Is it possible to use a range as a key for a hash in Ruby?

Question

I'm trying to create a script to go through an index, look at each page number, and tell me what chapter of the book that entry is in. Here's an approximation of what I'm doing:

@chapters = {
  1 => "introduction.xhtml",
  2..5 => "chapter1.xhtml",
  6..10 => "chapter2.xhtml",
  11..18 => "chapter3.xhtml",
  19..30 => "chapter4.xhtml" }

def find_chapter(number)
  @chapters.each do |page_range, chapter_name|
    if number === page_range
      puts "<a href=\"" + chapter_name + "\page" + number.to_s + "\">" + number.to_s + </a>"
    end
  end
end

find_chapter(1) will spit out the string I want, but find_chapter(15) doesn't return anything. Is it not possible to use a range as a key like this?

Answer 1

You can use a range for Hash keys and you can look up keys very easily using select like this:

@chapters = { 1 => "introduction.xhtml", 2..5 => "chapter1.xhtml", 
              6..10 => "chapter2.xhtml", 11..18 => "chapter3.xhtml",                                         
              19..30 => "chapter4.xhtml" } 

@chapters.select {|chapter| chapter === 5 }
 #=> {2..5=>"chapter1.xhtml"} 

If you only want the chapter name, just add .values.first like this:

@chapters.select {|chapter| chapter === 9 }.values.first
 #=> "chapter2.xhtml" 

Answer 2

Here's a concise way of returning just the value of the first matching key:

# setup
i = 17; 
hash = { 1..10 => :a, 11..20 => :b, 21..30 => :c }; 

# find key
hash.find { |k, v| break v if k.cover? i }

Answer 3

Sure, just reverse the comparison

if page_range === number

Like this

@chapters = {
  1 => "introduction.xhtml",
  2..5 => "chapter1.xhtml",
  6..10 => "chapter2.xhtml",
  11..18 => "chapter3.xhtml",
  19..30 => "chapter4.xhtml" }

def find_chapter(number)
  @chapters.each do |page_range, chapter_name|
    if page_range === number
      puts chapter_name
    end
  end
end

find_chapter(1)
find_chapter(15)
# >> introduction.xhtml
# >> chapter3.xhtml

It works this way because === method on Range has special behaviour: Range#===. If you place number first, then Fixnum#=== is called, which compares values numerically. Range isn't a number, so they don't match.

Answer 4

As @Sergio Tulentsev demonstrates, it can be done. The usual way to do this however is by using a case when . It is a bit more flexible because you can execute code in the then clause and you can use an else part handling everything unhandled . It uses the same === method under the hood.

def find_chapter(number)
  title = case number
    when 1      then "introduction.xhtml"
    when 2..5   then "chapter1.xhtml"
    when 6..10  then "chapter2.xhtml"
    when 11..18 then "chapter3.xhtml"
    when 19..30 then "chapter4.xhtml"
    else "chapter unknown"
  end
  #optionally: do something with title
end