Control Flow | Kotlin |Tutorial-8

Control Flow 

If Expression:

In Kotlin, if is an expression, i.e. it returns a value. Therefore there is no ternary operator (condition ? then : else), because ordinary if works fine in this role.

// Traditional usage 

 var max = a   
 if (a < b) max = b  
 // With else   
 var max: Int  
 if (a > b) {  
     max = a  
 } else {  
     max = b  
 }  

// As expression 

 val max = if (a > b) a else b  
 if branches can be blocks, and the last expression is the value of a block:  
 val max = if (a > b) {  
     print("Choose a")  
     a  
 } else {  
     print("Choose b")  
     b  
 }  

If you're using if as an expression rather than a statement (for example, returning its value or assigning it to a variable), the expression is required to have an else branch.

When Expression:

when replaces the switch operator of C-like languages. In the simplest form it looks like this

 when (x) {  
     1 -> print("x == 1")  
     2 -> print("x == 2")  
     else -> { // Note the block  
     print("x is neither 1 nor 2")  
   }  
 }  

when matches its argument against all branches sequentially until some branch condition is satisfied. when can be used either as an expression or as a statement. If it is used as an expression, the value of the satisfied branch becomes the value of the overall expression. If it is used as a statement, the values of individual branches are ignored. (Just like with if, each branch can be a block, and its value is the value of the last expression in the block.)

The else branch is evaluated if none of the other branch conditions are satisfied. If when is used as an expression, the else branch is mandatory, unless the compiler can prove that all possible cases are covered with branch conditions.

If many cases should be handled in the same way, the branch conditions may be combined with a comma:

 when (x) {  
     0, 1 -> print("x == 0 or x == 1")  
     else -> print("otherwise")  
 }  

We can use arbitrary expressions (not only constants) as branch conditions

 when (x) {  
     parseInt(s) -> print("s encodes x")  
     else -> print("s does not encode x")  
 }  

We can also check a value for being in or !in a range or a collection:

 when (x) {  
     in 1..10 -> print("x is in the range")  
     in validNumbers -> print("x is valid")  
     !in 10..20 -> print("x is outside the range")  
     else -> print("none of the above")  
 }  

Another possibility is to check that a value is or !is of a particular type. Note that, due to smart casts, you can access the methods and properties of the type without any extra checks.

 fun hasPrefix(x: Any) = when(x) {  
     is String -> x.startsWith("prefix")  
     else -> false  
 }  

when can also be used as a replacement for an if-else if chain. If no argument is supplied, the branch conditions are simply boolean expressions, and a branch is executed when its condition is true:

 when {  
     x.isOdd() -> print("x is odd")  
     x.isEven() -> print("x is even")  
     else -> print("x is funny")  
 }  

For Loops:

for loop iterates through anything that provides an iterator. The syntax is as follows:

 for (item in collection) print(item)  
 The body can be a block.  
 for (item: Int in ints) {  
     // ...  
 }  

As mentioned before, for iterates through anything that provides an iterator, i.e.

has a member- or extension-function iterator(), whose return type

has a member- or extension-function next(), and

has a member- or extension-function hasNext() that returns Boolean.

All of these three functions need to be marked as operator.

A for loop over an array is compiled to an index-based loop that does not create an iterator object.

If you want to iterate through an array or a list with an index, you can do it this way:

 for (i in array.indices) {  
     print(array[i])  
 }  

Note that this "iteration through a range" is compiled down to optimal implementation with no extra objects created.

Alternatively, you can use the withIndex library function:

While Loops:

while and do..while work as usual

 while (x > 0) {  
     x--  
 }  
 do {  
     val y = retrieveData()  
 } while (y != null) // y is visible here!  

Break and continue in loops

Kotlin supports traditional break and continue operators in loops.