Question

I’d like to split a list into two lists, one which can be handled directly, and the other being the remainder, which will be passed down a chain to other handlers.

Input:

• One list of items
• a filtering method to determine which list to include the item in.

Output:

• a “true” list
• a “false” list

Does this already exist? Perhaps a Linq method that I’m not thinking of at the moment? Otherwise, does anyone have a good C# example?

Answer 1

Here is one simple method. Note that ToLookup eagerly evaluates the input sequence.

List<int> list = new List<int> { 1, 2, 3, 4, 5, 6 };

var lookup = list.ToLookup(num => num % 2 == 0);

IEnumerable<int> trueList = lookup[true];
IEnumerable<int> falseList = lookup[false];

You can use GroupBy to get lazy evaluation of the input sequence, but it’s not quite as pretty:

var groups = list.GroupBy(num => num % 2 == 0);

IEnumerable<int> trueList = groups.Where(group => group.Key).FirstOrDefault();
IEnumerable<int> falseList = groups.Where(group => !group.Key).FirstOrDefault();