Select lists from lists with linq

Question

I have an list with x items. I wish to get an results that groups this list based of a number and not a property.

For example.

I have a list of 8 items. I want to group them by 3. I want to get a List thats contains three lists, where the first two lists contains each three items and the last list the remaining two.

I want a more elegant solution than this:

private static List<List<string>> GroupBy(List<string> pages, int groupSize)
{
    var result = new List<List<TrimlinePage>>();
    while (!(result.Count != 0 && result.Last().Count % 3 > 0))
    {
        int skip = result.Count*groupSize;
        var group = pages.Skip(skip).Take(groupSize).ToList();
        result.Add(group);
    }
    return result;
}

Tim Schmelter · Accepted Answer · 2014-04-02 14:53:53Z

You can use the integer divison trick:

List<List<string>> lists = pages
    .Select((str, index) => new { str, index }) 
    .GroupBy(x => x.index / groupSize)
    .Select(g => g.Select(x => x.str).ToList())
    .ToList();

Example:

int groupSize = 3;
var pages = new List<string> { "A", "B", "C", "D", "E", "F", "G" };
List<List<string>> lists = pages
    .Select((str, index) => new { str, index })
    .GroupBy(x => x.index / groupSize)
    .Select(g => g.Select(x => x.str).ToList())
    .ToList();

Result:

foreach(var list in lists)
    Console.WriteLine(string.Join(",", list));

Output:

A,B,C
D,E,F
G

So this approach will give you lists with the specified max-size, in this case 3. If you instead want to ensure that you always get three lists you need to use % instead of /:

List<List<string>> lists = pages
    .Select((str, index) => new { str, index }) 
    .GroupBy(x => x.index % groupSize)
    .Select(g => g.Select(x => x.str).ToList())
    .ToList();

Hamlet Hakobyan · Accepted Answer · 2014-04-02 14:51:00Z

0

Try this:

var list = Enumerable.Range(1,100);
var query = list
              .Select((x, i) => new {x, i})
              .GroupBy(v => v.i / 3).Select(g => g.Select(v =>v.x.ToList()))
              .ToList();

answered Apr 2, 2014 at 14:51

Hamlet Hakobyan

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Comments

p.s.w.g · Accepted Answer · 2014-04-02 14:51:02Z

0

Here's a simple solution using side effects (which is generally discouraged):

private static List<List<string>> GroupBy(List<string> pages, int groupSize)
{
    var i = 0;
    return pages.GroupBy(p => i++ / 3, (k, g) => g.ToList()).ToList();
}

Or if you want to avoid relying on side effects, you could use this:

private static List<List<string>> GroupBy(List<string> pages, int groupSize)
{
    return pages.Select(p => new { p, i })
                .GroupBy(x => x.i / 3)
                .Select(g => g.Select(x => x.p).ToList())
                .ToList();
}

answered Apr 2, 2014 at 14:51

p.s.w.g

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Comments

Sasha · Accepted Answer · 2014-04-02 14:51:10Z

0

LINQ is not the best solution. Often good old indexing is much more readable and efficient.

private static List<List<T>> GroupBy(List<T> pages, int groupSize)
{
    var result = new List<List<T>>();
    List<T> l;
    for (int i=0; i < pages.Count; i++)
    {
        if (i%groupSize == 0)
        {
            l = new List<T>();
            result.Add(l);
        }
        l.Add(pages[i]);
    }
    return result;
}

answered Apr 2, 2014 at 14:51

Sasha

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Comments

devdigital · Accepted Answer · 2014-04-02 14:54:20Z

0

You could also have a look at morelinq which contains the Partition method.

It's available via NuGet.

answered Apr 2, 2014 at 14:54

devdigital

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