Check for missing number in sequence

Question

I have an List which contains 1,2,4,7,9 for example.

I have a range from 0 to 10.

Is there a way to determine what numbers are missin

Answer 1

An alternative method which works in general for any two IEnunumerable<T> where T :IComparable. If the IEnumerables are both sorted, this works in O(1) memory (i.e. there is no creating another ICollection and subtracting, etc.) and in O(n) time.

The use of IEnumerable<IComparable> and GetEnumerator makes this a little less readable, but far more general.

Implementation

/// <summary>
/// <para>For two sorted IEnumerable&lt;T&gt; (superset and subset),</para>
/// <para>returns the values in superset which are not in subset.</para>
/// </summary>
public static IEnumerable<T> CompareSortedEnumerables<T>(IEnumerable<T> superset, IEnumerable<T> subset)
    where T : IComparable
{
    IEnumerator<T> supersetEnumerator = superset.GetEnumerator();
    IEnumerator<T> subsetEnumerator = subset.GetEnumerator();
    bool itemsRemainingInSubset = subsetEnumerator.MoveNext();

    // handle the case when the first item in subset is less than the first item in superset
    T firstInSuperset = superset.First();
    while ( itemsRemainingInSubset && supersetEnumerator.Current.CompareTo(subsetEnumerator.Current) >= 0 )
        itemsRemainingInSubset = subsetEnumerator.MoveNext();

    while ( supersetEnumerator.MoveNext() )
    {
        int comparison = supersetEnumerator.Current.CompareTo(subsetEnumerator.Current);
        if ( !itemsRemainingInSubset || comparison < 0 )
        {
            yield return supersetEnumerator.Current;
        }
        else if ( comparison >= 0 )
        {
            while ( itemsRemainingInSubset && supersetEnumerator.Current.CompareTo(subsetEnumerator.Current) >= 0 )
                itemsRemainingInSubset = subsetEnumerator.MoveNext();
        }
    }
}

Usage

var values = Enumerable.Range(0, 11);
var list = new List<int> { 1, 2, 4, 7, 9 };
var notIncluded = CompareSortedEnumerables(values, list);

Answer 2

Using Unity i have tested two solutions on set of million integers. Looks like using Dictionary and two "for" loops gives better result than Enumerable.Except

FindMissing1 Total time: 0.1420 (Enumerable.Except)
FindMissing2 Total time: 0.0621 (Dictionary and two for loops)

public static class ArrayExtension
{
    public static T[] FindMissing1<T>(T[] range, T[] values)
    {
        List<T> result = Enumerable.Except<T>(range, values).ToList<T>();
        return result.ToArray<T>();
    }

    public static T[] FindMissing2<T>(T[] range, T[] values)
    {
        List<T> result = new List<T>();
        Dictionary<T, T> hash = new Dictionary<T, T>(values.Length);
        for (int i = 0; i < values.Length; i++)
            hash.Add(values[i], values[i]);

        for (int i = 0; i < range.Length; i++)
        {
            if (!hash.ContainsKey(range[i]))
                result.Add(range[i]);
        }

        return result.ToArray<T>();
    }
}

public class ArrayManipulationTest : MonoBehaviour
{
    void Start()
    {
        int rangeLength = 1000000;
        int[] range = Enumerable.Range(0, rangeLength).ToArray();
        int[] values = new int[rangeLength / 5];
        int[] missing;
        float start;
        float duration;

        for (int i = 0; i < rangeLength / 5; i ++)
            values[i] = i * 5;

        start = Time.realtimeSinceStartup;
        missing = ArrayExtension.FindMissing1<int>(range, values);
        duration = Time.realtimeSinceStartup - start;
        Debug.Log($"FindMissing1 Total time: {duration:0.0000}");

        start = Time.realtimeSinceStartup;
        missing = ArrayExtension.FindMissing2<int>(range, values);
        duration = Time.realtimeSinceStartup - start;
        Debug.Log($"FindMissing2 Total time: {duration:0.0000}");
    }
}

Answer 3

If the range is predictable I suggest the following solution:

public static void Main()
{
    //set up the expected range
    var expectedRange = Enumerable.Range(0, 10);

    //set up the current list
    var currentList = new List<int> {1, 2, 4, 7, 9};

    //get the missing items
    var missingItems = expectedRange.Except(currentList);       

    //print the missing items
    foreach (int missingItem in missingItems)
    {
        Console.WriteLine(missingItem);
    }

    Console.ReadLine();
}

Regards, y00daa

Answer 4

 int sum = 0,missingNumber;
        int[] arr = { 1,2,3,4,5,6,7,8,9};
        for (int i = 0; i < arr.Length; i++)
        {
            sum += arr[i];
        }
        Console.WriteLine("The sum from 1 to 10 is 55");
        Console.WriteLine("Sum is :" +sum);
        missingNumber =  55 - sum;
        Console.WriteLine("Missing Number is :-"+missingNumber);
        Console.ReadLine();

Answer 5

var list = new List<int>(new[] { 1, 2, 4, 7, 9 });
var result = Enumerable.Range(0, 10).Except(list);

Answer 6

Create an array of num items

const int numItems = 1000;
bool found[numItems] = new bool[numItems];


List<int> list;

PopulateList(list);

list.ForEach( i => found[i] = true );

// now iterate found for the numbers found
for(int count = 0; i < numItems; ++numItems){
  Console.WriteList("Item {0} is {1}", count, found[count] ? "there" : "not there");
}