Cool tricks and expressive snippets with ruby collections/enumerables [closed]

Question

What are your favorite code snippets with ruby collections? Preferably they should be discovery for you, be expressive, readable and introduce some fun in your coding practice.

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Pattern-matching in arrays (for local variables and parameters):

(a, b), c = [[:a, :b], :c]
[a,b,c]
=> [:a, :b, :c]

(a,), = [[:a]]
a
=> :a

Assigning from non-arrays to multiple variables:

abc, a, b =* "abc".match(/(a)(b)./)
=> ["abc", "a", "b"]

nil1, =* "abc".match(/xyz/)
=> []

Initialize array elements with the same expression:

5.times.map { 1 }    
=> [1,1,1,1]

Array.new(5) { 1 }
=> [1,1,1,1,1]

Initialize array with the same value:

[2]*5
=>[2,2,2,2,2]

Array.new 5, 2
=>[2,2,2,2,2]

Sum elements of an array:

[1,2,3].reduce(0, &:+)

=> 6

Find all indices that match condition:

a.each_with_index.find_all { |e, i| some_predicate(e) }.map(&:last)

Alternate CSS classes:

(1..4).zip(%w[cls1 cls2].cycle)

=> [[1, "cls1"], [2, "cls2"], [3, "cls1"], [4, "cls2"]]

Unzipping:

keys, values = {a: 1, b: 2}.to_a.transpose
keys
=> [:a, :b]

Exploring boolean member methods of a string:

"".methods.sort.grep(/\?/)

Exploring string-specific methods:

"".methods.sort - [].methods

Answer 1

Lazy Fibonacci series with memoization, taken from Neeraj Singh:

fibs = { 0 => 0, 1 => 1 }.tap do |fibs|
  fibs.default_proc = ->(fibs, n) { fibs[n] = fibs[n-1] + fibs[n-2] }
end

fibs.take(10).map(&:last).each(&method(:puts))

An implementation of Counting Sort:

module Enumerable
  def counting_sort(k)
    reduce(Array.new(k+1, 0)) {|counting, n| counting.tap { counting[n] += 1 }}.
    map.with_index {|count, n| [n] * count }.flatten
  end
end

An implementation of sum aka prefix sum:

module Enumerable
  def scan(initial=nil, sym=nil, &block)
    args = if initial then [initial] else [] end
    unless block_given?
      args, sym, initial = [], initial, first unless sym
      block = ->(acc, el) { acc.send(sym, el) }
    end
    [initial || first].tap {|res| 
      reduce(*args) {|acc, el| 
        block.(acc, el).tap {|e|
          res << e
        }
      }
    }
  end
end

Here, I experimented with having Hash#each yield KeyValuePairs instead of two-element Arrays. It's quite surprising, how much code still works, after doing such a brutal monkey-patch. Yay, duck typing!

class Hash
  KeyValuePair = Struct.new(:key, :value) do
    def to_ary
      return key, value
    end
  end

  old_each = instance_method(:each)
  define_method(:each) do |&blk|
    old_each.bind(self).() do |k, v|
      blk.(KeyValuePair.new(k, v))
    end
  end
end

Something I have been playing around with is making Enumerable#=== perform recursive structural pattern matching. I have no idea if this is in any way useful. I don't even know if it actually works.

module Enumerable
  def ===(other)
    all? {|el| 
      next true if el.nil?
      begin
        other.any? {|other_el| el === other_el }
      rescue NoMethodError => e
        raise unless e.message =~ /any\?/
        el === other
      end
    }
  end
end

Another thing I toyed around with recently was re-implementing all methods in Enumerable, but using reduce instead of each as the basis. In this case, I know it doesn't actually work properly.

module Enumerable
  def all?
    return reduce(true) {|res, el| break false unless res; res && el } unless block_given?
    reduce(true) {|res, el| break false unless res; res && yield(el) }
  end

  def any?
    return reduce(false) {|res, el| break true if res || el } unless block_given?
    reduce(false) {|res, el| break true if res || yield(el) }
  end

  def collect
    reduce([]) {|res, el| res << yield(el) }
  end
  alias_method :map, :collect

  def count(item=undefined = Object.new)
    return reduce(0) {|res, el| res + 1 if el == item } unless undefined.equal?(item)
    unless block_given?
      return size if respond_to? :size
      return reduce(0) {|res, el| res + 1 }
    end
    reduce(0) {|res, el| res + 1 if yield el }
  end

  def detect(ifnone=nil)
    reduce(ifnone) {|res, el| if yield el then el end unless res }
  end
  alias_method :find, :detect

  def drop(n=1)
    reduce([]) {|res, el| res.tap { res << el unless n -= 1 >= 0 }}
  end

  def drop_while
    reduce([]) {|res, el| res.tap { res << el unless yield el }}
  end

  def each
    tap { reduce(nil) {|_, el| yield el }}
  end

  def each_with_index
    tap { reduce(-1) {|i, el| (i+1).tap {|i| yield el, i }}}
  end

  def find_all
    reduce([]) {|res, el| res.tap {|res| res << el if yield el }}
  end
  alias_method :select, :find_all

  def find_index(item=undefined = Object.new)
    return reduce(-1) {|res, el| break res + 1 if el == item } unless undefined.equals?(item)
    reduce(-1) {|res, el| break res + 1 if yield el }
  end

  def grep(pattern)
    return reduce([]) {|res, el| res.tap {|res| res << el if pattern === el }} unless block_given?
    reduce([]) {|res, el| res.tap {|res| res << yield(el) if pattern === el }}
  end

  def group_by
    reduce(Hash.new {|hsh, key| hsh[key] = [] }) {|res, el| res.tap { res[yield el] = el }}
  end

  def include?(obj)
    reduce(false) {|res, el| break true if res || el == obj }
  end

  def reject
    reduce([]) {|res, el| res.tap {|res| res << el unless yield el }}
  end
end

Answer 2

Initialize multiple values from an array:

a = [1,2,3]
b, *c = a

assert_equal [b, c], [1, [2,3]]

d, = a
assert_equal d, a[0]