Scala’s flow control is more advanced than Java.

Condition

Scala provides the same if/else keywords as Java. They return a value, which is the value of the expression that is true after evaluation. For example,

val s = if (x > 0) 1 else -1

is the same as

if (x > 0) s = 1 else s = -1

In the first form, it can initialize a val but in the second form s needs to be a var.

The expression is similar to ?: operator in C++ and Java, but if/else in Scala can have statements inside.

An if statement without the else clause can yield a () if evaluation fails. So if (x > 0) 1 is equivalent to if (x > 0) 1 else ().

There are no switch statement in Scala. Use pattern matching instead. @switch can be applied to improve performance from decision tree to branch table at compile time, with a few constraints.

Loop

Scala has the same while and do-while loops as Java and C++:

while (n > 0) {
	r = r * n
	n -= 1
}

do {
	r = r * n
	n -= 1
} while (n > 0)

for loop is more like Python:

for (i <- 1 to n)
	r = r * i

Ranges created with the <- symbol in for loops are refereed to as generators. There are no break and continue keywords in Scala.

Note i to n returns a Range from 1 to n inclusive, and <- makes i traverse all values of the expression at its right side.

In Scala, it is possible to apply a function to all values in a sequence at once, reducing the use of loops.

Advanced for Loop Usage

Multiple counters

Multiple generators of the form variable <- expression separated by semicolon:

for (i <- 1 to 3; j <- 1 to 3) print(i + j)

It is preferred to write it in this form:

for {
    i <- 1 to 3
    j <- 1 to 3
} print(i + j)

Guard

Each generator can have a guard, a Boolean condition by if:

for (i <- 1 to 3; j <- 1 to 3 if i != j) print(i + j)

No semicolon before the if.

You can define variables to use in for without the var keyword:

for (i <- 1 to 3; from = 4 - i; j <- from to 3) print (i + j)

The content of a for loop can be expressed in braces, replacing semicolons with newlines:

for { i <- 1 to 3
	from = 4 - i
	j <- from to 3
}

for loop with counters (like Go): Scala collections offer a zipWithIndex method that create a new collection with (elem, counter) as elements.

for Comprehensions

When the body of the for loop starts with yield keyword, then the for loop constructs a collection of values. For example,

for {
	i <- 1 to 3
	square = i * i
} yield square

constructs a collection. Note variables inside for loop don’t need a var or val quantifier. If the statement after yield requires multiple lines, enclose them in {}. In most cases, the for/yield loop returns the same type of collections as input.

For comprehension is essentially the same as map method available on Iterable.

Patterns in for expressions

for ((k, v) <- map) println(k + v) traverses the maps and output keys and values bound to k and v. Match failures are silently ignored.

Guard can be applied, but it goes after the <- symbol.

How Scala Compiler works with for loops

  1. A simple for loop that iterates over a collection is translated to a foreach method call on the collection.
  2. A for loop with a yield expression is translated to a map call on the collection.
  3. The guard is translated to one or more withFilter calls on the collection prior to foreach or map call.

Thus it is more straightforward to avoid explicit for loop but use map, foreach, and filter directly.

break and continue

There are no break and continue keywords, but similar functionality can be implemented with scala.util.control.Breaks. It offers two methods, break and breakable. break will throw a BreakControl exception and breakable will catch it.

Break

breakable {
    for (x <- xs) {
        if (cond)
            break
    }
}

Continue

for (x <- xs) {
    breakable {
        if (cond)
            break
    }
}

Labeled Break

Breaks object can be instantiated, and breakable will only catch exception thown by break in the same object.

Enumeration

Unlike Java, Scala does not have enumeration. Standard library provides an Enumeration helper class.

Define enumeration as follow:

object TrafficLightColor extends Enumeration {
	val Red, Yellow, Green = Value
}

Each gets an ID and name: ID defaults to one more than the previous one, starting from zero; name is the variable name.

So the enumeration can be accessed by TrafficLightColor.Red, etc.

You can look up an enumeration value by its ID or name:

TrafficLightColor(0)
TrafficLightColor.withName("Red")

Exception

Scala exception works the same way as in Java. But Scala has no “checked” exceptions, i.e. you never have to declare a function of method might throw and exception. However, if you want, you can still do so, and it is required if the method will be called from Java code:

@throws(classOf[NumberFormatException])
def toInt(s: String) = s.toInt

A throw expression has the special type Nothing. If one branch in if/else is Nothing, the type of the if/else is the type of the other branch.

The syntax of catch exceptions is modeled after the pattern matching syntax.

finally clause is the same as in Java.