CODEWITHSUNDEEP
java
Ismail
Ismail

Reputation: 109

Find prime factors of range min to max

I am trying to find prime factors of a range starting from min number to max number. SO lets say I have min value: 7 and max value: 10. I will need to find prime factors starting from 7 to 10. I am able to find prime factors of single number like 10 or 12 but I cant seem to do it for range within numbers 7 - 10. Here is what I wrote:

 public static void primeFactors(int min, int max){
        for(int i = 2; i <= max; i++){
                while(min <= max && i % 2 == 0){
                    min = min / 2;

                }
                System.out.println("Factor: " + i);
            }
        }
    }

Upvotes: 0

Views: 177

Answers (1)

tkruse
tkruse

Reputation: 10695

If you wish to do this in a single loop, you need to keep track of all numbers of the input, e.g. by using an array:

public static void main(String[] args) {
        // range [7, 10]
        int[] inputs = IntStream.range(7, 11).toArray();
        System.out.println(factorize(inputs));
    }

    private static Set<Integer> factorize(final int[] input) {
        // no prime factors bigger than half the largest number
        final int limit = input[input.length - 1] / 2;
        final Set<Integer> factors = new HashSet<>();

        for (int f = 2; f <= limit; f++) {
            // even numbers are not primes
            if (((f % 2) == 0) && (f > 2)) continue;
            for (int i = 0; i < input.length; i++) {
                while (input[i] % f == 0) {
                    input[i] /= f;
                    factors.add(f);
                }
            }
        }
        return factors;
    }

Output:

[2, 3, 5]

However, if you have a method that computes prime factors of a number, calling that method could be simpler:

IntStream.range(7, 11).forEach(i -> factorizeSingleNumber(i));

Upvotes: 0

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