C# LINQ group by
last modified July 5, 2023
In this article we show how to group data in C# with LINQ queries.
Language-Integrated Query (LINQ) is a domain-specific language for querying data from various data sources, including arrays, lists, XML files, or databases.
The group, by, and into keywords are
used to group data in LINQ.
It is possible to group data by:
- a single key
- an attribute
- a compound key
- a computed numeric range
- an expression
C# LINQ group by a key
We can group data into categories based on a certain key.
List<Car> cars =
[
new ("Audi", "red", 52642),
new ("Mercedes", "blue", 57127),
new ("Skoda", "black", 9000),
new ("Volvo", "red", 29000),
new ("Bentley", "yellow", 350000),
new ("Citroen", "white", 21000),
new ("Hummer", "black", 41400),
new ("Volkswagen", "white", 21600),
];
var groups = from car in cars
group car by car.Colour;
foreach (var group in groups)
{
Console.WriteLine(group.Key);
foreach (var car in group)
{
Console.WriteLine($" {car.Name} {car.Price}");
}
}
record Car(string Name, string Colour, int Price);
We separate the cars into groups by their colour.
$ dotnet run red Audi 52642 Volvo 29000 blue Mercedes 57127 black Skoda 9000 Hummer 41400 yellow Bentley 350000 white Citroen 21000 Volkswagen 21600
C# LINQ group by attribute
We can group data by some attributes of the elements.
List<string> words =
[
"war", "water", "cup", "cloud", "atom", "abyss", "soup", "book",
"moon", "nice", "sky", "forest"
];
var groups = from word in words
group word by word.Length;
foreach (var group in groups)
{
Console.WriteLine(group.Key);
foreach (var e in group)
{
Console.WriteLine($"{e}");
}
Console.WriteLine("----------------");
}
The example groups words by their length.
$ dotnet run 3 war cup sky ---------------- 5 water cloud abyss ---------------- 4 atom soup book moon nice ---------------- 6 forest ----------------
C# LINQ group by and aggregate
In the following example, we perform a grouping and aggregation operations.
Revenue[] revenues =
[
new (1, "Q1", 2340),
new (2, "Q1", 1200),
new (3, "Q1", 980),
new (4, "Q2", 340),
new (5, "Q2", 780),
new (6, "Q3", 2010),
new (7, "Q3", 3370),
new (8, "Q4", 540),
];
var res = from revenue in revenues
group revenue by revenue.Quarter
into g
select new { Quarter = g.Key, Total = g.Sum(e => e.Amount) };
foreach (var line in res)
{
Console.WriteLine(line);
}
record Revenue(int Id, string Quarter, int Amount);
We have revenues for four quarters. We group the revenues by the quarters and sum the amounts.
$ dotnet run
{ Quarter = Q1, Total = 4520 }
{ Quarter = Q2, Total = 1120 }
{ Quarter = Q3, Total = 5380 }
{ Quarter = Q4, Total = 540 }
We can apply a filter on aggregated data with where clause.
Revenue[] revenues =
[
new (1, "Q1", 2340),
new (2, "Q1", 1200),
new (3, "Q1", 980),
new (4, "Q2", 340),
new (5, "Q2", 780),
new (6, "Q3", 2010),
new (7, "Q3", 3370),
new (8, "Q4", 540),
];
var res = from revenue in revenues
group revenue by revenue.Quarter
into g
where g.Count() == 2
select new { Quarter = g.Key, Total = g.Sum(c => c.Amount) };
foreach (var line in res)
{
Console.WriteLine(line);
}
record Revenue(int Id, string Quarter, int Amount);
In the example, we pick only those quarters, which have exactly two revenues.
$ dotnet run
{ Quarter = Q2, Total = 1120 }
{ Quarter = Q3, Total = 5380 }
C# LINQ group by composite key
We can group data by a composite key, consisting of multiple fields.
User[] users =
[
new ("John", "Doe", "gardener"),
new ("Jane", "Doe", "teacher"),
new ("Roger", "Roe", "driver"),
new ("Peter", "Doe", "teacher"),
new ("Pavol", "Novak", "programmer"),
new ("Albert", "Novak", "teacher"),
new ("Sam", "Novak", "driver"),
new ("Peter", "Horvath", "accountant"),
new ("Lucia", "Horvath", "accountant"),
new ("Michael", "Novak", "programmer"),
];
var groups = from user in users
group user by new { user.LastName, user.Occupation };
foreach (var group in groups)
{
Console.WriteLine(group.Key);
foreach (var e in group)
{
Console.WriteLine($"{e}");
}
Console.WriteLine("----------------");
}
record User(string FirstName, string LastName, string Occupation);
The program arranges groups of users by their lastname and occupation.
