Nth Prime

Saurabh Sharma

Prime number is an age old problem. I am trying to solve it via streams.

class PrimeCalculator {


    public int nth(int nth) {
        if (nth <= 0)
            throw new IllegalArgumentException();

        // Will skip the nth - 1
        // 2 to n-1
        return IntStream.iterate(2, i -> i + 1).filter(PrimeCalculator::isPrime).skip(nth - 1).findFirst().getAsInt();
    }

    public static boolean isPrime(int x) {
        // till half of x
        return IntStream.rangeClosed(2, (int) (x/2) ).dropWhile(i -> x % i != 0).findFirst().isEmpty();
    }
}

Explanation

public int nth(int nth) {
        if (nth <= 0)
            throw new IllegalArgumentException();

        // Will skip the nth - 1
        // 2 to n-1
        return IntStream.iterate(2, i -> i + 1).filter(PrimeCalculator::isPrime).skip(nth - 1).findFirst().getAsInt();
    }

The method nth(int) allows you to return the nth prime starting at 2, 3, 5....

  • If the nth is negative or l.t.e 0 return an illegal argument exception.
  • Else, Iterate 2 onwards
  • Filter all the elements that are prime.
  • Skip all n - 1 elements.
  • Find first and return it as int.
static <T> Stream<T> iterate(T seed,
                             UnaryOperator<T> f)

Returns an infinite sequential ordered Stream produced by iterative application of a function f to an initial element seed.

Stream<T> skip(long n)

Skip is a stateful intermediate operation.

`Skip()` Returns a stream consisting of the remaining elements of this stream after discarding the first n elements of the stream. If this stream contains fewer than n elements then an empty stream will be returned.

from java docs
Optional<T> findFirst()

findFirst is a short-circuiting terminal operation

java docs

An intermediate operation is short-circuiting if, when presented with infinite input, it may produce a finite stream as a result. A terminal operation is short-circuiting if, when presented with infinite input, it may terminate in finite time.

java docs

Helpers

  • https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html
  • https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#iterate-T-java.util.function.UnaryOperator-
  • https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#skip-long-
  • https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#StreamOps

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