about_arrays.rb

require File.expand_path(File.dirname(__FILE__) + '/neo')

class AboutArrays < Neo::Koan
  def test_creating_arrays
    empty_array = Array.new
    assert_equal Array, empty_array.class
    assert_equal 0, empty_array.size
  end

  def test_array_literals
    array = Array.new
    assert_equal [], array

    array[0] = 1
    assert_equal [1], array

    array[1] = 2
    assert_equal [1, 2], array

    array << 333
    assert_equal [1, 2, 333], array
  end

  # Note:
    # "<<" appends an element to the end of an Array
    # Array starts from the 0th element

  def test_accessing_array_elements
    array = [:peanut, :butter, :and, :jelly]

    assert_equal :peanut, array[0]
    assert_equal :peanut, array.first
    assert_equal :jelly, array[3]
    assert_equal :jelly, array.last
    assert_equal :jelly, array[-1]
    assert_equal :butter, array[-3]
  end

  # Note:
    # From RubyDoc:
      # Array indexing starts at 0, as in C or Java. A negative index is assumed to be relative to the end of the array---that is, an index of -1 indicates the last element of the array, -2 is the next to last element in the array, and so on.

  def test_slicing_arrays
    array = [:peanut, :butter, :and, :jelly]

    assert_equal [:peanut], array[0,1]
    assert_equal [:peanut, :butter], array[0,2]
    assert_equal [:and, :jelly], array[2,2]
    assert_equal [:and, :jelly], array[2,20]
    assert_equal [], array[4,0]
    assert_equal [], array[4,100]
    assert_equal nil, array[5,0]
  end

  # Note:
  #   From RubyDoc:
  #     Elements in an array can be retrieved using the Array#[] method. It can take a single integer argument (a numeric index), a pair of arguments (start and length) or a range.

  def test_arrays_and_ranges
    assert_equal Range, (1..5).class
    assert_not_equal [1,2,3,4,5], (1..5)
    assert_equal [1, 2, 3, 4, 5], (1..5).to_a
    assert_equal [1, 2, 3, 4], (1...5).to_a
  end

  # Note
  #   From RubyDoc:
  #     A Range represents an interval—a set of values with a beginning and an end. Ranges may be constructed using the s..e and s...e literals, or with ::new. Ranges constructed using .. run from the beginning to the end inclusively. Those created using ... exclude the end value. When used as an iterator, ranges return each value in the sequence.

  def test_slicing_with_ranges
    array = [:peanut, :butter, :and, :jelly]

    assert_equal [:peanut, :butter, :and], array[0..2]
    assert_equal [:peanut, :butter], array[0...2]
    assert_equal [:and, :jelly], array[2..-1]
  end

  def test_pushing_and_popping_arrays
    array = [1,2]
    array.push(:last)

    assert_equal [1, 2, :last], array

    popped_value = array.pop
    assert_equal :last, popped_value
    assert_equal [1, 2], array
  end

  # Note:
  #   Push an element to the end of an Array using either << or push()
  #   pop() removes and returns the last Array element

  def test_shifting_arrays
    array = [1,2]
    array.unshift(:first)

    assert_equal [:first, 1, 2], array

    shifted_value = array.shift
    assert_equal :first, shifted_value
    assert_equal [1,2], array
  end

  # Note
  #   From RubyDoc:
  #     shift: Removes the first element and returns it (shifting all other elements down by one). Returns nil if the array is empty.
  #     unshift: Prepends objects to the front of array, moving other elements upwards.
end