Check for consecutive numbers

Question

I have an array m of integers. I'm looking for a method to check if the elements of m are consecutive. Is there a way to test for consecutive numbers?

I came up with this code intended to work when the array length is four:

m.count == 4 && (m.max-m.min) == 3

which incorrectly returns true for [1,1,1,4] or [0,0,0,3].

Answer 1

Enumerable has a really handy method called each_cons that works like this:

[1,2,3,4].each_cons(2).to_a # => [ [1, 2], [2, 3], [3, 4] ]

That is, it yields each consecutive set of n elements. In our case n is 2.

Of course, as the name implies, it returns an Enumerator, so we can chain it with other Enumerable methods like all?:

def four_consecutive?(arr)
  return false unless arr.size == 4
  arr.each_cons(2).all? {|a, b| b == a + 1 }
end

four_consecutive?([2,3,4,5])   # => true
four_consecutive?([2,2,2,5])   # => false
four_consecutive?([1,2,3,4,5]) # => false

This method has the advantage above others that, because all? short-circuits as soon as the block returns false, it will only test numbers until it finds a pair that don't meet the condition (b == a + 1). Of course, with only four elements this doesn't really make a difference—unless you're calling this method thousands of times in situation where performance matters.

Answer 2

You can try this:

a == (a.min..a.max).to_a && a.count == 4

This only works when the array is in increasing order. [3, 4, 5, 6] will pass but [4, 3, 5, 6] won't.