Printing prime number from 1 to 100

Question

This program is supposed to output the prime numbers between 1 and 100. Will anyone please explain me the flow the programme below? I am having difficulty in writing the progra

Answer 1

If you split the various parts out into their own methods with appropriate names it becomes a bit easier to understand:

for (int n = 1; n < 100; n++)
    if (isPrime(n))
        System.out.println(n);

private boolean isPrime(int n) {
    for (int f = 2; f < n; f++) {
        if (isFactor(f, n))
            return false;
    }
    return true;
}

private boolean isFactor(int factor, int number) {
    return number % factor == 0;
}

This is also an area where Java 8 streams can make things a bit clearer:

List primes = IntStream.range(1, 100)
    .filter(this::hasNoFactors)
    .collect(Collectors.toList());

private boolean hasNoFactors(int number) {
    return IntStream.range(2, number)
        .noneMatch(f -> number % f == 0);
}

Also note that this is a horribly inefficient algorithm. You don't need to check every possible factor from 2 to n, just the primes. You can also take advantage of multi-processor machines:

List primes = new ArrayList<>();
IntStream.range(2, 100)
    .filter(n -> primes.parallelStream().noneMatch(p -> n % p == 0))
    .forEach(primes::add);