$ dotnet run
{ LastName = Doe, Occupation = gardener }
User { FirstName = John, LastName = Doe, Occupation = gardener }
----------------
{ LastName = Doe, Occupation = teacher }
User { FirstName = Jane, LastName = Doe, Occupation = teacher }
User { FirstName = Peter, LastName = Doe, Occupation = teacher }
----------------
{ LastName = Roe, Occupation = driver }
User { FirstName = Roger, LastName = Roe, Occupation = driver }
----------------
{ LastName = Novak, Occupation = programmer }
User { FirstName = Pavol, LastName = Novak, Occupation = programmer }
User { FirstName = Michael, LastName = Novak, Occupation = programmer }
----------------
{ LastName = Novak, Occupation = teacher }
User { FirstName = Albert, LastName = Novak, Occupation = teacher }
----------------
{ LastName = Novak, Occupation = driver }
User { FirstName = Sam, LastName = Novak, Occupation = driver }
----------------
{ LastName = Horvath, Occupation = accountant }
User { FirstName = Peter, LastName = Horvath, Occupation = accountant }
User { FirstName = Lucia, LastName = Horvath, Occupation = accountant }
----------------
C# LINQ group by boolean expression
In the following example, we group data using a boolean expression.
Student[] students =
[
new ("John", "Doe", 78),
new ("Roger", "Roe", 89),
new ("Peter", "Doe", 90),
new ("Pavol", "Novak", 34),
new ("Albert", "Novak", 66),
new ("Peter", "Horvath", 89),
new ("Lucia", "Horvath", 88),
new ("Michael", "Novak", 99),
];
var groups = from student in students
group student by new
{
Passed = student.Score > 70,
};
foreach (var group in groups)
{
if (group.Key.Passed)
{
Console.WriteLine("passed");
}
else
{
Console.WriteLine("failed");
}
foreach (var e in group)
{
Console.WriteLine(e);
}
Console.WriteLine("----------------------------");
}
record Student(string FirstName, string LastName, int Score);
We have an array of student records having firstname, lastname, and score fields. To pass an exam, the student needs to have a score over 70. With a LINQ expression, we divide the students into two groups: passed and failed.
$ dotnet run
passed
Student { FirstName = John, LastName = Doe, Score = 78 }
Student { FirstName = Roger, LastName = Roe, Score = 89 }
Student { FirstName = Peter, LastName = Doe, Score = 90 }
Student { FirstName = Peter, LastName = Horvath, Score = 89 }
Student { FirstName = Lucia, LastName = Horvath, Score = 88 }
Student { FirstName = Michael, LastName = Novak, Score = 99 }
----------------------------
failed
Student { FirstName = Pavol, LastName = Novak, Score = 34 }
Student { FirstName = Albert, LastName = Novak, Score = 66 }
----------------------------
C# LINQ group by range
In the next example, we use a numeric range as a group key.
List<Student> students =
[
new ("John", "Doe", 78),
new ("Roger", "Roe", 89),
new ("Peter", "Doe", 90),
new ("Pavol", "Novak", 34),
new ("Albert", "Novak", 66),
new ("Peter", "Horvath", 89),
new ("Lucia", "Horvath", 88),
new ("Michael", "Novak", 99),
];
var groups = from std in students
let avg = students.Average(e => e.Score)
group std by (std.Score / 10) into g
orderby g.Key
select g;
foreach (var group in groups)
{
Console.WriteLine(group.Key * 10);
foreach (var e in group)
{
Console.WriteLine(e);
}
Console.WriteLine("----------------------------");
}
record Student(string FirstName, string LastName, int Score);
The example groups students into percentile ranges.
$ dotnet run
30
Student { FirstName = Pavol, LastName = Novak, Score = 34 }
----------------------------
60
Student { FirstName = Albert, LastName = Novak, Score = 66 }
----------------------------
70
Student { FirstName = John, LastName = Doe, Score = 78 }
----------------------------
80
Student { FirstName = Roger, LastName = Roe, Score = 89 }
Student { FirstName = Peter, LastName = Horvath, Score = 89 }
Student { FirstName = Lucia, LastName = Horvath, Score = 88 }
----------------------------
90
Student { FirstName = Peter, LastName = Doe, Score = 90 }
Student { FirstName = Michael, LastName = Novak, Score = 99 }
----------------------------
C# LINQ word frequency
In the following example, we count the frequency of words in a file.
$ wget https://raw.githubusercontent.com/janbodnar/data/main/the-king-james-bible.txt
We read data from the King James Bible.
using System.Text.RegularExpressions;
var fileName = "the-king-james-bible.txt";
var text = File.ReadAllText(fileName);
var dig = new Regex(@"\d");
var matches = new Regex("[a-z-A-Z']+").Matches(text);
var words =
from match in matches
let val = match.Value
where !dig.IsMatch(val)
select match.Value;
var topTen =
(from word in words
group word by word into wg
orderby wg.Count() descending
select new {word = wg.Key, Total = wg.Count()}
).Take(10);
foreach (var e in topTen)
{
Console.WriteLine($"{e.word}: {e.Total}");
}
We count the frequency of the words from the King James Bible.
var matches = new Regex("[a-z-A-Z']+").Matches(text);
var words = matches.Select(m => m.Value).ToList();
We find all the matches witch Matches method. From the match
collection, we get all the words into a list.
var words =
from match in matches
let val = match.Value
where !dig.IsMatch(val)
select match.Value;
In the first query, we find all the matches. From the match collection, we get all the words.
var topTen =
(from word in words
group word by word into wg
orderby wg.Count() descending
select new {word = wg.Key, Total = wg.Count()}
).Take(10);
The words are grouped and ordered by frequency in descending order. We take the first ten most common words.
$ dotnet run the 62103 and 38848 of 34478 to 13400 And 12846 that 12576 in 12331 shall 9760 he 9665 unto 8942
Source
In this article we have grouped data in C# LINQ.
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