Scala Vector class: Method examples (map, filter, fold, reduce)

By Alvin Alexander. Last updated: August 18, 2021

This page contains over 170 examples of how to use the Scala Vector class, covering most of the methods that are available on a Vector, including map, filter, foldLeft, reduceLeft, and many more.

Introduction

Use the Vector class when you want to use a general-purpose, immutable indexed sequence in your Scala applications:

Immutable: the elements in the collection cannot be changed, and the collection cannot be resized
Indexed: you can access elements quickly by their element number, such as people(12345432) (a linked list would be very slow for this purpose)

Just as the ArrayBuffer is the recommended “go to” class for mutable indexed sequential collections, the Vector class is recommended as the general-purpose collections class for immutable indexed sequential collections.

An important note about multithreaded applications

Under certain conditions the Vector is not safe to use in multithreaded applications. These notes come from the Vector class Scaladoc:

“Despite being an immutable collection, the implementation uses mutable state internally during construction. These state changes are invisible in single-threaded code but can lead to race conditions in some multi-threaded scenarios. The state of a new collection instance may not have been "published" (in the sense of the Java Memory Model specification), so that an unsynchronized non-volatile read from another thread may observe the object in an invalid state (see scala/bug#7838 for details).”

An important note about the examples

Remember that Vector is immutable, so in all of the examples that follow you need to assign the result of the operation shown to a new variable, like this:

val x = nums.distinct

Create a new Vector with initial elements

To create a new Vector with initial elements:

val nums = Vector(1, 2, 3)

val people = Vector(
    Person("Emily"),
    Person("Hannah"),
    Person("Mercedes")
)

When you need to be clear about what’s in the vector:

val x = Vector(1, 1.0, 1F)                   # Vector[Double] = Vector(1.0, 1.0, 1.0)
val x: Vector[Number] = Vector(1, 1.0, 1F)   # Vector[Number] = Vector(1, 1.0, 1.0)

trait Animal
case class Dog(name: String) extends Animal
case class Cat(name: String) extends Animal
val animalHouse: Vector[Animal] = Vector(    # Vector[Animal] = Vector(Dog(Rover), Cat(Felix))
    Dog("Rover"),
    Cat("Felix")
)

If you ever need to create an empty vector:

val nums = Vector[Int]()

Remember the constructor syntax is just syntactic sugar for apply:

val nums = Vector(1, 2, 3)                   # Vector(1, 2, 3)
val nums = Vector.apply(1, 2, 3)             # Vector(1, 2, 3)

Create a new Vector by populating it

You can also create a new Vector that’s populated with initial elements using a Range:

# to, until
(1 to 5).toVector                   # Vector(1, 2, 3, 4, 5)
(1 until 5).toVector                # Vector(1, 2, 3, 4)

(1 to 10 by 2).toVector             # Vector(1, 3, 5, 7, 9)
(1 until 10 by 2).toVector          # Vector(1, 3, 5, 7, 9)
(1 to 10).by(2).toVector            # Vector(1, 3, 5, 7, 9)

('d' to 'h').toVector               # Vector(d, e, f, g, h)
('d' until 'h').toVector            # Vector(d, e, f, g)

('a' to 'f').by(2).toVector         # Vector(a, c, e)

# range method
Vector.range(1, 3)                  # Vector(1, 2)
Vector.range(1, 6, 2)               # Vector(1, 3, 5)

You can also use the fill and tabulate methods:

Vector.fill(3)("foo")               # Vector(foo, foo, foo)
Vector.tabulate(3)(n => n * n)      # Vector(0, 1, 4)
Vector.tabulate(4)(n => n * n)      # Vector(0, 1, 4, 9)

How to add (append and prepend) elements to a Vector

Because Vector is immutable, you can’t add elements to an existing Vector. The way you work with Vector is to modify the elements it contains as you assign the results to a new Vector.

