Question

I would like to split a list in parts, without knowing how much items I will have in that list. The question is different from those of you who wants to split a list into chunk of fixed size.

int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

I would like the values to be splitted vertically.

Splitted in 2 :

-------------------
| item 1 | item 6 |
| item 2 | item 7 |
| item 3 | item 8 |
| item 4 | item 9 |
| item 5 |        |

Splitted in 3:

| item 1 | item 4 | item 7 |
| item 2 | item 5 | item 8 |
| item 3 | item 6 | item 9 |

Splitted in 4:

| item 1 | item 4 | item 6 | item 8 |
| item 2 | item 5 | item 7 | item 9 |
| item 3 |        |        |        |

I’ve found a few c# extensions that can do that but it doesn’t distribute the value the way I want. Here’s what I’ve found:

// this technic is an horizontal distribution
public static IEnumerable<IEnumerable<T>> Split<T>(this IEnumerable<T> list, int parts)
{
    int i = 0;
    var splits = from item in list
                    group item by i++ % parts into part
                    select part.AsEnumerable();
    return splits;
}

the result is this but my problem is that the value are distributed horizontally:

| item 1 | item 2 |
| item 3 | item 4 |
| item 5 | item 6 |
| item 7 | item 8 |
| item 9 |        |

or

| item 1 | item 2 | item 3 |
| item 4 | item 5 | item 6 |
| item 7 | item 8 | item 9 |

any idea how I can distribute my values vertically and have the possibility to choose the number of parts that i want?

In real life

For those of you who want to know in which situation I would like to split a list vertically, here’s a screenshot of a section of my website:

Answer 1

With .Take() and .Skip() you can:

int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

int splitIndex = 4; // or (a.Length / 2) to split in the middle.

var list1 = a.Take(splitIndex).ToArray(); // Returns a specified number of contiguous elements from the start of a sequence.
var list2 = a.Skip(splitIndex).ToArray(); // Bypasses a specified number of elements in a sequence and then returns the remaining elements.

You can use .ToList() instead of .ToArray() if you want a List<int>.

---

EDIT:

After you changed (clarified maybe) your question a bit, I guess this is what you needed:

public static class Extensions
{
    public static IEnumerable<IEnumerable<T>> Split<T>(this IEnumerable<T> source, int parts)
    {
        var list = new List<T>(source);
        int defaultSize = (int)((double)list.Count / (double)parts);
        int offset = list.Count % parts;
        int position = 0;

        for (int i = 0; i < parts; i++)
        {
            int size = defaultSize;
            if (i < offset)
                size++; // Just add one to the size (it's enough).

            yield return list.GetRange(position, size);

            // Set the new position after creating a part list, so that it always start with position zero on the first yield return above.
            position += size;
        }
    }
}

Using it:

int[] a = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var lists = a.Split(2);

This would generate:

split in 2 : a.Split(2);

| item 1 | item 6 |
| item 2 | item 7 |
| item 3 | item 8 |
| item 4 | item 9 |
| item 5 |        |

split in 3 : a.Split(3);

| item 1 | item 4 | item 7 |
| item 2 | item 5 | item 8 |
| item 3 | item 6 | item 9 |

split in 4 : a.Split(4);

| item 1 | item 4 | item 6 | item 8 |
| item 2 | item 5 | item 7 | item 9 |
| item 3 |        |        |        |

Also, if you would have:

int[] b = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; // 10 items

and split in 4 : b.Split(4);

| item 1 | item 4 | item 7 | item 9 |
| item 2 | item 5 | item 8 | item 10|
| item 3 | item 6 |        |        |