Reduce

Reduce is an operation performed on collections to accumulate a single value out of it.

Sum is the simplest example of Reduce operation where we add up all the items in the collection to get a single value.

// Syntax : coll.reduce { acc, i -> /* return computed value */ }

The lambda passed to reduce function is invoked for each item in the collection. i is the current item & acc is the value accumulated so far.

Lambda computes the new accumulated value and returns it.

For the first iteration, acc is taken as 0. If you want explicitly specify another value for initial acc, you can use fold() function :

// Syntax : coll.fold(/* initial acc */) { acc, i -> /* return computed value */ }

Example

Suppose we have a list of digits of a number and we need to compute the number using it i.e. we need to create a single Int from List<Int>. Example : [3, 4, 2, 1] → 3421

This is a perfect usecase for reduce operation. Algorithm :

Initially, the acc will be 0. So, we use reduce() instead of fold()
On each iteration, we compute the accumulated value as acc * 10 + i
The final acc will be our required number

 fun main() {
    val digits = listOf(3, 4, 2, 1)
    
    val num = digits.reduce { acc, i ->
        acc * 10 + i
    }
    println(num) // Prints 3421
}

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Reduce

Example