With the release of System.Linq when breaking a list into multiple lists

Question

I wrote a method for splitting a list of items into multiple lists with System.Linq. When I run this method for 50,000 primes, it takes about 59.862 seconds .

Stopwatch watchresult0 = new Stopwatch();
watchresult0.Start();
var result0 = SubDivideListLinq(Enumerable.Range(0, 50000), 100).ToList();
watchresult0.Stop();
long elapsedresult0 = watchresult0.ElapsedMilliseconds;

So, I tried to increase it and wrote it in a simple loop repeating every element in my list, and it only takes 4 milliseconds :

Stopwatch watchresult1 = new Stopwatch();
watchresult1.Start();
var result1 = SubDivideList(Enumerable.Range(0, 50000), 100).ToList();
watchresult1.Stop();
long elapsedresult1 = watchresult1.ElapsedMilliseconds;

This is my Subdivide method using Linq:

private static IEnumerable<List<T>> SubDivideListLinq<T>(IEnumerable<T> enumerable, int count)
{
    while (enumerable.Any())
    {
        yield return enumerable.Take(count).ToList();
        enumerable = enumerable.Skip(count);
    }
}

And this is my Subdivide method with a loop foreachover each element:

private static IEnumerable<List<T>> SubDivideList<T>(IEnumerable<T> enumerable, int count)
{
    List<T> allItems = enumerable.ToList();

    List<T> items = new List<T>(count);
    foreach (T item in allItems)
    {
        items.Add(item);

        if (items.Count != count) continue;
        yield return items;
        items = new List<T>(count);
    }

    if (items.Any())
        yield return items;
}

do you have an idea why my own implementation is much faster than dividing into Linq? Or am I doing something wrong?

: , , , . linq .

Answer 1

, . , linq. .

private static IEnumerable<List<T>> SubDivideListLinq<T>(IEnumerable<T> enumerable, int count)
{
    int index = 0;
    return enumerable.GroupBy(l => index++/count).Select(l => l.ToList());
}

:

private static IEnumerable<List<T>> SubDivideListLinq<T>(IEnumerable<T> enumerable, int count)
{
    int index = 0;
    return from l in enumerable
        group l by index++/count
        into l select l.ToList();
}

:

private static IEnumerable<List<T>> SubDivideListLinq<T>(IEnumerable<T> enumerable, int count)
{
    int index = 0;
    return enumerable.GroupBy(l => index++/count, 
                             item => item, 
                             (key,result) => result.ToList());
}

linq 0.006 sec non-linq 0.002 sec, linq.

, - . , , , .

Answer 2

- , :

, , , System.Linq :

, , (14 /50 . ):

T[] allItems = enumerable as T[] ?? enumerable.ToArray();
while (allItems.Any())
{
    yield return allItems.Take(count);
    allItems = allItems.Skip(count).ToArray();
}

, linq, . 3 50 . :

public static class EnumerableExtensions
{
    public static IEnumerable<List<T>> Subdivide<T>(this IEnumerable<T> enumerable, int count)
    {

        List<T> items = new List<T>(count);
        int index = 0;
        foreach (T item in enumerable)
        {
            items.Add(item);
            index++;
            if (index != count) continue;
            yield return items;
            items = new List<T>(count);
            index = 0;
        }
        if (index != 0 && items.Any())
            yield return items;
    }
}

@AndreasNiedermair, MoreLinq -Library, Batch. ( )