Check to see if an array is already sorted?

Question

I know how to put an array in order, but in this case I just want to see if it is in order. An array of strings would be the easiest, I imagine, and answer

Answer 1

These are all way too hard. You don't have to sort, but you can use sort to check. Scrambled array below for demonstration purposes.

arr = [["b",3],["a",53],["c",2]]
arr.sort == arr # => false
p arr.sort # => [["a",53],["b",3],["c",2]]

Answer 2

It looks like a generic abstraction, let's open Enumerable:

module Enumerable
  def sorted?
    each_cons(2).all? { |a, b| (a <=> b) <= 0 }
  end
end

[["a", 3], ["b", 53],["c", 2]].sorted? #=> true

Notice that we have to write (a <=> b) <= 0 instead of a <= b because there are classes that support <=> but not the comparator operators (i.e. Array), since they do not include the module Comparable.

You also said you'd like to have the ability "to check for order based on some arbitrary parameter":

module Enumerable  
  def sorted_by?
    each_cons(2).all? { |a, b| ((yield a) <=> (yield b)) <= 0 }    
  end
end

[["a", 3], ["b", 1], ["c", 2]].sorted_by? { |k, v| v } #=> false

Using lazy enumerables (Ruby >= 2.1), we can reuse Enumerable#sorted?:

module Enumerable  
  def sorted_by?(&block)
    lazy.map(&block).sorted?
  end
end

Answer 3

For this to work efficiently you will want to sort during insertion. If you are dealing with unique items, a SortedSet is also an option.

For clarification, if we patch array to allow for a sorted insertion, then we can keep the array in a sorted state:

class Array
  def add_sorted(o)
    size = self.size
    if size == 0
      self << o
    elsif self.last < o
      self << o
    elsif self.first > o
      self.insert(0, o)
    else
      # This portion can be improved by using a binary search instead of linear
      self.each_with_index {|n, i| if n > o; self.insert(i, o); break; end}
    end
  end
end

a = []
12.times{a.add_sorted(Random.rand(10))}
p a # => [1, 1, 2, 2, 3, 4, 5, 5, 5, 5, 7]

or to use the built in sort:

class Array
  def add_sorted2(o)
    self << o
    self.sort
  end
end

or, if you are dealing with unique items:

require "set"
b = SortedSet.new
12.times{b << Random.rand(10)}
p b # => #<SortedSet: {1, 3, 4, 5, 6, 7, 8, 9}>

Answer 4

You can compare them two by two:

[["a", 3],["b",53],["c",2]].each_cons(2).all?{|p, n| (p <=> n) != 1} # => true

Answer 5

reduce can compare each element to the one before, and stop when it finds one out of order:

array.reduce{|prev,l| break unless l[0] >= prev[0]; l}

Answer 6

Iterate over the objects and make sure each following element is >= the current element (or previous is <=, obviously) the current element.