Detecting sequence of at least 3 sequential numbers from a given list?

Question

I have a list of numbers e.g. 21,4,7,9,12,22,17,8,2,20,23 I want to be able to pick out sequences of sequential numbers (minimum 3 items in length), so from the example above it would be 7,8,9 and 20,21,22,23. I have played around with a few ugly sprawling functions but I am wondering if there is a neat LINQ-ish way to do it. Any suggestions? **UPDATE:** Many thanks for all the responses, much appriciated. Im am currently having a play with them all to see which would best integrate into our project.

Answer 1

We can solve this by first sorting the list from smallest to greatest so that the numbers are easier to compare in order. Then we create an empty list to store the numbers that are currently consecutive. We can loop through the sorted numbers, we can check if the current number was exactly 1 more than the last number in the list. If it was, we can add it to the empty list we created. If it wasn't, that means the sequence ended, so we checked whether the sequence had at least 3 numbers. If it does, we print it. Then we clear the list and start a new one with the current number. Finally, after the loop finishes, we check the last sequences one last time in case it ended at the very end of the list. This process will allow us to find sequences of sequential numbers. Lets implement it in C#:

using System;

using System.Collections.Generic;

class Program

{

static void Main()

{

// Creating list

List<int> numbers = new List<int>

{

21, 4, 7, 9, 12, 22, 17, 8, 2, 20, 23

};

// Sorting list from smallest to greatest

numbers.Sort();

// Storing the current sequence we are checking

List <int> sequences = new List<int>();

// Loop through each number in sorted list

for (int i = 0; i < numbers.Count; i++)

{

// If the sequence is empty, add the first number

if (sequence.Count == 0)

sequence.Add(numbers[i]);

// Check if the current number is 1 more than the last number in the sequence

else if (numbers[i] == sequence[sequence.Count - 1] + 1)

sequence.Add(numbers[i]);

// If the current number is not consecutive, the sequence has ended

else

{

// If the sequence has at least 3 numbers, print it

if (sequence.Count >= 3)

Console.WriteLine(string.Join(", ", sequence));

// Clear the old sequence and start a new one

sequence.Clear();

sequence.Add(numbers[i]);

}

}

// After the loop ends, check the last sequence

if (sequence.Count >= 3)

Console.Write(String.Join(", ", sequence));

}

}