Kotlin reduce method explained

The Kotlin reduce method is used to apply a reducer function to a set of collections that you have in your code.

A reducer function is a function that performs a custom operation for each element in a collection with the goal to return a value.

The value returned by the reducer function then accumulates until a single final value is returned from the reduce method.

It’s a bit confusing to learn about the reducer in theory, so let’s see an example of a reducer in action.

Imagine that you have a List of Int values, and you want to sum all the values in the list.

You can use the reduce method as follows:

val numbers = listOf(1, 2, 3, 4)

val aSum = numbers.reduce { accumulator, currentEl ->
    println("accumulator: $accumulator")
    println("currentEl: $currentEl")
    accumulator + currentEl
}

println(aSum) // 10

The output of the println statement in the reduce method above looks as follows:

accumulator: 1
currentEl: 2
accumulator: 3
currentEl: 3
accumulator: 6
currentEl: 4

As you can see, the reduce method iterates over a collection of elements where it was called and pass the values of your collection into the reducer function, which is the entire code inside the curly brackets {} above.

The curly brackets are Kotlin lambda function expression. It’s simply a shorter way to pass a function without a name to the reduce method.

Here’s an equivalent to the code above without lambda function expression:

val aSum = numbers.reduce(fun(accumulator: Int, currentEl: Int): Int {
    println("accumulator: $accumulator")
    println("currentEl: $currentEl")
    return accumulator + currentEl
})

The lambda function expression is shorter and more concise than a regular anonymous function as shown above.

For the first iteration of the reducer function, the first index value in your list will be passed as the accumulator and the second index value as the currentEl (or current element)

Then, you are free to perform operations on these values, as long as you return a single value that will be passed as the accumulator value of the next iteration.

The Kotlin reduce method is available for all types of collections, so you can use it on Int, String, Char, or Boolean lists as you need.

val letters = listOf("ab", "cd")

val combinedLetters = letters.reduce { accumulator, currentEl ->
    accumulator + currentEl
}

println(combinedLetters) // abcd

Now you’ve learned how the reduce method works in Kotlin. Great work! 👍