", "b", "c", "d"]