Difference between revisions of "Ruby Ranges"

Revision as of 12:06, 1 December 2007

Ruby Ranges allow data to be represented in the form of a range (in other words a data set with start and end values and a logical sequence of values in between). The values in a range can be numbers, characters, strings or objects. In this chapter we will look at the three types of range supported by Ruby, namely sequences, conditions and intervals.

Ruby Sequence Ranges

Sequence ranges in Ruby are used to create a range of successive values - consisting of a start value, an end value and a range of values in between.

Two operators are available for creating such ranges, the inclusive two-dot (..) operator and the exclusive three-dot operator (...). The inclusive operator includes both the begin and end values in the range. The exclusive range operator excludes the end value from the sequence. For example:

1..10    # Creates a range from 1 to 10 inclusive
1...10   # Creates a range from 1 to 9

A range may be converted to an array using the Ruby to_a method. For example:

(1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

(1...10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

As mentioned previously, ranges are not restricted to numerical values. We can also create a character based range:

('a'..'l').to_a
=> ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l"]

Also, we can create range based on string values:

('cab'..'car').to_a
=> ["cab", "cac", "cad", "cae", "caf", "cag", "cah", "cai", "caj", "cak", "cal", "cam", 
"can", "cao", "cap", "caq", "car"]

Range values may also be objects. The only requirements for placing an object in a range is that the object provides a mechanism for returning the next object in the sequence via succ and it must be possible to compare the objects using the <=> operator.

Using Range Methods

Given the object-oriented nature of Ruby it should come as no surprise to you that a range is actually an object. As such, there are a number of methods of the Range class which can be accessed to interrogate work with a range object.

words = 'cab'..'car'

words.min         # get lowest value in range
=> "cab"

words.max         # get highest value in range
=> "car"

words.include?('can') # check to see if a value exists in the range
=> true

words.reject {|subrange| subrange < 'cal'} # reject values below a specified range value
=> ["cal", "cam", "can", "cao", "cap", "caq", "car"]

words.each {|word| puts "Hello " + word} # iterate through each value and perform a task
Hello cab
Hello cac
Hello cad
Hello cae
Hello caf
Hello cag
Hello cah
Hello cai
Hello caj
Hello cak
Hello cal
Hello cam
Hello can
Hello cao
Hello cap
Hello caq
Hello car

Ruby Ranges as Conditional Expressions

Ruby Ranges can also be used as conditional expressions in looping conditions. The range start value represents the start of the of loop, which runs until the range end marker is detected.

 while input = gets
 puts input + " triggered" if input =~ /start/ .. input =~ /end/
 end

Ruby Range Intervals

Ranges are ideal for identifying if a value falls within a particular range. For example, we might want to know whether a number is within a certain range, or a character within a certain group of letters arranged in alphabetical order. This is achieved using the === equality operator:

(1..20) === 15       # Does the number fit in the range 1 to 20
=> true
('k'..'z') === 'm'   # Does the letter fall between the letters 'k' and 'z' in the alphabet?
=> true

Ranges in case Statements

Ranges are perhaps at their most powerful when they are used in conjunction with a case statement (for more information see The Ruby case Statement chapter:

score = 70

result = case score
   when 0..40: "Fail"
   when 41..60: "Pass"
   when 61..70: "Pass with Merit"
   when 71..100: "Pass with Distinction"
   else "Invalid Score"
end

puts result