I have a list contains duplicate items.

List<string> filterList = new List<string>()
{
     "postpone", "access", "success", "postpone", "success"
};

I get the output which is postpone, access, success by using

List<string> filter = filterList.Distinct().ToList();
string a = string.Join(",", filter.Select(a => a).ToArray());
Console.WriteLine(a);

I had saw other example, they can use groupby to get the latest element since they have other item like ID etc. Now I only have the string, how can I get the latest item in the list which is access, postpone, success? Any suggestion?

CodePudding user response：

One way to do this would be use the Index of the item in original collection along with GroupBy. For example,

    var lastDistinct = filterList.Select((x,index)=> new {Value=x,Index=index})
                                 .GroupBy(x=>x.Value)
                                 .Select(x=> x.Last())
                                 .OrderBy(x=>x.Index)
                                 .Select(x=>x.Value);
    var result = string.Join(",",lastDistinct);

Output

access,postpone,success

CodePudding user response：

Your input list is only of type string, so using groupBy doesn't really add anything. If you consider your code, your first line gives you the distinct list, you only lose the distinct items because you did a string.join on line 2. All you need to do is add a line before you join:

List<string> filter = filterList.Distinct().ToList();
string last = filter.LastOrDefault();
string a = string.Join(",", filter.Select(a => a).ToArray());
Console.WriteLine(a);

I suppose you could make your code more terse because you need neither .Select(a => a) nor .ToArray() in your call to string.Join.

GroupBy would be used if you had a list of class/struct/record/tuple items, where you might want to group by a specific key (or keys) rather than using Distinct() on the whole thing. GroupBy is very useful and you should learn that, and also the ToDictionary and ToLookup LINQ helper functionality.