Can I split an IEnumerable into two by a boolean criteria without two queries?

Question

Can I split an IEnumerable into two IEnumerable using LINQ and only a single query/LINQ statement?

I want to avoid iter

Answer 1

Copy pasta extension method for your convenience.

public static void Fork(
    this IEnumerable source,
    Func pred,
    out IEnumerable matches,
    out IEnumerable nonMatches)
{
    var groupedByMatching = source.ToLookup(pred);
    matches = groupedByMatching[true];
    nonMatches = groupedByMatching[false];
}

Or using tuples in C# 7.0

public static (IEnumerable matches, IEnumerable nonMatches) Fork(
    this IEnumerable source,
    Func pred)
{
    var groupedByMatching = source.ToLookup(pred);
    return (groupedByMatching[true], groupedByMatching[false]);
}

// Ex.
var numbers = new [] { 1, 2, 3, 4, 5, 6, 7, 8 };
var (numbersLessThanEqualFour, numbersMoreThanFour) = numbers.Fork(x => x <= 4);