将字节列表拆分为位置列表C＃

Question

So I have a list of bytes

List<byte> s = {1,2,3,2,2,2,3,1,2,4,2,1,4,.....};

I want to get new position lists using index of element.To something like this...

List<byte> 1 = {0,7,11};
List<byte> 2 = {1,3,4,5,8,10};
List<byte> 3 = {2,6};
List<byte> 4 = {9,12};
List<byte> 5 = ..... and so on

What`s the best way of doing this?

thank you.

Answer 1

You can use GroupBy and ToDictionary to get Dictionary<byte, List<int>> :

var dict = s.Select((value, index) => new { value, index })
            .GroupBy(x => x.value)
            .ToDictionary(g => g.Key, g => g.Select(x => x.index).ToList());

Answer 2

With LINQ, you can create an ILookup<TKey, TElement> with the desired results like this:

var indicesByByte = s.Select((item, index) => new { Item = item, Index = index } )
                     .ToLookup(tuple => tuple.Item, tuple => tuple.Index);

Now,

• indicesByByte[0] will be a sequence containing {0,7,11}
• indicesByByte[1] will be a sequence containing {1,3,4,5,8,10}
• etc.

Answer 3

One way to do this is with LINQ, using the overload of Enumerable<T>.Select which contains the index, then grouping:

var groups = s.Select((item, index) => new {index, item})
              .GroupBy(x => x.item, x => x.index)
              .ToDictionary(x => x.Key, x => x.ToList());

This will return a Dictionary<byte, List<int>> where the key is the value (1, 2, 3, 4, 5 in your example) and the value contains a list of positions.

---

You could also do it using a for loop, in a single pass:

var groups = new Dictionary<byte, List<int>>();

for (int i = 0; i < s.Count; i++)
{
    if(!groups.ContainsKey(s[i]))
        groups[s[i]] = new List<int>();

    groups[s[i]].Add(i);
}