Method	Description	Example
`:+`	append 1 item	`oldSeq :+ e`
`++`	append N items	`oldSeq ++ newSeq`
`+:`	prepend 1 item	`e +: oldSeq`
`++:`	prepend N items	`newSeq ++: oldSeq`

Append and prepend examples

These examples show how to use the append and prepend methods:

val v1 = Vector(4,5,6)     # Vector(4, 5, 6)
val v2 = v1 :+ 7           # Vector(4, 5, 6, 7)
val v3 = v2 ++ Seq(8,9)    # Vector(4, 5, 6, 7, 8, 9)

val v4 = 3 +: v3           # Vector(3, 4, 5, 6, 7, 8, 9)
val v5 = Seq(1,2) ++: v4   # Vector(1, 2, 3, 4, 5, 6, 7, 8, 9)

About the `:` character in the method names

Note that during these operations the : character is always next to the old (original) sequence. I use that as a way to remember these methods.

The correct technical way to think about this is that a Scala method name that ends with the : character is right-associative, meaning that the method comes from the variable on the right side of the expression. Therefore, with +: and ++:, these methods comes from the Vector that’s on the right of the method name.

Filtering methods (how to “remove” elements from an Vector)

A Vector is an immutable sequence, so you don’t remove elements from a Vector. Instead, you describe how to remove elements as you assign the results to a new collection. These methods let you “remove” elements during this process:

Method	Description
`distinct`	Return a new sequence with no duplicate elements
`drop(n)`	Return all elements after the first `n` elements
`dropRight(n)`	Return all elements except the last `n` elements
`dropWhile(p)`	Drop the first sequence of elements that matches the predicate `p`
`filter(p)`	Return all elements that match the predicate `p`
`filterNot(p)`	Return all elements that do not match the predicate `p`
`find(p)`	Return the first element that matches the predicate `p`
`head`	Return the first element; can throw an exception if the `Vector` is empty
`headOption`	Returns the first element as an `Option`
`init`	All elements except the last one
`intersect(s)`	Return the intersection of the vector and another sequence `s`
`last`	The last element; can throw an exception if the `Vector` is empty
`lastOption`	The last element as an `Option`
`slice(f,u)`	A sequence of elements from index `f` (from) to index `u` (until)
`tail`	All elements after the first element
`take(n)`	The first `n` elements
`takeRight(n)`	The last `n` elements
`takeWhile(p)`	The first subset of elements that matches the predicate `p`

Examples

val a = Vector(10, 20, 30, 40, 10)    # Vector(10, 20, 30, 40, 10)
a.distinct                            # Vector(10, 20, 30, 40)
a.drop(2)                             # Vector(30, 40, 10)
a.dropRight(2)                        # Vector(10, 20, 30)
a.dropWhile(_ < 25)                   # Vector(30, 40, 10)
a.filter(_ < 25)                      # Vector(10, 20, 10)
a.filter(_ > 100)                     # Vector()
a.filterNot(_ < 25)                   # Vector(30, 40)
a.find(_ > 20)                        # Some(30)
a.head                                # 10
a.headOption                          # Some(10)
a.init                                # Vector(10, 20, 30, 40)
a.intersect(Seq(19,20,21))            # Vector(20)
a.last                                # 10
a.lastOption                          # Some(10)
a.slice(2,4)                          # Vector(30, 40)
a.tail                                # Vector(20, 30, 40, 10)
a.take(3)                             # Vector(10, 20, 30)
a.takeRight(2)                        # Vector(40, 10)
a.takeWhile(_ < 30)                   # Vector(10, 20)

As noted, head and last can throw exceptions:

scala> val a = Vector[Int]()
a: scala.collection.immutable.Vector[Int] = Vector()

scala> a.head
java.lang.UnsupportedOperationException: empty.head
  at scala.collection.immutable.Vector.head(Vector.scala:185)
  ... 28 elided

scala> a.last
java.lang.UnsupportedOperationException: empty.last
  at scala.collection.immutable.Vector.last(Vector.scala:197)
  ... 28 elided

How to “update” Vector elements

Because Vector is immutable, you can’t update elements in place, but depending on your definition of “update,” there are a variety of methods that let you update a Vector as you assign the result to a new variable:

