CODEWITHSUNDEEP
scala
jcm
jcm

Reputation: 5659

Do if else statements in Scala always require an else?

I have the following function which works fine: 

def power(x: Double, n: Int) : Double = {
  if (n > 0 && n % 2 == 0) power(x, n/2) * power(x, n/2)
  else if (n > 0 && n % 2 == 1) x * power(x, n-1)
  else if (n < 0) 1 / power(x, -n)
  else 1
}

If I change it to be:

def power(x: Double, n: Int) : Double = {
  if (n > 0 && n % 2 == 0) power(x, n/2) * power(x, n/2)
  else if (n > 0 && n % 2 == 1) x * power(x, n-1)
  else if (n < 0) 1 / power(x, -n)
  else if (n==0 ) 1
}

I.e. change the final else statement to be an else if, then I get the following error trying to call the function: 

> <console>:8: error: not found: value power
                power(2, 1)
                ^

I'm guessing this is because there is a possible return type of Unit because the value of n could meet none of the conditions?

Upvotes: 2

Views: 2337

Answers (3)

lutzh
lutzh

Reputation: 4965

Your guess is correct. The method must return a Double, yours would return Unit if non of the "if.." cases match. Actually, when you paste the second definition into the repl, you should get

<console>:11: error: type mismatch;
found : Unit
required: Double

Upvotes: 0

Nikita Volkov
Nikita Volkov

Reputation: 43310

In Java "if-else" is a statement. In Scala it is an expression (think of Java's ternary ?:), meaning that it always produces some value. As mentioned by som-snytt in comments, in case of a missing else block the compiler supplies else (), which is of type Unit, which obviously conflicts with the expected type Double in your example.

Some valid examples of missing else are provided in Chris's answer.

Upvotes: 7

Chris Martin
Chris Martin

Reputation: 30736

No - In general, an if expression does not require an else clause.

Examples:

if (true) println("a")
// prints "a"

if (false) println("a") else if (true) println("b")
// prints "b"

As Nikita Volkov's answer says, though, it is necessary if you need the expression to have some type other than Unit.

Upvotes: 3

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