C# split list into two lists using bool function [duplicate]

Question

Say I have a List<string> listOfStrings and I want to divide this list into two lists based on some predicate. E.g., the first list should contain all strings that start with a letter, and second is a list of strings that don't.

Now I would do this like this:

var firstList = listOfStrings.Where(str => predicate(str));
var secondList = listOfStrings.Where(str => !predicate(str));

Is there a better way of doing this in one line?

Answer 1

You can use Linq's GroupBy():

var splitted = listOfStrings.GroupBy(s => Char.IsLetter(s[0]));

And with your predicate, it would be:

Func<string, bool> predicate;

var splitted = listOfStrings.GroupBy(predicate);

Usage:

The easiest way would be to convert the grouped data into a Dictionary<bool, IEnumerable<string>>, when the key is a bool that denotes whether the items in it start with a letter:

var splitted = list.GroupBy(x => Char.IsLetter(x[0]))
                   .ToDictionary(x => x.Key, z => z.ToArray());  

var startWithLetter = splitted[true];
var dontStartWithLetter = splitted[false];

Of course, there are many ways to massage the data into your desired structure, but the above is pretty concise in my opinion.

See MSDN