Method	Returns
`collect(pf)`	A new collection by applying the partial function `pf` to all elements of the vector, returning elements for which the function is defined
`distinct`	A new sequence with no duplicate elements
`flatten`	Transforms a list of lists into a single list
`flatMap(f)`	When working with sequences, it works like `map` followed by `flatten`
`map(f)`	Return a new sequence by applying the function `f` to each element in the `Vector`
`updated(i,v)`	A new vector with the element at index `i` replaced with the new value `v`
`union(s)`	A new vector that contains elements from the current vector and the sequence `s`

val x = Vector(Some(1), None, Some(3), None)
x.collect{case Some(i) => i}          # Vector(1, 3)

val x = Vector(1,2,1,2)
x.distinct                            # Vector(1, 2)
x.map(_ * 2)                          # Vector(2, 4, 2, 4)
x.updated(0,100)                      # Vector(100, 2, 1, 2)

val a = Vector(Seq(1,2), Seq(3,4))
a.flatten                             # Vector(1, 2, 3, 4)

val fruits = Vector("apple", "pear")
fruits.map(_.toUpperCase)             # Vector(APPLE, PEAR)
fruits.flatMap(_.toUpperCase)         # Vector(A, P, P, L, E, P, E, A, R)

Vector(2,4).union(Vector(1,3))        # Vector(2, 4, 1, 3)

Transformer methods

A transformer method is a method that constructs a new collection from an existing collection.

Method	Returns
`collect(pf)`	Creates a new collection by applying the partial function `pf` to all elements of the vector, returning elements for which the function is defined
`diff(c)`	The difference between this vector and the collection `c`
`distinct`	A new sequence with no duplicate elements
`flatten`	Transforms a list of lists into a single list
`flatMap(f)`	When working with sequences, it works like `map` followed by `flatten`
`map(f)`	A new sequence by applying the function `f` to each element in the `Vector`
`reverse`	A new sequence with the elements in reverse order
`sortWith(f)`	A new sequence with the elements sorted with the use of the function `f`
`updated(i,v)`	A new `Vector` with the element at index `i` replaced with the new value `v`
`union(c)`	A new sequence that contains all elements of the vector and the collection `c`
`zip(c)`	A collection of pairs by matching the vector with the elements of the collection `c`
`zipWithIndex`	A vector of each element contained in a tuple along with its index

val x = Vector(Some(1), None, Some(3), None)
x.collect{case Some(i) => i}          # Vector(1, 3)

# diff
val oneToFive = (1 to 5).toVector     # val oneToFive = (1 to 5).toVector
val threeToSeven = (3 to 7).toVector  # Vector(3, 4, 5, 6, 7)
oneToFive.diff(threeToSeven)          # Vector(1, 2)
threeToSeven.diff(oneToFive)          # Vector(6, 7)

Vector(1,2,1,2).distinct              # Vector(1, 2)

val a = Vector(Seq(1,2), Seq(3,4))
a.flatten                             # Vector(1, 2, 3, 4)

# map, flatMap
val fruits = Vector("apple", "pear")
fruits.map(_.toUpperCase)             # Vector(APPLE, PEAR)
fruits.flatMap(_.toUpperCase)         # Vector(A, P, P, L, E, P, E, A, R)

Vector(1,2,3).reverse                 # Vector(3, 2, 1)

val nums = Vector(10, 5, 8, 1, 7)
nums.sorted                           # Vector(1, 5, 7, 8, 10)
nums.sortWith(_ < _)                  # Vector(1, 5, 7, 8, 10)
nums.sortWith(_ > _)                  # Vector(10, 8, 7, 5, 1)

Vector(1,2,3).updated(0,10)           # Vector(10, 2, 3)
Vector(2,4).union(Vector(1,3))        # Vector(2, 4, 1, 3)

# zip
val women = Vector("Wilma", "Betty")  # Vector(Wilma, Betty)
val men = Vector("Fred", "Barney")    # Vector(Fred, Barney)
val couples = women.zip(men)          # Vector((Wilma,Fred), (Betty,Barney))

val a = Vector.range('a', 'e')        # Vector(a, b, c, d)
a.zipWithIndex                        # Vector((a,0), (b,1), (c,2), (d,3))

Informational and mathematical methods

These methods let you obtain information from a collection.

