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Simple reduce on an empty array will throw:
Exception in thread "main" java.lang.UnsupportedOperationException: Empty iterable can't be reduced.
The same exception when chaining:
val a = intArrayOf() val b = a.reduce({ memo, next -> memo + next }) // -> throws an exception val a1 = intArrayOf(1, 2, 3) val b1 = a.filter({ a -> a < 0 }).reduce({ a, b -> a + b }) // -> throws an exception Is it the expected operation of the reduce or is it a bug?
Are there any workarounds?
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Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element. Throws an exception if this array is empty. If the array can be empty in an expected way, please use reduceOrNull instead.
Kotlin – Array isEmpty() To check if an Array is empty in Kotlin language, use Array. isEmpty() method. Call isEmpty() method on this array, and the method returns True if the array is empty, or False if the array is not empty.
Fold and reduce The difference between the two functions is that fold() takes an initial value and uses it as the accumulated value on the first step, whereas the first step of reduce() uses the first and the second elements as operation arguments on the first step.
fold takes an initial value, and the first invocation of the lambda you pass to it will receive that initial value and the first element of the collection as parameters. For example, take the following code that calculates the sum of a list of integers: listOf(1, 2, 3).fold(0) { sum, element -> sum + element }
The exception is correct, reduce does not work on an empty iterable or array. What you're probably looking for is fold, which takes a starting value and an operation which is applied successively for each element of the iterable. reduce takes the first element as a starting value, so it needs no additional value to be passed as an argument, but requires the collection to be not empty.
Example usage of fold:
println(intArrayOf().fold(0) { a, b -> a + b }) // prints "0" If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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