Linq to Objects - return pairs of numbers from list of numbers

Question

var nums = new[]{ 1, 2, 3, 4, 5, 6, 7};
var pairs  = /* some linq magic here*/ ;

=> pairs = { {1, 2}, {3, 4}, {5, 6}, {7, 0} }

The elements of pairs should be either two-element lists, or instances of some anonymous class with two fields, something like new {First = 1, Second = 2}.

Answer 1

None of the default linq methods can do this lazily and with a single scan. Zipping the sequence with itself does 2 scans and grouping is not entirely lazy. Your best bet is to implement it directly:

public static IEnumerable<T[]> Partition<T>(this IEnumerable<T> sequence, int partitionSize) {
    Contract.Requires(sequence != null)
    Contract.Requires(partitionSize > 0)

    var buffer = new T[partitionSize];
    var n = 0;
    foreach (var item in sequence) {
        buffer[n] = item;
        n += 1;
        if (n == partitionSize) {
            yield return buffer;
            buffer = new T[partitionSize];
            n = 0;
        }
    }
    //partial leftovers
    if (n > 0) yield return buffer;
}

Answer 2

Try this:

int i = 0;
var pairs = 
  nums
    .Select(n=>{Index = i++, Number=n})
    .GroupBy(n=>n.Index/2)
    .Select(g=>{First:g.First().Number, Second:g.Last().Number});

Answer 3

int[] numbers = new int[] { 1, 2, 3, 4, 5, 6, 7 };
var result = numbers.Zip(numbers.Skip(1).Concat(new int[] { 0 }), (x, y) => new
        {
            First = x,
            Second = y
        }).Where((item, index) => index % 2 == 0);