How to convert a Scala collection to a String with mkString

By Alvin Alexander. Last updated: January 13, 2020

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 10.29, “How to Convert a Scala Collection to a String with mkString”

Problem

You want to convert elements of a collection to a String, possibly adding a field separator, prefix, and suffix.

Solution

Use the mkString method to print a collection as a String. Given a simple collection:

val a = Array("apple", "banana", "cherry")

you can print the collection elements using mkString:

scala> a.mkString
res1: String = applebananacherry

That output doesn’t look too good, so add a separator:

scala> a.mkString(" ")
res2: String = apple banana cherry

That’s better. Use a comma and a space to create a CSV string:

scala> a.mkString(", ")
res3: String = apple, banana, cherry

The mkString method is overloaded, so you can also add a prefix and suffix:

scala> a.mkString("[", ", ", "]")
res4: String = [apple, banana, cherry]

If you happen to have a list of lists that you want to convert to a String, such as the following array of arrays, first flatten the collection, and then call mkString:

scala> val a = Array(Array("a", "b"), Array("c", "d"))
a: Array[Array[java.lang.String]] = Array(Array(a, b), Array(c, d))

scala> a.flatten.mkString(", ")
res5: String = a, b, c, d

Discussion

You can also use the toString method on a collection, but it returns the name of the collection with the elements in the collection listed inside parentheses:

scala> val v = Vector("apple", "banana", "cherry")
v: scala.collection.immutable.Vector[String] = Vector(apple, banana, cherry)

scala> v.toString
res0: String = Vector(apple, banana, cherry)