For loop and ranges

Often you need to do some action repeatedly. For example, if you want to find the sum of numbers from 1 to 100 (without formula), you need to add the numbers sequentially. To do this, you may use the for loop and ranges, which you are already familiar with. Let's take a closer look at this!

For loop

Kotlin provides the for loop to iterate through ranges, arrays, and other collections of elements. We will meet them later.

This is the syntax the for loop has:

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

The body of this loop consists of one or more statements that are executed for each element from the specified source. The loop stops after having processed the last element.

Iterating through a range

The simplest example of using the for loop is printing each element of an integer range.

for (i in 1..4) {
    println(i)    
}

This loop prints each number from 1 to 4.

1
2
3
4

It is also possible to iterate through a range of characters:

for (ch in 'a'..'c') {
    println(ch)
}

This loop prints:

a
b
c

Note that we cannot use a string range such as "da".."dd" to get the desired result "da" "db" "dc" "dd". In the range, we can only use single characters. From now on, all the examples will be about numbers because iterating through characters is always the same.

Iterating through a String

Also, you may iterate over strings. The following code prints each symbol of a String:

val str = "Hello!"
for (ch in str) {
    println(ch)    
}

This code prints:

H
e
l
l
o
!

Iterating in the backward order

You can also iterate a range in the backward order.

for (i in 4 downTo 1) {
    println(i)
}

This loop prints numbers from 4 to 1.

4
3
2
1

Excluding the upper limit

If we need to exclude the upper limit from a range, we can subtract one from it or write until instead of ..:

for (i in 1 until 4) {
    println(i)
}

This loop prints numbers from 1 to 3.

Specifying a step

If we do not specify a step, it is assumed that the step is equal to one (e.g. 1, 2, 3, ...). Although if we want to change the step, we need to specify it explicitly.

In the example below, we print only odd numbers from the range 1..7.

for (i in 1..7 step 2) {
    println(i)
}

This loop prints four numbers:

1
3
5
7

You can also use it for backward iteration.

for (i in 7 downTo 1 step 2) {
    println(i)
}

This loop prints:

7
5
3
1

An example: the factorial of a number

Let's write a program that calculates the factorial of a given integer number. It's a classic. The factorial of n is a product of integer numbers from 1 to n inclusive. Assumed that factorial of 0 is 1. The factorial of 1 is 1 as well.

fun main() {
    val n = readln().toInt()
    var result = 1 // starting value of the factorial

    for (i in 2..n) { // the product from 2 to n
        result *= i
    }

    println(result)
}

The program above reads an integer number from the standard input. We suppose that the starting value of the factorial is 1 and then sequentially multiply it by numbers from 2 to n. If the input number is 1, the result is 1. If the input number is 5, the result is 120.

An example: the multiplication table of even numbers

You can nest one for loop in the body of another for loop because loops are just regular statements. Programmers often use nested loops to process multidimensional structures like tables (matrices), data cubes, and so on.

For example, the code below prints the multiplication table of even numbers from 2 to 10.

fun main() {
    for (i in 2..10 step 2) {
        for (j in 2..10 step 2) {
            print(i * j)
            print('\t')  // print the product of i and j followed by one tab
        }
        println()
    }
}

It prints:

4   8   12  16  20  
8   16  24  32  40  
12  24  36  48  60  
16  32  48  64  80  
20  40  60  80  100  

Idiom

Almost everything in this topic is an idiom! Different types of ranges may be difficult to understand, but they offer a really convenient and easy-to-read way to write code. Here we give you a quick reminder of the syntax for iterating over basic ranges:

for (i in 1..6) { ... }        // closed range: 1, 2, 3, 4, 5, 6
for (i in 1 until 6) { ... }   // half-open range: 1, 2, 3, 4, 5
for (x in 1..6 step 2) { ... } // step 2: 1, 3, 5
for (x in 6 downTo 1) { ... }  // closed range, backward order: 6, 5, 4, 3, 2, 1 

Conclusion

The for loop is a very powerful instrument. You will use it in almost all of your programs. Remember that all these types of ranges are available to you; they can be very convenient for some special cases. Good luck with the tasks!