List of most occurrences of element

Question

I have a list let's say {1,1,2,2,3,3,3,4,4,4}. I want to find a List of the elements that occur the most often (it has to be a list as there can be a situation like here that 3 and 4 occur most and I need to get that information. How can I achieve this using LINQ?

it can be that all the elements appear only once, then I would get a whole list back. So most often is just the element(s) that appear maximum times. — cAMPy, Jun 04 '17 at 18:14
int[] numbers = { 1, 1, 2, 2, 3, 3, 3, 4, 4, 4 }; var groups = numbers.GroupBy(i => i).OrderByDescending(g => g.Count()); — Gururaj, Jun 04 '17 at 18:34
https://stackoverflow.com/questions/43403243/c-sharp-how-to-find-the-most-common-and-the-least-common-integers-in-an-array/43403684#43403684 — Slai, Jun 04 '17 at 18:46

Amy B · Answer 1 · 2017-06-07T19:13:53.673

3

var highCountItems = source
  .GroupBy(item => item)
  .GroupBy(g => g.Count(), g => g.Key)
  .OrderByDescending(counts => counts.Key)
  .First();

int theCount = highCountItems.Key;
var theItems = highCountItems.ToList();

edited Jun 07 '17 at 19:13

answered Jun 04 '17 at 19:04

Amy B

108,202
21
135
185

Your code doesn't compile because of the second GroupBy – cAMPy Jun 07 '17 at 16:08
@cAMPy fixed missing parens on g.Count – Amy B Jun 07 '17 at 19:14

Salah Akbari · Accepted Answer · 2017-06-05T06:00:21.673

2

By Grouping:

var grp = list.GroupBy(i => i).ToList();
int max = grp.Max(c => c.Count());
var most = grp.Where(d => d.Count() == max)
              .Select(c => c.Key).ToList();

edited Jun 05 '17 at 06:00

answered Jun 04 '17 at 18:33

Salah Akbari

39,330
10
79
109

Dangit, got there before me :P – Clint Jun 04 '17 at 18:34

score 0 · Answer 3 · answered Jun 04 '17 at 18:42

First you will have to group the numbers followed by ordering them so you will get the most frequently occurring number on the top.

int[] numbers = { 1, 1, 2, 2, 3, 3, 3, 4, 4, 4 };
var groups = numbers.GroupBy(i => i).OrderByDescending(g => g.Count());

foreach (var group in groups)
{
      // group.Key -> Represents the number in the list
}

The groups variable will contain all the groups formed from the numbers list ordered by their occurrence, meaning the first group will be the top most occurring group followed by the next. In case of same occurrences, the groups will be ordered by their occurrence in the list for example 3 & 4 have equal occurrences whereas 3 comes first before 4 and hence the group formation will be in same order.

Rogger Tân · Answer 4 · 2017-06-04T18:56:40.360

-1

A little complicated

var lst_input = new List<int>(new int[] { 1, 1, 2, 2, 3, 3, 3, 4, 4, 4 });
        var result = lst_input.Where(x => lst_input.Max(y => lst_input.Count(z => z == y)) == lst_input.Count(z => z == x)).Distinct().ToList();

But the above code is not effective when dealing with a really big array, since finding max is re-run for each element, and we could distinct the list in the first place. Another more efficient way:

var lst_input = new List<int>(new int[] { 1, 1, 2, 2, 3, 3, 3, 4, 4, 4 });
var tmp = lst_input.Distinct();
var max_val = tmp.Max(y => lst_input.Count(z => z == y));
var result = tmp.Where(x => max_val == lst_input.Count(z => z == x)).ToList();

edited Jun 04 '17 at 18:56

answered Jun 04 '17 at 18:16

Rogger Tân

71
2
6

This would do the trick, but because you're doing several passes over the list it would be quite inefficient. – Clint Jun 04 '17 at 18:35
@Clint edited for dealing with larger array. Thank you! – Rogger Tân Jun 04 '17 at 19:00

List of most occurrences of element

4 Answers4