class Array

  # Sum the size of single arrays in an array of arrays
  # @param [Array] Array of arrays
  # @return [Integer] Sum of size of array elements
  def sum_size
    self.inject(0) { |s,a|
      if a.respond_to?('size')
        s+=a.size
      else
        internal_server_error "No size available: #{a.inspect}"
      end
    }
  end

  # Check if the array has just one unique value.
  # @param [Array] Array to test.
  # @return [TrueClass,FalseClass] 
  def zero_variance?
    return self.uniq.size == 1
  end

  # Get the median of an array
  # @return [Numeric]
  def median
    sorted = self.sort
    len = sorted.length
    (sorted[(len - 1) / 2] + sorted[len / 2]) / 2.0
  end

  # Get the mean of an array
  # @return [Numeric]
  def mean
    self.compact.inject{ |sum, el| sum + el }.to_f / self.compact.size
  end

  # Get the variance of an array
  # @return [Numeric]
  def sample_variance
    m = self.mean
    sum = self.compact.inject(0){|accum, i| accum +(i-m)**2 }
    sum/(self.compact.length - 1).to_f
  end

  # Get the standard deviation of an array
  # @return [Numeric]
  def standard_deviation
    Math.sqrt(self.sample_variance)
  end
  
  # Calculate dot product 
  # @param [Array]
  # @return [Numeric]
  def dot_product(a)
    products = self.zip(a).map{|a, b| a * b}
    products.inject(0) {|s,p| s + p}
  end

  # Calculate magnitude
  # @return [Numeric]
  def magnitude
    squares = self.map{|x| x ** 2}
    Math.sqrt(squares.inject(0) {|s, c| s + c})
  end

  # Convert array values for R
  # @return [Array]
  def for_R
    if self.first.is_a?(String) 
      #"\"#{self.collect{|v| v.sub('[','').sub(']','')}.join(" ")}\"" # quote and remove square brackets
      "NA"
    else
      self.median
    end
  end

  # Collect array with index
  #   in analogy to each_with_index
  def collect_with_index
    result = []
    self.each_with_index do |elt, idx|
      result << yield(elt, idx)
    end
    result
  end
end