Can you share some Scala List class examples?

Posted 2020-08-03 尧字节

Scala List FAQ: Can you share some Scala List class examples?

The Scala List class may be the most commonly used data structure in Scala applications. Therefore, it‘s very helpful to know how create lists, merge lists, select items from lists, operate on each element in a list, and so on.

In this tutorial, I‘ll try to share examples of the most common List operations (methods).

1) Scala List class - Introduction

The Scala List class (scala.List) holds a sequenced, linear list of items. In a List, each element must be of the same type.

At some point I‘ll add a class diagram here, but until then, here‘s a simplified version of the Scala List class hierarchy:

• The scala.List class is a pointer to the scala.collection.immutable.List class.
• The List class extends LinearSeq with Product (and some others).
• The trait LinearSeq extends Seq
• The trait Seq extends Iterable
• Iterable extends trait Traversable

(I need to update this, but this is the Scala class hierarchy, as shown by thecurrent Scala API documentation.

2) Creating a Scala List object

You can create a Scala List object in several different way. Here‘s the Lisp-style approach to creating a List:

scala> val list = 1 :: 2 :: 3 :: Nil
list: List[Int] = List(1, 2, 3)

Here‘s the Java-style approach to creating a Scala List:

scala> val list = List(1,2,3)
x: List[Int] = List(1, 2, 3)

A few notes about these approaches:

• The first approach shows the "cons" syntax, which, as I mentioned, is the Lisp style of creating a list.
• The :: method takes two arguments, a "head", which is a single element, and a "tail", which is a List.
• As you can see, Scala can usually infer the type of a List very well.

If you‘re mixing types in a List constructor, you may need to manually specify the List type. This example demonstrates that syntax:

scala> val x = List[Number](1, 2.0, 33d, 0x1)
x: List[java.lang.Number] = List(1, 2.0, 33.0, 1)

In this example, I‘m explicitly saying that I want the values in the List to be saved as the Number type.

2.1) The List range method

You can also create a List with the List‘s range method:

scala> val x = List.range(1,10)
x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9)

The range function can also take a third argument which serves as a "step" value when creating the List:

scala> val x = List.range(0,10,2)
x: List[Int] = List(0, 2, 4, 6, 8)

2.2) The List fill method

You can also create a new List with the fill method:

scala> val x = List.fill(3)("foo")
x: List[java.lang.String] = List(foo, foo, foo)

2.3) The List tabulate method

Finally, you can create a new List with the tabulate method of the List class. The tabulate method creates a new List whose elements are created according to the function you supply. The book Programming in Scala shows how to create a List using a simple "squares" function:

scala> val x = List.tabulate(5)(n => n * n)
x: List[Int] = List(0, 1, 4, 9, 16)

As you can see, that example creates a List of five elements, where the element values are the square of the index of each element (0 becomes 0, 1 becomes 1, 2 becomes 4, 3 becomes 9, and 4 becomes 16).

In summary, you can create a new Scala List with these approaches:

• Lisp style
• Java style
• range method
• tabulate method

3) Prepend items to a List

You can prepend items to a Scala List using the :: method:

// create a List
scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

// prepend an element to the list
scala> val y = 0 :: x
y: List[Int] = List(0, 1, 2, 3)

According to the Beginning Scala book (and several other sources), prepending items to a list is a "very fast, constant-time, O(1) operation."

(Again, I recall this approach and syntax from using Lisp many years ago.)

4) Appending and merging Lists

There are at least two ways to merge Scala Lists. First, you can merge two Scala lists using the ::: method of the List class:

scala> val a = List(1,2,3)
a: List[Int] = List(1, 2, 3)

scala> val b = List(4,5,6)
b: List[Int] = List(4, 5, 6)

scala> val c = a ::: b
c: List[Int] = List(1, 2, 3, 4, 5, 6)

This operation is said to have O(n) speed, where n is the number of elements in the first List.

You can also merge two Scala lists using the List‘s concat method:

scala> val a = List(1,2,3)
a: List[Int] = List(1, 2, 3)

scala> val b = List(4,5,6)
b: List[Int] = List(4, 5, 6)

scala> val c = List.concat(a, b)
c: List[Int] = List(1, 2, 3, 4, 5, 6)

5) Iterating lists with foreach

A very common way to iterate over Scala lists is with the foreach method. Here‘s a quote about the foreach method from the book Programming in Scala:

foreach takes a procedure (a function with a result type Unit) as the right operand. It simply applies the procedure to each List element. The result of the operation is again Unit; no list of results is assembled.

