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C# LINQ for Scala heads

This is a memo of C# Linq features for Scala programmers. Or vice versa.

Type inference

C# has type inference. I try to use var when I can for local variables.

var x = 1;

Scala also has var, but the preferred way is to use immutable val if possible.

val x = 1

Creating a new List or an Array

C# can create collections in-line.

using System.Collections.Generic;

var list = new List { “Adam”, “Alice”, “Bob”, “Charlie” }; var array = new [] { 0, 1, 2 };

All collections in Scala comes with a factory method.

val list = List("Adam", "Alice", "Bob", "Charlie")
val array = Array(0, 1, 2)

Filtering using lambda expression

C# has “enrich-my-library” monkey patching that adds Where method to a normal Array.

using System; using System.Collections.Generic; using System.Collections.Linq;

var xs = array.Where(x => x >= 1);

There are several ways to write this in Scala.

array.filter(x => x >= 1)
array filter { _ >= 1 }
array filter { 1 <= }

Projection

Projection in C# is done by Select and SelectMany.

var xs = array.Select(x => x + 1); var yx = array.SelectMany(x => new [] { x, 3 });

These correspond to map and flatMap.

array map { _ + 1 }
array flatMap { Array(_, 3) }

Sorting

C# can sort things using OrderBy.

var xs = list.OrderBy(x => x.Length);

I can’t remember the last time I had to sort something in Scala, but you can do that using sortBy.

list sortBy { _.length }

Filtering using query expression

Now comes the query expression.

var results = from x in array where x >= 1 select x;

The closest thing Scala got probably is for-comprehension.

for (x <- array if x >= 1)
  yield x

You can write something similar in C#, but unlike Scala foreach does not return a value, so the whole thing needs to be wrapped in a method.

static IEnumerable Foo(int[] array) { foreach (var x in array) if (x >= 1) yield return x; }

Projection using query expression

Let’s try projection to an anonymous type in C#.

var results = from x in array select new { Foo = x + 1 };

Scala using for-comprehension.

for (x <- array)
  yield new { def foo = x + 1 }

Sorting by intermediate values

Here’s how to sort by intermediate values.

using System; using System.Collections.Generic; using System.Collections.Linq; using System.Text.RegularExpression;

var results = from x in list let cs = new Regex(@"[aeiou]").Replace(x.ToLower(), “”) orderby cs.Length select x;

Scala’s for-comprehension does not support sorting, but you can always sort things afterwards.

list sortBy { x =>
  val cs = """[aeiou]""".r.replaceAllIn(x.toLowerCase, "")
  cs.length
}

Cross join

The SQL-ness comes in handy when you join with C#.

var results = from x in list from c in x.ToCharArray() where c != 'a' && c != 'e' select c;

Using Scala for-comprehension.

for {
  x <- list
  c <- x.toCharArray
  if c != 'a' && c != 'e'
} yield c

Inner join

Inner joining using C#.

var results = from name in list join n in array on name.Length equals n + 3 select new { name, n };

Using Scala for-comprehension.

for {
  name <- list
  n <- array if name.length == n + 3
} yield (name, n)

Grouping

Grouping using C#.

var results = from x in list group x by x[0] into g where g.Count() > 1 select g;

Not for-comprehension, but still doable in Scala.

list groupBy { _(0) } filter { case (k, vs) =>
  vs.size > 1 }

Quantifiers

Quantifiers work more or less the same way.

var hasThree = list.Any(x => x.Length == 3) var allThree = list.All(x => x.Length == 3)

In Scala.

val hasThree = list exists { _.length == 3 }
val allThree = list forall { _.length == 3 }

Pattern matching

One unique aspect of Scala is that it accepts a partial function where a lambda expression is expected.

array map {
  case 1 => "foo"
  case n if n % 2 == 0 => n.toString + "!"
}

You probably have to throw an exception to mimic this in C#.

Notes

For Scala I prefer normal calls to filter and map over for-comprehension. Infix operator syntax and placeholder syntax makes array filter { _ >= 1 } concise enough that for-comprehension ends up becoming more bulkier unless they are nested.

On the other hand in C#, query expression syntax rids of some of the symbols (., (), =>) from fluent syntax.

Rahul (@missingfaktor) wrote a nice list of Enumerable methods and their equivalent ones in Scala, which covers everything I couoldn’t here.