Method	Returns
`contains(e)`	True if the vector contains the element `e`
`containsSlice(s)`	True if the vector contains the sequence `s`
`count(p)`	The number of elements in the vector for which the predicate is true
`endsWith(s)`	True if the vector ends with the sequence `s`
`exists(p)`	True if the predicate returns true for at least one element in the vector
`find(p)`	The first element that matches the predicate `p`, returned as an `Option`
`forall(p)`	True if the predicate `p` is true for all elements in the vector
`hasDefiniteSize`	True if the vector has a finite size
`indexOf(e)`	The index of the first occurrence of the element `e` in the vector
`indexOf(e,i)`	The index of the first occurrence of the element `e` in the vector, searching only from the value of the start index `i`
`indexOfSlice(s)`	The index of the first occurrence of the sequence `s` in the vector
`indexOfSlice(s,i)`	The index of the first occurrence of the sequence `s` in the vector, searching only from the value of the start index `i`
`indexWhere(p)`	The index of the first element where the predicate `p` returns true
`indexWhere(p,i)`	The index of the first element where the predicate `p` returns true, searching only from the value of the start index `i`
`isDefinedAt(i)`	True if the vector contains the index `i`
`isEmpty`	True if the vector contains no elements
`lastIndexOf(e)`	The index of the last occurrence of the element `e` in the vector
`lastIndexOf(e,i)`	The index of the last occurrence of the element `e` in the vector, occurring before or at the index `i`
`lastIndexOfSlice(s)`	The index of the last occurrence of the sequence `s` in the vector
`lastIndexOfSlice(s,i)`	The index of the last occurrence of the sequence `s` in the vector, occurring before or at the index `i`
`lastIndexWhere(p)`	The index of the first element where the predicate `p` returns true
`lastIndexWhere(p,i)`	The index of the first element where the predicate `p` returns true, occurring before or at the index `i`
`max`	The largest element in the vector
`min`	The smallest element in the vector
`nonEmpty`	True if the vector is not empty (i.e., if it contains 1 or more elements)
`product`	The result of multiplying the elements in the collection
`segmentLength(p,i)`	The length of the longest segment for which the predicate `p` is true, starting at the index `i`
`size`	The number of elements in the vector
`startsWith(s)`	True if the vector begins with the elements in the sequence `s`
`startsWith(s,i)`	True if the vector has the sequence `s` starting at the index `i`
`sum`	The sum of the elements in the vector
`fold(s)(o)`	“Fold” the elements of the vector using the binary operator `o`, using an initial seed `s` (see also `reduce`)
`foldLeft(s)(o)`	“Fold” the elements of the vector using the binary operator `o`, using an initial seed `s`, going from left to right (see also `reduceLeft`)
`foldRight(s)(o)`	“Fold” the elements of the vector using the binary operator `o`, using an initial seed `s`, going from right to left (see also `reduceRight`)
`reduce`	“Reduce” the elements of the vector using the binary operator `o`
`reduceLeft`	“Reduce” the elements of the vector using the binary operator `o`, going from left to right
`reduceRight`	“Reduce” the elements of the vector using the binary operator `o`, going from right to left

Examples

First, some sample data:

val evens = Vector(2, 4, 6)
val odds = Vector(1, 3, 5)
val fbb = "foo bar baz"
val firstTen = (1 to 10).toVector          # Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val fiveToFifteen = (5 to 15).toVector     # Vector(5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
val empty = Vector[Int]()                  # Vector[Int] = Vector()
val letters = ('a' to 'f').toVector        # Vector(a, b, c, d, e, f)

The examples:

evens.contains(2)                          # true
firstTen.containsSlice(Seq(3,4,5))         # true
firstTen.count(_ % 2 == 0)                 # 5
firstTen.endsWith(Seq(9,10))               # true
firstTen.exists(_ > 10)                    # false
firstTen.find(_ > 2)                       # Some(3)
firstTen.forall(_ < 20)                    # true
firstTen.hasDefiniteSize                   # true
empty.hasDefiniteSize                      # true
letters.indexOf('b')                       # 1 (zero-based)
letters.indexOf('d', 2)                    # 3
letters.indexOf('d', 3)                    # 3
letters.indexOf('d', 4)                    # -1
letters.indexOfSlice(Seq('c','d'))         # 2
letters.indexOfSlice(Seq('c','d'),2)       # 2
letters.indexOfSlice(Seq('c','d'),3)       # -1
firstTen.indexWhere(_ == 3)                # 2
firstTen.indexWhere(_ == 3, 2)             # 2
firstTen.indexWhere(_ == 3, 5)             # -1
letters.isDefinedAt(1)                     # true
letters.isDefinedAt(20)                    # false
letters.isEmpty                            # false
empty.isEmpty                              # true

