C# Enumerable.Take with default value

Question

What is the best way to get exactly x values from an Enumerable in C#. If i use Enumerable .Take() like this:

var myList = Enumerable.Range(0,10);
var result = myList.Take(20);

The result will only have 10 elements.

I want to fill the missing entries with a default value. Something like this:

var myList = Enumerable.Range(0,10);
var result = myList.TakeOrDefault(20, default(int));  //Is there anything like this?

Is there such a function in C# and if not, what would be the best way to achieve this?

Answer 1

You could do something like:

var result = myList.Concat(Enumerable.Repeat(default(int), 20)).Take(20); 

And it would be easy to turn this into an extension method:

public static IEnumerable<T> TakeOrDefault<T>(this IEnumerable<T> list, int count, T defaultValue)
{
    return  list.Concat(Enumerable.Repeat(defaultValue, count)).Take(count);
}

But there is a subtle gotcha here. This would work perfectly fine for value types, for a reference type, if your defaultValue isn't null, you are adding the same object multiple times. Which probably isn't want you want. For example, if you had this:

var result = myList.TakeOrDefault(20, new Foo());

You are going to add the same instance of Foo to pad your collection. To solve that problem, you'd need something like this:

public static IEnumerable<T> TakeOrDefault<T>(this IEnumerable<T> list, int count, Func<T> defaultFactory)
{
    return  list.Concat(Enumerable.Range(0, count).Select(i => defaultFactory())).Take(count);
}

Which you'd call like this:

var result = myList.TakeOrDefault(20, () => new Foo())

Of course, both methods can co-exist, so you could easily have:

// pad a list of ints with zeroes
var intResult = myIntList.TakeOrDefault(20, default(int));
// pad a list of objects with null
var objNullResult = myObjList.TakeOrDefault(20, (object)null);
// pad a list of Foo with new (separate) instances of Foo
var objPadNewResult = myFooList.TakeOrDefault(20, () => new Foo());

Answer 2

Its not there by default, but it's easy enough to write as an extension method

public static IEnumerable<T> TakeOrDefault<T>(this IEnumerable<T> items, int count, T defaultValue)
{
    var i = 0;
    foreach(var item in items)
    {
        i++;
        yield return item;
        if(i == count)
             yield break;
    }
    while(i++<count)
    {
        yield return defaultValue;
    }
}

Live example: http://rextester.com/XANF91263

Answer 3

What you're looking for is a general-purpose PadTo method, which extends the collection's length if needed using a given value.

public static IEnumerable<T> PadTo<T>(this IEnumerable<T> source, int len)
{
    return source.PadTo(len, default(T));
}

public static IEnumerable<T> PadTo<T>(this IEnumerable<T> source, int len, T elem)
{
    return source.PadTo(len, () => elem);
}

public static IEnumerable<T> PadTo<T>(this IEnumerable<T> source, int len, Func<T> elem)
{
    int i = 0;
    foreach(var t in source)
    {
        i++;
        yield return t;
    }

    while(i++ < len)
        yield return elem();
}

You can now express:

myList.Take(20).PadTo(20);

This is analogous to Scala's List[A].padTo

Answer 4

You could use Concat for this purpose. You can use a simple helper method to join this all together:

public IEnumerable<T> TakeSpawn(this IEnumerable<T> @this, int take, T defaultElement)
{
  return @this.Concat(Enumerable.Repeat(defaultElement, take)).Take(take);
}

The idea is that you always append another enumerable on the end of the original enumerable, so if the input doesn't have enough elements, it will start enumerating from the Repeat.

Answer 5

There isn't anything in the .NET Framework, not that I'm aware of. This can be achieved easily using an extension method though and it works for all types if you supply a default value yourself:

public static class ListExtensions
{
    public static IEnumerable<T> TakeOrDefault<T>(this List<T> list, int count, T defaultValue)
    {
        int missingItems = count - list.Count;
        List<T> extra = new List<T>(missingItems);

        for (int i = 0; i < missingItems; i++)
            extra.Add(defaultValue);

        return list.Take(count).Concat(extra);
    }
}

Answer 6

Why not just write an extension method that checks the count and returns the default value for remaining entries:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> values = new List<int>{1, 2, 3, 4};

            IEnumerable<int> moreValues = values.TakeOrDefault(3, 100);
            Console.WriteLine(moreValues.Count());

            moreValues = values.TakeOrDefault(4, 100);
            Console.WriteLine(moreValues.Count());

            moreValues = values.TakeOrDefault(10, 100);
            Console.WriteLine(moreValues.Count());

        }
    }

    public static class ExtensionMethods
    {
        public static IEnumerable<T> TakeOrDefault<T>(this IEnumerable<T> enumerable, int count, T defaultValue)
        {
            int returnedCount = 0;
            foreach (T variable in enumerable)
            {
                returnedCount++;
                yield return variable;
                if (returnedCount == count)
                {
                    yield break;
                }
            }

            if (returnedCount < count)
            {
                for (int i = returnedCount; i < count; i++)
                {
                    yield return defaultValue;
                }
            }
        }
    }
}

Answer 7

I wrote a quick extension for this which depends on T being a value type.

  public static class Extensions
    {
        public static IEnumerable<T> TakeOrDefault<T>(this IEnumerable<T> list, int totalElements)
        {
            List<T> finalList = list.ToList();

            if (list.Count() < totalElements)
            {
                for (int i = list.Count(); i < totalElements; i++)
                {
                    finalList.Add(Activator.CreateInstance<T>());
                }
            }

            return finalList;
        }
    }