reduce

Java Stream: divide into two lists by boolean predicate

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Collectors.partitioningBy: Map<Boolean, List<Employee>> partitioned = listOfEmployees.stream().collect( Collectors.partitioningBy(Employee::isActive)); The resulting map contains two lists, corresponding to whether or not the predicate was matched: List<Employee> activeEmployees = partitioned.get(true); List<Employee> formerEmployees = partitioned.get(false); There are a couple of reasons to use partitioningBy over groupingBy (as suggested by Juan Carlos Mendoza): Firstly, the parameter of groupingBy is a Function<Employee, … Read more

Count the number of true members in an array of boolean values

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Seems like your problem is solved already, but there are plenty of easier methods to do it. Excellent one: .filter(Boolean); // will keep every truthy value in an array const arr = [true, false, true, false, true]; const count = arr.filter(Boolean).length; console.log(count); Good one: const arr = [true, false, true, false, true]; const count = … Read more

How to call reduce on an empty Kotlin array?

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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 … Read more

python histogram one-liner [duplicate]

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Python 3.x does have reduce, you just have to do a from functools import reduce. It also has “dict comprehensions”, which have exactly the syntax in your example. Python 2.7 and 3.x also have a Counter class which does exactly what you want: from collections import Counter cnt = Counter(“abracadabra”) In Python 2.6 or earlier, … Read more