Question

I’m trying to create a dictionary collection from a single collection below the key in the dictionary is every file of type “k” and the values for each key are files of type “a”. In other words, I’m trying to build a parent-child relationship but the file names are unique and do not denote the relationship between “a” and “k” file types. The only relationship is the file date. For example, file 4 will be a key b/c it’s of type “k” and it’s values will be files 3 and 2 because their file date is greater than file 3’s date. file 1 should not be included as a child of file 4 since it’s of type “k” even though it’s date is greater than file 3.

Single Collection to work with:

IEnumerable<IFile>

file name   file type   file date
file1       k           2013-01-01
file2       a           2012-03-30
file3       a           2012-02-27
file4       k           2012-02-23
file5       a           2011-03-31
file6       k           2011-02-24
file7       a           2010-08-24
file8       a           2010-03-31
file9       k           2010-02-26

Desired output:

Dictionary<IFile, IEnumerable<IFile>>

key     value
file1   none b/c no files of type "a" exist with a date greater than file1
file4   file3, file2
file6   file5
file9   file8, file7

Answer 1

You could do something like:

var result = data.Where(x => x.Type == 'k')
                 .ToDictionary(x => x.Name,
                               x => data.Where(a => a.Type == 'a' &&
                                               a.Date >= x.Date)
                                        .Select(a => a.Name)
                                        .ToList());

That wouldn’t quite get what you want though – because the entry for 2010-02-26 would include all of the future ones. So it’s not just a case of this relationship:

For example, file 4 will be a key b/c it’s of type “k” and it’s values will be files 3 and 2 because their file date is greater than file 3’s date.

It sounds like it’s actually:

For example, file 4 will be a key b/c it’s of type “k” and it’s values will be files 3 and 2 because their file date is greater than file 3’s date and their file date is less than file 1’s date.

That’s going to be trickier. You might want something like:

var orderedKeys = data.Where(x => x.Type == 'k')
                      .OrderBy(x => x.Date)
                      .Concat(null); // You'll see why in a minute...

// Just for convenience. Could make this more efficient, admittedly.
var values = data.Where(x => x.Type == 'a').ToList();

var result = orderedKeys.Zip(orderedKeys.Skip(1),
                             (current, next) => new { current, next })
                        .ToDictionary(pair => pair.current.Name,
     // Sorry for the formatting...
     pair => values.Where(v => v.Date >= pair.current.Date &&
                               pair.next == null || v.Date < pair.next.Date)
                   .Select(v => v.Name)
                   .ToList());

That’s if you want to be really LINQ-y. It would be more efficient to walk through both keys and values ordered by date though:

var ordered = data.OrderBy(x => x.Date);
var result = new Dictionary<string, List<string>>();
var currentList = null;
foreach (var item in ordered)
{
    if (item.Type == 'a' && currentList != null)
    {
        currentList.Add(item.Name);
    }
    else if (item.Type == 'k')
    {
        currentList = new List<string>();
        result[item.Name] = currentList;
    }
}