In Scala, why does Array("1") ++ "-3" output Array[Any] = Array(1, -, 3)?

Question

Why in Scala 2.12.6 does Array("1") ++ "-3" output res1: Array[Any] = Array(1, -, 3)?

How do I get the result Array("1", "-3")?

Answer 1

The ++ operator concatenates two Lists (or other collections) together. As a String is essentially just a list of Chars, as @senjin.hajrulahovic points out the compiler treats it this way (and returns an Array[Any].

You can get around this in multiple ways. For example, Array("1") ++ Array("-3") (to add two arrays together) or Array("1") :+ "-3" to put the value on the end of the Array.

Answer 2

++ is binary operator(also called method) that is used to concatenate two collections.eg. Arrays or Lists. If the first operand is an Array, the result defaults to Array. If the first operand is a List, the result defaults to List.

The compiler, by default, treats "-3" as String. All Strings can be treated as collections also (by implicit conversions when required ). But what type of collection? Is it Array? Is it List? depends on its usage with the type of collection it is operated on.

For Example, While using it as Array("1") ++ "-3", the String "-3" by default will be treated by the compiler as Array[Char] because the first operand of ++ is an Array ( though the type of elements will be always Char by default , by the compiler) While using it as List("1") ++ "-3", the String "-3" by default will be treated as List[Char] because the first operand of ++ is a List ( though the type of elements will be always Char by default , by the compiler)

When the ++ operator encounters both operands are collections of different types of elements, their closest super-type in type-hierarchy, will be the type of elements of the resultant collection. The super-type of String, which is the type of elements of first operand(collection) and Char, which is the type of elements of second operand(collection) is Any. So the result of Array("1") ++ "-3" is Array[Any].

If you want to add the String "-3" as an element to the collection, Array("1"), then you must add it as an element to this collection. There is another method ie., :+ or +: to append or prepend an element to the collection as below.

To append to the collection:

scala> Array("1") :+ "-3"
res6: Array[String] = Array(1, -3)

To prepend to the collection:

scala> "-3" +: Array("1")
res7: Array[String] = Array(-3, 1)

Another way of doing the above is as below(converting element to a collection and using ++ method):

scala> Array("1") ++ Array("-3")
res9: Array[String] = Array(1, -3)

scala> Array("-3") ++ Array("1")
res10: Array[String] = Array(-3, 1)

scala>

Answer 3

Because scala treats "-3" as a sequence of chars '-' and '3'.

If you want to append the string "-3" as one element use :+.