Getting count of duplicate items in a List

Question

I am trying to get the count for the most common duplicate item in a list.

So far, I have:

List<string> brandList = new List<string>();

which contains 5 different soft drink brands, and the count is 100. I need to find out which brand has the most duplicates in the list and count how many duplicates there are.

(Duplicate?) See http://stackoverflow.com/questions/19406242/select-distinct-using-linq — cdsln, Jul 16 '15 at 09:00
@cdslnte He needs the number of duplicates, not the distinct values. — MakePeaceGreatAgain, Jul 16 '15 at 09:01
That gets rid of duplicates, not what im looking for. but thanks anyway.. — chersharp, Jul 16 '15 at 09:02
I´m voting to close this question as its unclear what OP really needs. New requirements are coming every second... — MakePeaceGreatAgain, Jul 16 '15 at 10:14
how hard is it for you to give me code that gives me the frequency the duplicate items in a list in visual studio C# windows forms application code that is NOT linq ? but in basic format? lol — chersharp, Jul 16 '15 at 10:19

Tim Schmelter · Answer 1 · 2015-07-16T09:54:57.647

4

Presuming that your pseudo code actually was:

List<Brand> brandList=new List<Brand>();
// fill list

and your Brand class either overrides Equals+getHashCode or has a property like BrandID which is the identifier. N youow want to get the count of the most popular brand, you can use LINQ:

var mostPopularBrand = brandList.GroupBy(b => g.BrandID)
    .OrderByDescending(g => g.Count())
    .Select(g => new { BrandID = g.Key, Count =  g.Count()})
    .First();

Console.WriteLine("Most poular brand: {0} Count: {1}", 
                   mostPopularBrand.BrandID, mostPopularBrand.Count);

Update: If it's actually a List<string>(question was edited):

var mostPopularBrand = brandList.GroupBy(str => str)
    .OrderByDescending(g => g.Count())
    .Select(g => new { Brand = g.Key, Count =  g.Count()})
    .First();

edited Jul 16 '15 at 09:54

answered Jul 16 '15 at 09:09

Tim Schmelter

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thats not for visual studio ?? :( – chersharp Jul 16 '15 at 09:09
@chersharp: it's for C#. You can use LINQ if you use at least Visual Studio 2008(.NET 3.5). – Tim Schmelter Jul 16 '15 at 09:10
i have to use windows forms application, do you think you could tell me how to do it on there? – chersharp Jul 16 '15 at 09:21
@chersharp: i don't see how it is related that it's winforms. The issue is not related to gui controls. – Tim Schmelter Jul 16 '15 at 09:22
i mean the coding format, i have to do it the simple way not Linq – chersharp Jul 16 '15 at 09:28
@chersharp: you need to add `using System.Linq;` at the top of the code file, then you are able to use Linq extension methods like `Enumerable.GroupBy`. Btw, is it really a `List`? – Tim Schmelter Jul 16 '15 at 09:32
Yup i know that, but im at uni and we are not allowed to do our work that way – chersharp Jul 16 '15 at 09:33
yes it is list it removed the '<' for some reason when i posted it :O haha – chersharp Jul 16 '15 at 09:33

score 3 · Answer 2 · answered Jul 16 '15 at 09:11

Your code doesn't compile. Providing that you mean List<String> as a storage of brands:

  var ListbrandList = new List<String>() {
    "Cola",
    "Juice",
    "Cola",
    "Water",
    "Milk",
    "Water",
    "Cola",
  };

  var result = ListbrandList
    .GroupBy(item => item)
    .Select(item => new {
         Name = item.Key,
         Count = item.Count()
       })
    .OrderByDescending(item => item.Count)
    .ThenBy(item => item.Name);

  String report = String.Join(Environment.NewLine, result
    .Select(item => String.Format("{0} appears {1} time(s)", item.Name, item.Count)));

you'll have report as

  Cola appears 3 time(s)
  Water appears 2 time(s)
  Juice appears 1 time(s)
  Milk appears 1 time(s)

MMZurita · Answer 3 · 2015-07-16T09:53:49.903

1

var result = brandList
            .Distinct()
            .GroupJoin(brand,
            k => k,
            b => b,
            (k, b) => new { BrandName = k, Count = b.Count() });

// An usage could be...

foreach (var r in result)
{
    Debug.WriteLine("{0} Brand has {1}", r.BrandName, r.Count);
}

Without LinQ:

var result = new Dictionary<string, int>();

foreach (var brand in brandList)
{
    if (!result.ContainsKey(brand))
    {
        var count = brandList.FindAll(x => x.Equals(brand)).Count;
        result.Add(brand, count);
    }
}

foreach (var r in result)
{
    Console.WriteLine("{0} Brand has {1}", r.Key, r.Value);
}

edited Jul 16 '15 at 09:53

answered Jul 16 '15 at 09:16

MMZurita

125
8

I am on VISUAL STUDIO C# Windows forms application...... How do you get the count for the number of times an item appears in a list??????? because i have no idea of that other form of coding yet – chersharp Jul 16 '15 at 09:17
You have to include this in your file: `using System.Linq;` Then you will have the Count() extension method. The Count (without parenthesis) is a property of the List class. – MMZurita Jul 16 '15 at 09:20
Ohh ok, and then i would be able to type it in as you said? – chersharp Jul 16 '15 at 09:22
Yes, I used that code without troubles. – MMZurita Jul 16 '15 at 09:24
do you know it in the other format, without using linq? – chersharp Jul 16 '15 at 09:35
check out the answer again – MMZurita Jul 16 '15 at 09:54
That still has LINQ-code within... – MakePeaceGreatAgain Jul 16 '15 at 09:58
yes it still has linq – chersharp Jul 16 '15 at 10:00
Maybe I'm mistaken but I'm not using the methods of LinQ in the answer below of my first one. I don't have `System.Linq` in my imports. – MMZurita Jul 16 '15 at 10:14

MakePeaceGreatAgain · Answer 4 · 2015-07-16T09:54:34.113

0

Something like this maybe:

var res = brandList.GroupyBy(x => x)
    .Select(x => new {Key = x.Key, Count = x.Count});

Now you have a list containing the actual drink-number and the count for this number.

