find_the_first_non_consecutive_number.md

Find the first non-consecutive number

Description

Your task is to find the first element of an array that is not consecutive.

By not consecutive we mean not exactly 1 larger than the previous element of the array.

E.g. If we have an array [1,2,3,4,6,7,8] then 1 then 2 then 3 then 4 are all consecutive but 6 is not, so that's the first non-consecutive number.

If the whole array is consecutive then return null².

The array will always have at least 2 elements¹ and all elements will be numbers. The numbers will also all be unique and in ascending order. The numbers could be positive or negative and the first non-consecutive could be either too!

If you like this Kata, maybe try this one next: https://www.codewars.com/kata/represent-array-of-numbers-as-ranges

¹ Can you write a solution that will return null² for both [] and [ x ] though? (This is an empty array and one with a single number and is not tested for, but you can write your own example test.)

² Swift, Ruby and Crystal: nil
Haskell: Nothing
Python, Rust, Scala: None
Julia: nothing
Nim: none(int) (See options)

My Solution

def first_non_consecutive(arr)
  el = (arr.first..arr.last).to_a - arr
  arr[arr.index(el.first+1)] unless el.empty?
end

Better/Alternative solution from Codewars

def first_non_consecutive(arr)
  arr.each_index do |i|
    return arr[i + 1] if arr[i].next != arr[i + 1]
  end
end