Distinct based on first and last characters on the items in list C#

Question

I have a list of strings:

"55002",
"81512",
"81612",
"84515",
"84615",
"86358",
"86358",
"91128",
"94002"

I want to find in that list items that have the same 2 first characters and the same last characters and then remove from list the one which has lower middle character. In this example the same would be: 81512 and 81612 and I want to remove 81512 , the same would be 84515 and 84615 and I want to remove 84515 etc.

Answer 1

You can try this :

var g = new string[] {"55002",  ...};

var t=g.GroupBy(f => f.Substring(0, 2))
       .Select(f =>f.OrderByDescending(a =>a ).First());

Result :

55002 
91128 
81612 
84615 
86358 
94002 

update :

I probably missed that groupby should be also mathing the last two charcters .

If that's the case then :

var t=g.GroupBy(f => f.Substring(0, 2)+f.Substring( f.Length-2))
       .Select(f =>f.OrderByDescending(a =>a ).First());

For visualization :

Looking at this code :

    var g = new string[] {"55002",
"91128",
"81512",
"81712",
"81612",
"84516",
"84216",
"84615",
"86358",
"86358",
"94002"};

    var t = g.GroupBy(f => f.Substring(0, 2) + f.Substring(f.Length - 2)).Select(f => new { key = f.Key, group = f.OrderByDescending(a => a)});
    Console.Write(t);

We're going to group by the "key" and to take the first top one in each right group :

Answer 2

You can do it like this:

var input = new []{ "1234", "2345" /*...*/};
var res = input.GroupBy(x=> x.Substring(0,2)).Select(x=> x.OrderByDescending(y=>y).ToList()).ToList();
foreach(var l in res)
{
    l.RemoveAt(l.Count-1);
}

This will remove last string in literal order, but it will cover your case