for_loop_and_lists.md

Theory: For loop and lists

In Kotlin, the for loop is a handy tool that allows you to iterate through an entire mutable list. Let's take a look at several ways of using it.

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Iterating through a MutableList

The simplest way to process each mutable list element is to use the following template:

for (element in mutList) {
    // body of loop
}

Suppose we have a mutable list that includes weekdays. Let's print each day of the week:

fun main() {
    val daysOfWeek = mutableListOf("Sun", "Mon", "Tues", "Wed", "Thur", "Fri", "Sat")
    
    for (day in daysOfWeek){
        println(day)
    }
}

After that, the program will print the following:

Sun
Mon
Tues
Wed
Thur
Fri
Sat

In the same way, you can process a mutable list of integers, characters, or any other data type.

• List 에서 in 을 사용해서 for loop 에서 순회할 수 있다.

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Iterating by indexes

It is possible to access elements by their index directly from the loop. To do that, you must use the mutList.indices property, which represents a range of valid mutList indexes.

Take a look at the daysOfWeek mutable list:

fun main() {
    val daysOfWeek = mutableListOf("Sun", "Mon", "Tues", "Wed", "Thur", "Fri", "Sat")

    for (index in daysOfWeek.indices){
        println("$index: ${daysOfWeek[index]}")
    }
}

The program will print the following:

0: Sun
1: Mon
2: Tues
3: Wed
4: Thur
5: Fri
6: Sat

• List.indices 를 통해서 index 를 가져와서 for loop 에서 사용하는 것도 가능하다.

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Iterating by range indexes

We have discussed two ways of iterating through a mutable list. They are useful when you need to process each list element. Sometimes, however, you may need to access a particular sublist. In such case, you can specify the range of the indexes you need.

The first element of a mutable list always has index 0.

Take a look at the program below:

fun main() {
    val daysOfWeek = mutableListOf("Sun", "Mon", "Tues", "Wed", "Thur", "Fri", "Sat")

    for (index in 1..5) {
        println("$index: ${daysOfWeek[index]}")
    }
}

It will print only the workdays:

1: Mon
2: Tues
3: Wed
4: Thur
5: Fri

To use the last index of a mutable list in ranges, you need to access mutList.lastIndex. So, we can modify the code this way:

for (index in 1 until daysOfWeek.lastIndex) {
    println("$index: ${daysOfWeek[index]}")
}

It displays the same days as before:

1: Mon
2: Tues
3: Wed
4: Thur
5: Fri

If you want to iterate through a mutable list in reverse order, use downTo in a range. You can also specify the offset between indexes using step.

The program below will print days in reverse order with a step of 2:

fun main() {
    val daysOfWeek = mutableListOf("Sun", "Mon", "Tues", "Wed", "Thur", "Fri", "Sat")

    for (index in daysOfWeek.lastIndex downTo 0 step 2) {
        println("$index: ${daysOfWeek[index]}")
    }
}

Output:

6: Sat
4: Thur
2: Tues
0: Sun

So, you can iterate through an entire mutable list or a part of it in both direct and reverse order with any step you need.

• List 의 index 요소와 range 표현식을 통해서 루프를 순회하는 것도 가능하다.
• 주로 list 를 순회할 때 lastIndex 와 step 같은 요소를 사용할 때 이 방법을 쓰지 않을까 싶다.

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