Ruby: How to add to new hash of array? - ruby

Question

I have the following code and it works fine.
months = {"Feb"=>["day1", "day2"]}
i = 0
while i < 5 do
months["Feb"][i] = "day#{i}"
i += 1
end
puts months
$> {"Feb"=>["day0", "day1", "day2", "day3", "day4"]}
But if I remove the first line where I intialize the hash or try to add values to a different hash key on the fly I get an error that 'months' in undefined.
So I am confused. Will Ruby not allow you to arbitrarily add keys to a hash? I am used to Perl where you can just start making hashes and arrays as you please. But I know that Perl treats hashes & arrays as seperate objects where as Ruby everything is considered the same, so I didn't know if it had something to do with that (although the Perl way is probably "sloppy" ^_^ )

Answer 1

In Ruby you must initialize a variable before using it (looks like a sound policy...). Note also that what you wrote is not idiomatic Ruby, an alternative:
months = {"Feb" => 0.upto(4).map { |i| "day#{i}" }}
To update:
months["Feb"] = 0.upto(4).map { |i| "day#{i}" }

Answer 2

You always need to initialize variables in Ruby.
But you can initialize hashes as following:
# Using {} to creates an empty hash
months = {}
# or create a new empty hash object from Hash class
months = Hash.new
# instead of
months = {"Feb"=>["day1", "day2"]}
To initialize a Array within a Hash:
# Array.new creates a new Array with size 5
# And values within Hash.new block are the default values for the hash
# i.e. When you call the Hash#[], it creates a new array of size 5
months = Hash.new { |hash, key| hash[key] = Array.new(5) }
puts months #=> {}
puts months["Feb"] # Here the Hash creates a new Array inside "Feb" key
puts months #=> {"Feb" => [nil, nil, nil, nil, nil]}
puts months["Feb"][3] = "Day3"
puts months #=> {"Feb" => [nil, nil, nil, "Day3", nil]}
To do the same using an undefined array size:
months = Hash.new { |hash, key| hash[key] = [] }
puts months #=> {}
months["Feb"].push "Day0"
puts months #=> {"Feb" => ["Day0"]}
months["Feb"].push "Day1"
puts months #=> {"Feb" => ["Day0", "Day1"]}
I think a more elegant approach is to use the map method to build your array of days before bind it to "Feb" key:
months = {"Feb" => (0..4).map{ |i| "day#{i}" }}
# => {"Feb"=>["day0", "day1", "day2", "day3", "day4"]}
If you don't want to type the month name you can require the Date class and get the month name passing an Fixnum:
require 'date'
months = {Date::ABBR_MONTHNAMES[2] => (0..4).map{ |i| "day#{i}"}}
# => {"Feb"=>["day0", "day1", "day2", "day3", "day4"]}
To generate the same structure for all months you can do:
days = (0..4).map{ |d| "day#{d}"}
months = (1..12).map{ |m| {Date::ABBR_MONTHNAMES[m] => days }}
# => {"Jan"=>["day0", "day1", "day2", "day3", "day4"],
# "Feb"=>["day0", "day1", "day2", "day3", "day4"],
# ...
# "Dec"=>["day0", "day1", "day2", "day3", "day4"]}
Some useful documentation:
Hash#new
Array
Enumerable#map
Range
Date

Answer 3

# the ||= set this to an empty hash({}) if it hasn't been initialized yet
months ||= {}
#updated as you like, use symbols :feb over string "feb" as the key (read this in other questions in this website)
months[:feb] = (0..4).map {|n| "day#{n}"}
p months #==> {:feb=>["day0", "day1", "day2", "day3", "day4"]}

Answer 4

Based on what you guys said I messed around and it looks like Ruby wants the key to exist before you can start assigning values to that key.
hashname = Hash.new
hashname[:foo] = {}
hashname[:foo][3] = "test"
hashname[:foo][1] = "test1"
hashname[:bar] = {}
hashname[:bar][3] = "test3"
puts hashname
puts hashname[:foo][1]
C:\Ruby>scratch.rb
{:foo=>{3=>"test", 1=>"test1"}, :bar=>{3=>"test3"}}
test1
Once the key exists, you can start assigning values as you please. Although in this case you can see that I am creating a "hash of hashes" instead of a "hash of arrays" as the .map method does which still eludes me as to why.
Thanks for the help!