The First N Primes

October 14, 2011

The Sieve of Eratosthenes computes all primes less than some given n. But sometimes you need the opposite, the first n primes, not knowing the largest prime in the set. One method is to calculate the nth prime, using a prime-counting formula, then use the Sieve of Eratosthenes to generate the list. Or instead of calculating the nth prime directly, you can over-estimate the nth prime using the formula Pn < n log (n log n), then trim the list as needed; that’s the natural logarithm to the base e.

Another method is described by Melissa O’Neill in her article The Genuine Sieve of Eratosthenes. Her method is based on sifting each prime, and makes exactly the same sequence of calculations as the regular Sieve of Eratosthenes, but instead of marking bits in a bitarray it uses a priority queue to keep track of composites, one item in the priority queue for each prime; thus, instead of marking all the composite multiples of each sifting prime all at once, in the bitarray, O’Neill’s method keeps a priority queue of the next item in each sifting sequence, inserts a new sifting sequence in the priority queue as each new prime is encountered, and resets the next item in each sifting sequence as it is reached.

Thus, ignoring 2 and the even numbers, O’Neill’s method begins with i = 3 and an empty priority queue. Since 3 is not in the priority queue, it is prime, so it is output, the pair i×i/i×2 = 9/6 is added to the queue, and i is advanced to i + 2 = 5. Now since 5 is not in the queue, it is prime, so it is output, the pair 25/10 is added to the queue, and i is advanced to 7. Now since 7 is not in the queue, it is prime, so it is output, the pair 49/14 is added to the queue, and i is advanced to 9; at this point the queue is [9/6 25/10 49/7]. Now i = 9 is in the queue, so it is composite, and the pair 9/6 is removed from the priority queue and replaced by the pair 15/6, where 15 is calculated as the current composite 9 plus the skip value 6. And so on, until the desired number of primes has been generated. Note that in some cases a composite will be a member of multiple sifting sequences, and each must be replaced; for instance, when i reaches 45, it is a member of the sifting sequences for both 3 and 5, so the priority queue will have pairs 45/6 and 45/10, which should be replaced by 51/6 and 55/10, respectively.

Your task is to implement O’Neill’s algorithm to compute the first n primes. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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