hash example

[Kaye Ng]

Hello. First of all, I'm reading this ebook,
http://www.ruby-doc.org/docs/ProgrammingRuby/
is it a good book?

---------------------------------------------------------------------------------
instSection['oboe'] » "woodwind"
instSection['cello'] » "string"
instSection['bassoon'] » nil

As the last example shows, a hash by default returns nil when indexed by
a key it doesn't contain. Normally this is convenient, as nil means
false when used in conditional expressions. Sometimes you'll want to
change this default. For example, if you're using a hash to count the
number of times each key occurs, it's convenient to have the default
value be zero. This is easily done by specifying a default value when
you create a new, empty hash.

histogram = Hash.new(0)
histogram['key1'] » 0
histogram['key1'] = histogram['key1'] + 1
histogram['key1'] » 1

-------------------------------------------------------------------
I don't understand the histogram example (starting from "For example, if
you're using a hash to count the...") What does Hash.new(0) do and what
is it used for?

A practical and simple example would be great. Thank you Ruby experts!

 

[Brian Candler]

Hello. First of all, I'm reading this ebook,
http://www.ruby-doc.org/docs/ProgrammingRuby/
is it a good book?

It is. You're reading the 1st edition, which was written for ruby 1.6.
The 2nd edition is for ruby 1.8 and the 3rd edition for ruby 1.9. The
2nd and 3rd editions are pay-for only.

If you're using ruby 1.8, the version you're reading will be perfectly
adequate. 1.8 was a minor update which in the main added more core
classes, like StringIO, without changing much of what was already there.

ruby 1.9 is a substantially different language. If you do use 1.9,
you'll find a lot of information missing from that book or is simply
wrong. (Personally I am sticking with 1.8)

What does Hash.new(0) do and what
is it used for?

It's a Hash which returns 0 for non-existent keys.

$ irb --simple-prompt

h = Hash.new(0) => {}
h[:foo] = 3 => 3
h[:foo] => 3
h[:bar]

=> 0

It's useful when counting things to have the value default to 0, since
you don't have to worry about nils

words = %w(the cat sat on the mat) => ["the", "cat", "sat", "on", "the", "mat"]
count = Hash.new(0) => {}
words.each { |w| count[w] += 1 } => ["the", "cat", "sat", "on", "the", "mat"]
count

=> {"mat"=>1, "cat"=>1, "sat"=>1, "the"=>2, "on"=>1}

Try this again with count = {} and you'll see what I mean.

count = {} => {}
words.each { |w| count[w] += 1 }

NoMethodError: undefined method `+' for nil:NilClass
from (irb):10
from (irb):10:in `each'
from (irb):10
from :0

Here's one possible solution which doesn't use Hash.new(0)

words.each { |w| count[w] = (count[w] || 0) + 1 } => ["the", "cat", "sat", "on", "the", "mat"]
count

=> {"mat"=>1, "cat"=>1, "sat"=>1, "the"=>2, "on"=>1}

 

[Kaye Ng]

The one I'm reading is the free ebook. I am using Ruby 1.9, do you
recommend I use 1.8? Or are there any free ebooks for Ruby 1.9?