Scala Cookbook

2.4. Replacements for ++ and −−

Problem

You want to increment or decrement numbers using operators like ++ and −− that are available in other languages, but Scala doesn’t have these operators.

Solution

Because val fields are immutable, they can’t be incremented or decremented, but var Int fields can be mutated with the += and −= methods:

scala> var a = 1
a: Int = 1

scala> a += 1

scala> println(a)
2

scala> a −= 1

scala> println(a)
1

As an added benefit, you use similar methods for multiplication and division:

scala> var i = 1
i: Int = 1

scala> i *= 2

scala> println(i)
2

scala> i *= 2

scala> println(i)
4

scala> i /= 2

scala> println(i)
2

Note that these symbols aren’t operators; they’re implemented as methods that are available on Int fields declared as a var. Attempting to use them on val fields results in a compile-time error:

scala> val x = 1
x: Int = 1

scala> x += 1
<console>:9: error: value += is not a member of Int
              x += 1
                ^

Note

As mentioned, the symbols +=, −=, *=, and /= aren’t operators, they’re methods. This approach of building functionality with libraries instead of operators is a consistent pattern in Scala. Actors, for instance, are not built into the language, but are instead implemented as a library. See the Dr. Dobbs link in the See Also for Martin Odersky’s discussion of this philosophy.

Discussion

Another benefit of this approach is that you can call methods of the same name on other types besides Int. For instance, the Double and Float classes have methods of the same name: ...