Part 3 : Loops and Exceptions in Kotlin

Motivation

This article is written for more advanced developers which are looking to get a quick grasp on the Kotlin Language for work or for some interesting projects.

In this article, we cover loops and exceptions and how to best use them. This is part 3 in a longer series.

Part 1 : Getting Started with Kotlin
Part 2 : Control Flow in Kotlin
Part 3 : Loops and Exceptions in Kotlin
Part 4 : Working with Types in Kotlin ( Coming Soon )

While Loops

While loops follow a general syntax of

while (Boolean Expression is true){
   //Perform some action
}

At each iteration, Kotlin checks to see if the boolean expression is still true. If it is, then it continues iterating through the block.

Note : This means that if your condition is not true, your loop will never run

We can see an example of this below

fun main(){
    val repetitions = 10;
    var counter = 0
    while(counter < repetitions){
        println("Counter : $counter ")
        counter++
    }
}

which gives us the output of

Counter : 0 
Counter : 1 
Counter : 2 
Counter : 3 
Counter : 4 
Counter : 5 
Counter : 6 
Counter : 7 
Counter : 8 
Counter : 9 

Do-While

If you want your loop to run at least once, a do-while loop is what you'll probably want to use.

do{
  // Perform some action
} while(Boolean Expression is true)

We can rewrite the while loop above into a do-while as

fun main(){
    val repetitions = 10;
    var counter = 0
    do{
        println("Counter : $counter ")
        counter++
    }while(counter < repetitions)
}

Note : This means that if your condition is not true, your loop will run at least once

We can verify that our do while loop runs at least once using the code-snippet below

fun main(){
    val condition = false
    do{
        println("Loop Ran")
    }while(condition)
}

For-Loops

We can also use For-Loops if we have a known-number of iterations that we want to execute our blocks of code for.

The general syntax is

for(x in something){
  //Do Something
}

There are two main ways that we can work with for loops - using repeat or with ranges

Repeat

The repeat function call allows us to perform an action a set number of times.

fun main(){
    repeat(3){
        println("Loop Ran")
    }
}

Which gives the output of

Loop Ran
Loop Ran
Loop Ran

Ranges

We can see a more concrete example below using ranges with

fun main(){
    val range = 3..5
    for(i in range){
        println(i)
    }
}

Note : 3..5 is a shorthand syntax which we use to generate a range. So in this case, 3..5 is equivalent to a range with the values 3,4,5.

We can generate the same range in different ways as seen below

3..5 
3 until 5 
3..5 step 1

We can also generate ranges that go in the reverse direction (eg. 5,4,3 )

5 downTo 3 step 1

Note : Ranges can also be used for characters (Eg. "a","b","c" which can be generated by 'a'..'c' )