003_prime_finder.py

# Let's find all prime numbers between 2 and n
# prime number ►
# n.	A positive integer that is greater than 1
# and is not divisible without a remainder by any positive integer other than itself and 1.

n = 600
number_range = set(range(2, n + 1))
# {2, 3, 4, 5,...,n}
# print(f"Original number range: \n{number_range}")

primes_list = []

while number_range:
    # The pop() method removes an arbitrary element from the set and returns the element removed.

    # The pop() method will usually extract the lowest element of a set.
    # Sets however are, by definition, unordered.
    # The items are stored internally with some order, but this internal order is determined
    # by the hash code of the key (which is what allows retrieval to be so fast).
    # This hashing method means that we can't 100% rely on it successfully getting the lowest value.
    # In very rare cases, the hash provides a value that is not the lowest.

    # for this small example, we'll leave this as is

    # we'll start by initializing prime with the value of 2
    prime = number_range.pop()

    primes_list.append(prime)

    # will create multiples based on the prime we found
    # will use step -- we want to increment in steps of our prime number
    # range([start], stop[, step])
    multiples = set(range(prime * 2, n + 1, prime))
    # print(multiples)

    # The difference_update() method removes the items that exist in both sets.
    # removes the unwanted items from the **original set**.
    number_range.difference_update(multiples)
    # print(number_range)

# print(primes_list)
prime_count = len(primes_list)
largest_prime = max(primes_list)
print(f"There are {prime_count} prime numbers between 2 and {n}, the largest of which is {largest_prime}.")