LINQ GroupBy on count

Question

I am learning LINQ in C#. I have the following JSON data:

[ { "product": [ "a", "b", "c" ] }, { "product": [ "a","b" ] }, { "product": [ "b","c" ] }, { "product": [ "b", "c" ] }, { "product": [ "a","b" ] }, { "product": [ "b", "c" ] }, { "product": [ "a" ] }, ]

I would like to perform a LINQ by grouping product over its count then order by descending and finally selecting the top 3.

How can I do that? In the result, I have to show the product and its count like below:

 "b","c" 3 "a","b" 2 "a","b","c" 1

My current code is:

public class Product { public List<string> Name { get; set; } } string json = File.ReadAllText("products.json"); var products = JsonConvert.DeserializeObject<List<Product>>(json); var result = (from p in products .GroupBy(pt => pt.Name) .OrderByDescending(pt => pt.Count()) .SelectMany(pt => pt) select p).Take(3);

But I am getting all 1 as count. Can you please help to correct the result? Thank you.

yes I tried but it came out with all 1. I think I am doing mistake — aliceangel
– aliceangel, Commented Mar 18, 2018 at 9:47

Aleks Andreev · Accepted Answer · 2018-03-18 10:59:22Z

You are trying to group by a sequence but without custom comparator. That will not work.
To achive your desired output try do this:

Define your custom equality comparer:

public class SequenceComparer : IEqualityComparer<string[]> { public bool Equals(string[] x, string[] y) { if (ReferenceEquals(x, y)) return true; if (x == null || y == null) return false; return x.SequenceEqual(y); } public int GetHashCode(string[] obj) { return obj.Aggregate(42, (c, n) => c ^ n.GetHashCode()); } }

Parse your json to anonymous object

var json = "[\r\n {\r\n \"product\": [\r\n \"a\", \"b\", \"c\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"a\",\"b\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"b\",\"c\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"b\", \"c\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"a\",\"b\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"b\", \"c\"\r\n ]\r\n },\r\n {\r\n \"product\": [\r\n \"a\"\r\n ]\r\n },\r\n]"; var products = JsonConvert.DeserializeAnonymousType(json, new[] {new {product = new string[] { }}});

Group by product with your custom equality comparer

var result = products.GroupBy(p => p.product, new SequenceComparer()) .Select(g => new {g.Key, Count = g.Count()}) .OrderByDescending(x => x.Count) .Take(3);

Now you can see a result:

foreach (var row in result) Console.WriteLine($"{string.Join(",", row.Key)} {row.Count}");

will output

'b,c' 3 'a,b' 2 'a,b,c' 1

Live demo

Thank you very much. It worked. :) If you have time, could you please tell me why we need the hash function and how did you calculate the hashcode?
@shahnaj This question may help you to understand why GetHashCode is important. And this question may guide you to "how to calculate"

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LINQ GroupBy on count

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Define your custom equality comparer:

Parse your json to anonymous object

Group by product with your custom equality comparer

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Collectives™ on Stack Overflow

1 Answer 1

Define your custom equality comparer:

Parse your json to anonymous object

Group by product with your custom equality comparer

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