# lastIndex...
val fbb = "foo bar baz"
fbb.indexOf('a')                           # 5
fbb.lastIndexOf('a')                       # 9
fbb.lastIndexOf('a', 10)                   # 9
fbb.lastIndexOf('a', 9)                    # 9
fbb.lastIndexOf('a', 6)                    # 5
fbb.lastIndexOf('a', 5)                    # 5
fbb.lastIndexOf('a', 4)                    # -1

fbb.lastIndexOfSlice("ar")                 # 5
fbb.lastIndexOfSlice(Seq('a','r'))         # 5
fbb.lastIndexOfSlice(Seq('a','r'), 4)      # -1
fbb.lastIndexOfSlice(Seq('a','r'), 5)      # 5
fbb.lastIndexOfSlice(Seq('a','r'), 6)      # 5

fbb.lastIndexWhere(_ == 'a')               # 9
fbb.lastIndexWhere(_ == 'a', 4)            # -1
fbb.lastIndexWhere(_ == 'a', 5)            # 5
fbb.lastIndexWhere(_ == 'a', 6)            # 5
fbb.lastIndexWhere(_ == 'a', 8)            # 5
fbb.lastIndexWhere(_ == 'a', 9)            # 9

firstTen.max                               # 10
letters.max                                # f
firstTen.min                               # 1
letters.min                                # a
letters.nonEmpty                           # true
empty.nonEmpty                             # false
firstTen.product                           # 3628800
letters.size                               # 6

val x = Vector(1,2,9,1,1,1,1,4)
x.segmentLength(_ < 4, 0)                  # 2
x.segmentLength(_ < 4, 2)                  # 0
x.segmentLength(_ < 4, 3)                  # 4
x.segmentLength(_ < 4, 4)                  # 3

firstTen.startsWith(Seq(1,2))              # true
firstTen.startsWith(Seq(1,2), 0)           # true
firstTen.startsWith(Seq(1,2), 1)           # false
firstTen.sum                               # 55

firstTen.fold(100)(_ + _)                  # 155
firstTen.foldLeft(100)(_ + _)              # 155
firstTen.foldRight(100)(_ + _)             # 155
firstTen.reduce(_ + _)                     # 55
firstTen.reduceLeft(_ + _)                 # 55
firstTen.reduceRight(_ + _)                # 55

firstTen.fold(100)(_ - _)                  # 45
firstTen.foldLeft(100)(_ - _)              # 45
firstTen.foldRight(100)(_ - _)             # 95
firstTen.reduce(_ - _)                     # -53
firstTen.reduceLeft(_ - _)                 # -53
firstTen.reduceRight(_ - _)                # -5

Grouping methods

These methods generally let you create multiple groups from a collection.

Method	Returns
`groupBy(f)`	A map of collections created by the function `f`
`grouped`	Breaks the vector into fixed-size iterable collections
`partition(p)`	Two collections created by the predicate `p`
`sliding(i,s)`	Group elements into fixed size blocks by passing a sliding window of size `i` and step `s` over them
`span(p)`	A collection of two collections; the first created by `vector.takeWhile(p)`, and the second created by `vector.dropWhile(p)`
`splitAt(i)`	A collection of two collections by splitting the vector at index `i`
`unzip`	The opposite of `zip`, breaks a collection into two collections by dividing each element into two pieces; such as breaking up a vector of `Tuple2` elements