EDIT: Getting the brand with most count is now easy, e.g. by using:

var mostPopular = res.Single(x => x.Count == res.Max(y => y.Count));

EDIT2: Without LINQ the following might work, which is way longer and more complicated:

// get the count for every brand

Dictionary<string, int> res = new Dictionary<string, int>();
foreach(var x in brands)
{
    if (!res.ContainsKey(x)) res[x] = 0;
    res[x]++;
}

// now get the max count

int currentMax = 0;
string key = "";
foreach (var kv in res)
{
    if (kv.Value > currentMax)
    {
        currentMax = kv.Value;
        key = kv.Key;
    }
}

Now the key should contain the brand with highest Count.

edited Jul 16 '15 at 09:54

answered Jul 16 '15 at 09:06

MakePeaceGreatAgain

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do you know how to do it in c# without doing it in linq? – chersharp Jul 16 '15 at 09:44
You should have mentioned that within your question. Wait a few minutes... – MakePeaceGreatAgain Jul 16 '15 at 09:46
OK, I´ve updated my answer now. – MakePeaceGreatAgain Jul 16 '15 at 09:54
i didnt even know what linq was until every answer was written in it, and still your second answer is not written in basic c# coding that i want – chersharp Jul 16 '15 at 10:02
Sure it is, compiles on .NET 3.5 (even on 2.0) also without LINQ. So what is your error here? – MakePeaceGreatAgain Jul 16 '15 at 10:05
im sure it is right but the style of coding youre writing it in is not the one i know, and im not allowed to use it in my work, i am just using original coding – chersharp Jul 16 '15 at 10:07
WHAT EXACTLY are you not allowed to do as this IS basic C#. What do you mean by "original coding"? – MakePeaceGreatAgain Jul 16 '15 at 10:09
well mine has int, double, bool, all that, not var? not GroupBy? – chersharp Jul 16 '15 at 10:11
i mean just basic original style of coding – chersharp Jul 16 '15 at 10:11
you may simply replace the `var` keyword by the actual type, although that IS basic C#. Just use VS to see what the actual type behind `var` is. Also there is no `GroupBy` within the last code. – MakePeaceGreatAgain Jul 16 '15 at 10:13
string[] popularBrands = { "Cola", "Pepsi" }; int[] quantities = new int[popularBrands.Length]; for (int i = 0; i < popularBrands.Length; i++) { quantities[i] = lst.Count(r => r.ToUpper() == popularBrands[i].ToUpper()); Console.WriteLine("{0} : {1}", popularBrands[i], quantities[i]); } ` – chersharp Jul 16 '15 at 10:15
thaats the kind of code that im looking for – chersharp Jul 16 '15 at 10:16
You should definitivly write all this stuff to your question to let other users know your actual requeirements. Furthermore if you already HAVE the code you´re looking for why ask here? – MakePeaceGreatAgain Jul 16 '15 at 10:17
i just copied and pasted it from the comments on this post? its in the right format but its wrong – chersharp Jul 16 '15 at 10:21

score 0 · Answer 5 · answered Jul 16 '15 at 09:06

0

You can try to do group using LINQ by Elements and calculate count

var groupedList = from l in ListbrandList
    group l by l into grp
    select new { key = grp.Key, cnt = grp.Count()};

Then you will have group by key (Element) and value (Count)

answered Jul 16 '15 at 09:06

Andrii Tsok

1,386
1
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score 0 · Answer 6 · edited Apr 20 '17 at 10:06

To count element with specific value in a list use :

int quantity = lst.Count(r => r == "Cola");

Example :

List<string> lst = new List<string>()
{
    "Sprite",
    "Cola",
    "Sprite",
    "Sprite",
    "Cola",
    "Sprite",
    "Sprite",
    "Cola",
    "Pepsi",
    "Sprite",
    "Pepsi",
    "Sprite",

};

string[] popularBrands = { "Cola", "Pepsi" };

int[] quantities = new int[popularBrands.Length];

for (int i = 0; i < popularBrands.Length; i++)
{
    quantities[i] = lst.Count(r => r.ToUpper() == popularBrands[i].ToUpper());
    Console.WriteLine("{0}  :   {1}", popularBrands[i], quantities[i]); 
}

Output

Cola : 3
Pepsi : 2

P.S.

About this code r => r.ToUpper() == popularBrands[i].ToUpper() : r is variable that holds a value from our list (that are taken one by one). We also use ToUpper() to make sure that our check is case insensitive.

So we basically loop through the collection taking values out of it one by one. Each time we put value to r variable and check if this variable satisfies condition. If so - we count it, if not - we just move to next value.

@chersharp r is a variable that contains value taken one by one from your list. I updated my record to explain it. — Fabjan, Jul 16 '15 at 09:46
Read this http://www.codeproject.com/Tips/298963/Understand-Lambda-Expressions-in-minutes — Fabjan, Jul 16 '15 at 10:17

Getting count of duplicate items in a List

6 Answers6