splitting a list into multiple lists in C#

Question

I have a list of strings which I send to a queue. I need to split up the list so that I end up with a list of lists where each list contains a maximum (user defined) number of strings. So for example, if I have a list with the following A,B,C,D,E,F,G,H,I and the max size of a list is 4, I want to end up with a list of lists where the first list item contains: A,B,C,D, the second list has: E,F,G,H and the last list item just contains: I. I have looked at the “TakeWhile” function but am not sure if this is the best approach. Any solution for this?

Answer 1

You can set up a List<IEnumerable<string>> and then use Skip and Take to split the list:

IEnumerable<string> allStrings = new[] { "A", "B", "C", "D", "E", "F", "G", "H", "I" };

List<IEnumerable<string>> listOfLists = new List<IEnumerable<string>>();
for (int i = 0; i < allStrings.Count(); i += 4)
{                
    listOfLists.Add(allStrings.Skip(i).Take(4)); 
}

Now listOfLists will contain, well, a list of lists.

Answer 2

/// <summary>
/// Splits a <see cref="List{T}"/> into multiple chunks.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="list">The list to be chunked.</param>
/// <param name="chunkSize">The size of each chunk.</param>
/// <returns>A list of chunks.</returns>
public static List<List<T>> SplitIntoChunks<T>(List<T> list, int chunkSize)
{
    if (chunkSize <= 0)
    {
        throw new ArgumentException("chunkSize must be greater than 0.");
    }

    List<List<T>> retVal = new List<List<T>>();
    int index = 0;
    while (index < list.Count)
    {
        int count = list.Count - index > chunkSize ? chunkSize : list.Count - index;
        retVal.Add(list.GetRange(index, count));

        index += chunkSize;
    }

    return retVal;
}

Reference: http://www.chinhdo.com/20080515/chunking/

Answer 3

Some related reading:

• Split a collection into `n` parts with LINQ?
• Split List into Sublists with LINQ
• LINQ Partition List into Lists of 8 members

Otherwise, minor variation on accepted answer to work with enumerables (for lazy-loading and processing, in case the list is big/expensive). I would note that materializing each chunk/segment (e.g. via .ToList or .ToArray, or simply enumerating each chunk) could have sideeffects -- see tests.

Methods

// so you're not repeatedly counting an enumerable
IEnumerable<IEnumerable<T>> Chunk<T>(IEnumerable<T> list, int totalSize, int chunkSize) {
    int i = 0;
    while(i < totalSize) {
        yield return list.Skip(i).Take(chunkSize);
        i += chunkSize;
    }
}
// convenience for "countable" lists
IEnumerable<IEnumerable<T>> Chunk<T>(ICollection<T> list, int chunkSize) {
    return Chunk(list, list.Count, chunkSize);
}
IEnumerable<IEnumerable<T>> Chunk<T>(IEnumerable<T> list, int chunkSize) {
    return Chunk(list, list.Count(), chunkSize);
}

Test (Linqpad)

(note: I had to include the Assert methods for linqpad)

void Main()
{
    var length = 10;
    var size = 4;

    test(10, 4);
    test(10, 6);
    test(10, 2);
    test(10, 1);

    var sideeffects = Enumerable.Range(1, 10).Select(i => {
        string.Format("Side effect on {0}", i).Dump();
        return i;
    });

    "--------------".Dump("Before Chunking");
    var result = Chunk(sideeffects, 4);
    "--------------".Dump("After Chunking");
    result.Dump("SideEffects");
    var list = new List<int>();
    foreach(var segment in result) {
        list.AddRange(segment);
    }
    list.Dump("After crawling");

    var segment3 = result.Last().ToList();
    segment3.Dump("Last Segment");
}

// test
void test(int length, int size) {
    var list = Enumerable.Range(1, length);

    var c1 = Chunk(list, size);

    c1.Dump(string.Format("Results for [{0} into {1}]", length, size));

    Assert.AreEqual( (int) Math.Ceiling( (double)length / (double)size), c1.Count(), "Unexpected number of chunks");
    Assert.IsTrue(c1.All(c => c.Count() <= size), "Unexpected size of chunks");
}