LINQ equivalent of F# List.partition - linq

Question

Using LINQ, how do you partition an IEnumerable in a way equivalent to F#'s List.parition function? The result should be the source sequence partitioned into two sequences based the predicate, with the predicate evaluated once per source item.

Answer 1

Enumerable.ToLookup comes close.
var l = new int[]{1,2,3,4,5};
var split = l.ToLookup( i => i > 2);
var a = split[true]; // 3,4,5
var b = split[false]; // 1,2
It's not too hard to wrap this in a function.
public static (IEnumerable<TSource> Yeses, IEnumerable<TSource> Nos) Partition<TSource>(IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
//Null handling?
var s = source.ToLookup(i => predicate(i));
return (s[true], s[false]);
}
public static void Main(string[] args)
{
var l = new int[] { 1,2,3,4,5 };
var s = Partition(l, i => { Console.WriteLine($"Called: {i}"); return i > 2;});
Console.WriteLine(string.Join(",", s.Yeses));
Console.WriteLine(string.Join(",", s.Nos));
}
Called: 1
Called: 2
Called: 3
Called: 4
Called: 5
3,4,5
1,2

Answer 2

I would suggest this way:
F# (test-case)
let list1 = [ 1 .. 10 ]
let listEven, listOdd = List.partition (fun elem -> elem % 2 = 0) list1
printfn "Evens: %A\nOdds: %A" listEven listOdd
/* Result:
Evens: [2; 4; 6; 8; 10]
Odds: [1; 3; 5; 7; 9]
*/
C# (test-case)
var list1 = Enumerable.Range(1, 10);
var (listEven, listOdd) = list1
.GroupBy(key => key % 2 == 0)
.Aggregate(
(part1: Enumerable.Empty<int>(), part2: Enumerable.Empty<int>()),
(accumulator, value) => {
if (value.Key)
{
accumulator.part1 = value.ToArray();
}
else
{
accumulator.part2 = value.ToArray();
}
return accumulator;
});
Console.WriteLine($"Evens: [{string.Join("; ", listEven)}]\nOdds: [{string.Join("; ", listOdd)}]");
/* Result:
Evens: [2; 4; 6; 8; 10]
Odds: [1; 3; 5; 7; 9]
*/
To reuse&generalize this method define the extension-method:
public static class EnumerableExtensions
{
public static (T[] part1, T[] part2) Partition<T>(this IEnumerable<T> self, Func<T, bool> predicate)
{
return self
.GroupBy(predicate)
.Aggregate(
(part1: Array.Empty<T>(), part2: Array.Empty<T>()),
(accumulator, value) => {
if (value.Key)
{
accumulator.part1 = value.ToArray();
}
else
{
accumulator.part2 = value.ToArray();
}
return accumulator;
});
}
}
Use it like this:
var list1 = Enumerable.Range(1, 10);
var (listEven, listOdd) = list1.Partition(v => v % 2 == 0);
Console.WriteLine($"Evens: [{string.Join("; ", listEven)}]\nOdds: [{string.Join("; ", listOdd)}]");
/* Result:
Evens: [2; 4; 6; 8; 10]
Odds: [1; 3; 5; 7; 9]
*/
var str = "aAbbBcCddD";
var (lowers, uppers) = str.Partition(v => char.IsLower(v));
Console.WriteLine($"Lowers: [{string.Join("; ", lowers)}]\nUppers: [{string.Join("; ", uppers)}]");
/* Result:
Lowers: [a; b; b; c; d; d]
Uppers: [A; B; C; D]
*/