LINQ：如何压缩2个列表并选择第一个列表不同的元素？

Question

I have 2 lists/arrays of values, x and y. There are duplicates in the x list. I need to select the elements of x and y that have unique x values. Using LINQ, how would I write the query to get the elements of x and y where x is unique? (I'd like the first y for every distinct x).

Example:

x = {1, 1, 2, 3, 4, 4, 5, 6}

y = {3, 4, 5, 6, 7, 8, 9, 10}

The result I want is:

newX = {1, 2, 3, 4, 5, 6}

newY = {3, 5, 6, 7, 9, 10}

Answer 1

You can get the first x with index, then look up for the index for y.

var xWithIndex = x.Select((value, index) => new { Value = value, Index = index })
         .GroupBy(item => item.Value)
         .Select(group => group.First())
var newX = xWithIndex.Select(item => item.Value).ToList();
var newY = xWithIndex.Select(item => y[item.Index]).ToList();

Answer 2

Zip two lists, group by x , and select the initial item from each group, like this:

var pairs = x.Zip(y, (first, second) => new {X = first, Y = second})
    .GroupBy(i => i.X)
    .Select(g => g.First());
var newX = pairs.Select(p => p.X).ToList();
var newY = pairs.Select(p => p.Y).ToList();

Answer 3

If you are willing to add MoreLINQ this can be achieved using DistinctBy :

int[] x = { 1, 1, 2, 3, 4, 4, 5, 6 };
int[] y = { 3, 4, 5, 6, 7, 8, 9, 10 };

var result = x.Zip(y, (a,b) => new { x = a, y = b})
    .DistinctBy(z => z.x).ToList();

var xresult = result.Select(z => z.x);
var yresult = result.Select(z => z.y);

// Verify the results
Console.WriteLine(String.Join(",", xresult));
Console.WriteLine(String.Join(",", yresult));