Good notes posted by Alex

Cheking if a number is prime?

It’s a class for generating an enumerator for prime numbers and traversing over them.

It’s really slow and will be replaced in ruby 1.9 with a faster one.

Note: if you just want to test whether a number is prime or not, you can use this piece of code:

class Fixnum
  def prime?
    ('1' * self) !~ /^1?$|^(11+?)\1+$/
  end
end

10.prime?

Optional Argument for detect/find [Not Documented]

detect/find’s optional argument lets you specify a proc or lambda whose return value will be the result in cases where no object in the collection matches the criteria.

classic_rock_bands = ["AC/DC", "Black Sabbath","Queen", "Ted Nugent and the Amboy Dukes","Scorpions", "Van Halen"]
default_band = Proc.new {"ABBA"}
classic_rock_bands.find(default_band) {|band| band > "Van Halen"}
=> "ABBA"

or

random_band = lambda do
  fallback_bands = ["Britney Spears", "Christina Aguilera", "Ashlee Simpson"]
  fallback_bands[rand(fallback_bands.size)]
end
classic_rock_bands.find(random_band) {|band| band > "Van Halen"}
=> "Britney Spears"

Convert a Hash to an Array of Arrays using map

Although you’ll always have to_a and it’s faster, this trick is too cool to ignore…

When the block is omitted, collect or map uses this implied block: {|item| item}, which means when applied on an hash without a block, collect/map returns an array containing a set of two-item arrays, one for each key/value pair in the hash. For each two-item array, item 0 is the key and item 1 is the corresponding value.

burgers = {"Big Mac" => 300, "Whopper with cheese" => 450, "Wendy's Double with cheese" => 320}

burgers.map
=> [["Wendy's Double with cheese", 320], ["Big Mac", 300], ["Whopper with cheese", 450]]

see also:

• Convert an Array of Arrays to a Hash using Enumerable#inject

• Convert an Array to a Hash using Hash#[]

Using any? on Empty Arrays and Hashes

When applied to an empty array or hash, with or without a block, any? always returns false. That’s because with an empty collection, there are no values to process and return a true value.

Testing Arrays for nils with Enumerable#all?

When the block is omitted, all? uses this implied block: {|item| item}.

Since everything in Ruby evaluates to true except for false and nil, using all? without a block on an array is effectively a test to see if all the items in the collection evaluate to true (or conversely, if there are any false or nil values in the array).

Using all? without a block on a hash is meaningless, as it will always return true.

Using all? on Empty Arrays and Hashes

When applied to an empty array or hash, with or without a block, all? always returns true. That’s because with an empty collection, there are no values to process and return a false value. so, watch out, if your array or hash is empty for any reason you will get a true which might not be what you expect it to be.