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Structure
Conditionals

Perform an action if a condition is true (IF .. THEN)

Given a variable name, if the value is "Bob", display the string "Hello, Bob!". Perform no action if the name is not equal.
scala
val name = "Bob"
if (name.equals("Bob")) printf("Hello, %s!\n", name)
val name = "Bob"

// Scala supports operator overloading, so the following works correctly
if (name == "Bob") printf("Hello, %s!\n", name)

Perform different actions depending on a boolean condition (IF .. THEN .. ELSE)

Given a variable age, if the value is greater than 42 display "You are old", otherwise display "You are young"
scala
val age = 42
if (age > 42) println("You are old") else println("You are young")
println( "You are " + ( if ( age > 42 ) "old" else "young" ) )

Perform different actions depending on several boolean conditions (IF .. THEN .. ELSIF .. ELSE)

scala
val age = 65

if (age > 84) println("You are really ancient")
else if (age > 30) println("You are middle-aged")
else println("You are young")

Replacing a conditional with many branches with a switch/case statement

Many languages support more compact forms of branching than just if ... then ... else such as switch or case or match. Use such a form to add an appropriate placing suffix to the numbers 1..40, e.g. 1st, 2nd, 3rd, 4th, ..., 11th, 12th, ... 39th, 40th
scala
object FourToTwenties {
def unapply (n: Int) = (4 to 20).contains(n % 100)
}

def suffix (n: Int) = {
n match {
case FourToTwenties() => "th"
case n if n % 10 == 1 => "st"
case n if n % 10 == 2 => "nd"
case n if n % 10 == 3 => "rd"
case _ => "th"
}
}

for (n <- 1 to 40) {
println(n.toString + suffix(n))
}

Loops

Perform an action multiple times based on a boolean condition, checked before the first action (WHILE .. DO)

Starting with a variable x=1, Print the sequence "1,2,4,8,16,32,64,128," by doubling x and checking that x is less than 150.
scala
var x = 1 ; while (x < 150) { printf("%d,", x) ; x *= 2 }

Perform an action multiple times based on a boolean condition, checked after the first action (DO .. WHILE)

Simulate rolling a die until you get a six. Produce random numbers, printing them until a six is rolled. An example output might be "4,2,1,2,6"
scala
val dice = new GenRandInt(1, 6) ; var rnd = 0 ; var fmt = ""
do { rnd = dice.next ; fmt = if (rnd != 6) "%d," else "%d" ; printf(fmt, rnd) } while (rnd != 6)

Perform an action a fixed number of times (FOR)

Display the string "Hello" five times like "HelloHelloHelloHelloHello"
scala
// Using overloaded '*' operator (String-specific)
print("Hello" * 5)
List.range(0, 5) foreach { (_) => print("Hello") }
for (_ <- List.range(0, 5)) print("Hello")
// Lazy version
for (_ <- Stream.range(0, 5)) print("Hello")
dotimes(5, _ => print("Hello"))
(0 until 5) foreach { (_) => print("Hello") }
5 times { print("Hello") }

Perform an action a fixed number of times with a counter

Display the string "10 .. 9 .. 8 .. 7 .. 6 .. 5 .. 4 .. 3 .. 2 .. 1 .. Liftoff!"
scala
for (i <- List.range(1, 11).reverse) printf("%d .. ", i) ; println("Liftoff!")
for (i <- List.range(-10, 0)) printf("%d .. ", (-i)) ; println("Liftoff!")
var i = 10 ; while (i > 0) { printf("%d .. ", i) ; i -= 1 } ; println("Liftoff!")
for (i <- -10 to -1) printf("%d .. ", (-i)) ; println("Liftoff!")