Here‘s a simple example showing how to use the foreach function to print every item in a List:

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x.foreach { println }
1
2
3

This next example shows a way to sum all the elements in a list using the foreach method:

scala> var sum = 0
sum: Int = 0

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x.foreach(sum += _)

scala> println(sum)
6

6) Scala Lists and the for comprehension

The Scala for comprehension is not specific to lists, but is an extremely powerful way to operate on lists. Here‘s a simple example of how to iterate over a list using the for comprehension:

scala> val names = List("Bob", "Fred", "Joe", "Julia", "Kim")
names: List[java.lang.String] = List(Bob, Fred, Joe, Julia, Kim)

scala> for (name <- names) println(name)
Bob
Fred
Joe
Julia
Kim

So far, so good. Now let‘s add a simple "if" clause to the for comprehension to print only the elements we want to print:

scala> val names = List("Bob", "Fred", "Joe", "Julia", "Kim")
names: List[java.lang.String] = List(Bob, Fred, Joe, Julia, Kim)

scala> for (name <- names if name.startsWith("J"))
     | println(name)
Joe
Julia

If you already know about the for comprehension, you know that you can add multiple if clauses, and much more functionality. I could easily write an entire tutorial on the Scala for comprehension, so to keep this tutorial short, I‘ll stop here for now.

Before leaving, I will add these notes however, from the book Programming in Scala:

Scala provides the for comprehension, which provides syntactically pleasing nesting of map, flatMap, and filter ... The for comprehension is not a looping construct, but is a syntactic construct the compiler reduces to map, flatMap, and filter.

7) Filtering Scala lists

A great thing about Scala is that it is a functional programming language. In the next examples we‘ll show some of the power of functional programming. In this section we‘ll focus on the filter method of the List class.

scala> val x = List(1,2,3,4,5,6,7,8,9,10)
x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

// create a list of all the even numbers in the list
scala> val evens = x.filter(a => a % 2 == 0)
evens: List[Int] = List(2, 4, 6, 8, 10)

takeWhile

scala> val x = List(1,2,3,4,5,6,7,8,9,10)
x: List[Int] = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

scala> val y = x.takeWhile(a => a < 6)
y: List[Int] = List(1, 2, 3, 4, 5)

Other list filtering methods:

partition - returns a pair of lists, one where the predicate is true,
            the other where the predicate is false
find      - returns the first element matching a predicate (as opposed to
            returning all such elements)

Others:   takeWhile, dropWhile, span

8) The List map function

The Scala List map function "transforms each element of a collection based on a function."

Here are a few map examples. First, doubling every item in a List:

scala> val x = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> val y = x.map(a => a * 2)
y: List[Int] = List(2, 4, 6)

Here‘s a slightly simpler version of that map example, using the Scala wildcard character (_) instead of a fake variable name:

scala> val y = x.map(_ * 2)
y: List[Int] = List(2, 4, 6)

Here‘s an example using a list of strings:

scala> val names = List("Fred", "Joe", "Bob")
names: List[java.lang.String] = List(Fred, Joe, Bob)

scala> val lower = names.map(_.toLowerCase)
lower: List[java.lang.String] = List(fred, joe, bob)

scala> val upper = names.map(_.toUpperCase)
upper: List[java.lang.String] = List(FRED, JOE, BOB)

A very nice example in the book Beginning Scala demonstrates how to convert a List into something useful in the html world, a list of <li> elements using the map function:

scala> val names = List("Fred", "Joe", "Bob")
names: List[java.lang.String] = List(Fred, Joe, Bob)

scala> val li = names.map(name => <li>{name}</li>)
li: List[scala.xml.Elem] = List(<li>Fred</li>, <li>Joe</li>, <li>Bob</li>)

As you can see, you can rapidly build a lot of functionality in just a little bit of code.

9) Sorting Scala Lists

I need to research why this code is deprecated, but for the time being, here‘s an example of how to sort a Scala List:

scala> val x = List(10, 2, 5)
x: List[Int] = List(10, 2, 5)

scala> val y = x.sort(_ < _)
warning: there were 1 deprecation warnings; re-run with -deprecation for details
y: List[Int] = List(2, 5, 10)

(TODO: Research the correct, current approach for sorting Scala lists.)

10) Scala List class and pattern matching

You can use the List class with the Scala pattern matching and case/match syntax. I‘ll add examples here as I create them in my own code. (TODO)

11) Other Scala List functions

The Scala List class has an incredible number of functions/methods, and over time I‘ll attempt to document them all. In the meantime, here‘s a short list of the many other Scala List methods I don‘t have examples for at this time:

length  - returns the length of a List
head    - returns the first element of a List
last    - returns the last element of a List
init    - returns a List consisting of all elements except the last one
tail    - returns every elements of a List except the first element
isEmpty - returns a Boolean indicating if the List is empty

reverse - returns a reversed version of the List
flatten - takes a list of lists and flattens it out to a single list

mkString - converts a List to a String

iterator
toArray

foldLeft
reduceLeft

map
flatMap
foreach

forall
exists

Folding lists: /: and :
sortWith

Again, I‘ll try to add examples of these List methods over time. (TODO)

12) Relatives of the Scala List

There are times you may want to use one of the relatives of the Scala List class, instead of using the List class itself. I‘ll add more to this over time, but for now, here are a few links:

• The ListBuffer class
• The MutableList class