calculating factorial using Java 8 IntStream?

Question

I am relatively new in Java 8 and lambda expressions as well as Stream , i can calculate factorial using for loop or recursion. But is there a way to use IntStream to calculate factorial of a number ? I am fine even with factorial in integer range.

I read through IntStream docs here, http://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html and i can see so many methods but not sure which one I can use to calculate factorial.

for example, there is rang method that says,

range(int startInclusive, int endExclusive) Returns a sequential ordered IntStream from startInclusive (inclusive) to endExclusive (exclusive) by an incremental step of 1.

so I can use it to provide the range of numbers to IntStream to be multiplied to calculate factorial.

number = 5;
IntStream.range(1, number)

but how to multiply those numbers to get the factorial ?

Answer 1

You can use IntStream::reduce for this job,

int number = 5;
IntStream.rangeClosed(2, number).reduce(1, (x, y) -> x * y)

Answer 2

To get a stream of all infinite factorials, you can do:

class Pair{
   final int num;
   final int value;

    Pair(int num, int value) {
        this.num = num;
        this.value = value;
    }

}

Stream<Pair> allFactorials = Stream.iterate(new Pair(1,1), 
                                   x -> new Pair(x.num+1, x.value * (x.num+1)));

allFactorials is a stream of factorials of number starting from 1 to ..... To get factorials of 1 to 10:

allFactorials.limit(10).forEach(x -> System.out.print(x.value+", "));

It prints: 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800,

Now say you only wish to have a factorial of a particular number then do:

allFactorials.limit(number).reduce((previous, current) -> current).get()

The best part is that you dont recompute again for new numbers but build on history.

Answer 3

With LongStream.range() you can calculate factorial for number less 20. If you need calculate for larger number create stream with BigInteger:

 public BigInteger factorial(int number) {
    if (number < 20) {
        return BigInteger.valueOf(
                LongStream.range(1, number + 1).reduce((previous, current) -> previous * current).getAsLong()
        );
    } else {
        BigInteger result = factorial(19);
        return result.multiply(Stream.iterate(BigInteger.valueOf(20), i -> i.add(BigInteger.ONE)).limit(number - 19)
                .reduce((previous, current) -> previous.multiply(current)).get()
        );
    }
}

Answer 4

We can solve the question with AtomicInteger like this:

  int num = 5;
  AtomicInteger sum = new AtomicInteger(1);
  IntStream.rangeClosed(2, num).forEach(i -> {
    sum.updateAndGet(v -> v * i);
        if (i == num) {
          System.out.println(sum.get());
        }
  });

Answer 5

I think we can change the main condition to: 1,v v-1,s t with the function reduce. Maybe we can filter before we did the main rule