Kotlin List sumBy()
Syntax & Examples

Syntax of List.sumBy()

The syntax of List.sumBy() extension function is:

fun <T> Iterable<T>.sumBy(selector: (T) -> Int): Int

This sumBy() extension function of List returns the sum of all values produced by selector function applied to each element in the collection.

✐ Examples

1 Example

In this example,

We create a list named list containing the integers 1, 2, 3, 4, 5.
We apply a selector function { it * it } which squares each element of the list.
The sumBy function calculates the sum of all squared values.
The sum, which is an integer, is stored in sum.
Finally, we print the value of sum to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(1, 2, 3, 4, 5);
    val sum = list.sumBy { it * it };
    println(sum);
}

Output

2 Example

In this example,

We create a list named list containing the strings "apple", "banana", "cherry", "date", "elderberry".
We apply a selector function { it.length } which calculates the length of each string in the list.
The sumBy function calculates the sum of all lengths.
The sum, which is an integer, is stored in sum.
Finally, we print the value of sum to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    val list = listOf(&quot;apple&quot;, &quot;banana&quot;, &quot;cherry&quot;, &quot;date&quot;, &quot;elderberry&quot;);
    val sum = list.sumBy { it.length };
    println(sum);
}

Output

3 Example

In this example,

We define a data class Person with properties name and age.
We create a list named people containing instances of Person.
We apply a selector function { it.age } which extracts the age property of each Person object.
The sumBy function calculates the sum of all ages.
The sum, which is an integer, is stored in sum.
Finally, we print the value of sum to standard output using the println function.

Kotlin Program

fun main(args: Array<String>) {
    data class Person(val name: String, val age: Int)
    val people = listOf(
        Person(&quot;Alice&quot;, 30),
        Person(&quot;Bob&quot;, 25),
        Person(&quot;Charlie&quot;, 40)
    );
    val sum = people.sumBy { it.age };
    println(sum);
}

Output

Summary

In this Kotlin tutorial, we learned about sumBy() extension function of List: the syntax and few working examples with output and detailed explanation for each example.