Examples

val firstTen = (1 to 10).toVector      # Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

firstTen.groupBy(_ > 5)                # Map(false -> Vector(1, 2, 3, 4, 5), true -> Vector(6, 7, 8, 9, 10))
firstTen.grouped(2)                    # Iterator[Vector[Int]] = non-empty iterator
firstTen.grouped(2).toVector           # Vector(Vector(1, 2), Vector(3, 4), Vector(5, 6), Vector(7, 8), Vector(9, 10))
firstTen.grouped(5).toVector           # Vector(Vector(1, 2, 3, 4, 5), Vector(6, 7, 8, 9, 10))

"foo bar baz".partition(_ < 'c')       # (" ba ba", foorz)  // a Tuple2
firstTen.partition(_ > 5)              # (Vector(6, 7, 8, 9, 10), Vector(1, 2, 3, 4, 5))

firstTen.sliding(2)                    # Iterator[Vector[Int]] = non-empty iterator
firstTen.sliding(2).toVector           # Vector(Vector(1, 2), Vector(2, 3), Vector(3, 4), Vector(4, 5), Vector(5, 6), Vector(6, 7), Vector(7, 8), Vector(8, 9), Vector(9, 10))  
firstTen.sliding(2,2).toVector         # Vector(Vector(1, 2), Vector(3, 4), Vector(5, 6), Vector(7, 8), Vector(9, 10))  
firstTen.sliding(2,3).toVector         # Vector(Vector(1, 2), Vector(4, 5), Vector(7, 8), Vector(10))
firstTen.sliding(2,4).toVector         # Vector(Vector(1, 2), Vector(5, 6), Vector(9, 10)) 

val x = Vector(15, 10, 5, 8, 20, 12)
x.groupBy(_ > 10)                      # Map(false -> Vector(10, 5, 8), true -> Vector(15, 20, 12))
x.partition(_ > 10)                    # (Vector(15, 20, 12), Vector(10, 5, 8))
x.span(_ < 20)                         # (Vector(15, 10, 5, 8), Vector(20, 12))
x.splitAt(2)                           # (Vector(15, 10), Vector(5, 8, 20, 12))

More information:

How to split sequences into subsets in Scala

Looping over a `Vector` with for and foreach

These examples show how to loop/iterate over a vector with for and foreach. (As I write in Functional Programming, Simplified, foreach is only used for side effects, and therefore I rarely use it.)

val oneToFive = Vector(1, 2, 3, 4, 5)

for (i <- oneToFive) yield i           # Vector(1, 2, 3, 4, 5)
for (i <- oneToFive) yield i * 2       # Vector(2, 4, 6, 8, 10)
for (i <- oneToFive) yield i % 2       # Vector(1, 0, 1, 0, 1)

for {                                  # Vector(3, 4, 5)
    i <- oneToFive
    if i > 2
} yield i

for {                                  # Vector(6, 8, 10)
    i <- oneToFive
    if i > 2
} yield {
    // could be multiple lines here
    i * 2
}

# foreach (which i rarely use)
val oneToThree = Vector(1, 2, 3)
oneToThree.foreach(print)              # 123
for (i <- oneToThree) print(i)         # 123

A few things you can do with a Vector of Options

The Option type is used a lot in idiomatic Scala code, so here are some ways to work with a Vector that contains Options.

val x = Vector(Some(1), None, Some(3), None)

x.flatten                              # Vector(1, 3)
x.collect{case Some(i) => i}           # Vector(1, 3)


# map, flatten, flatMap
import scala.util.Try
def toInt(s: String): Option[Int] = Try(Integer.parseInt(s)).toOption
val strings = Vector("1", "2", "foo", "3", "bar")

strings.map(toInt)                     # Vector(Some(1), Some(2), None, Some(3), None)
strings.map(toInt).flatten             # Vector(1, 2, 3)
strings.flatMap(toInt)                 # Vector(1, 2, 3)

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Scala Vector summary

I hope these Vector examples are helpful. Most of these examples will work very similarly with other immutable sequential collections classes like Seq, IndexedSeq, LinearSeq, List, and more. (Eventually I’ll create dedicated pages for each of those types, and others, such as ArrayBuffer and ListBuffer.)

As a final note, if I made a mistake, or you know another way to do something with an Vector I haven’t shown, leave a note in the Comments section.

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