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Pentagonal numbers are generated by the formula, \$P_n=\frac{n(3n−1)}{2}\$. The first ten pentagonal numbers are:

1, 5, 12, 22, 35, 51, 70, 92, 117, 145

It can be seen that \$P_4 + P_7 = 22 + 70 = 92 = P_8\$. However, their difference, \$70 − 22 = 48\$, is not pentagonal.

Find the pair of pentagonal numbers, \$P_j\$ and \$P_k\$, for which their sum and difference are pentagonal and \$D = |P_k − P_j|\$ is minimised; what is the value of D?

Awaiting feedback.

from time import time


def generate_pentagons(n):
    """generates next n pentagons"""
    pentagons = (num * (3 * num - 1) // 2 for num in range(1, n))
    for i in range(n - 1):
        yield next(pentagons)


def get_pentagons(n):
    """Assumes n is a range > 0.
    generates pentagons that obey to the + - rules."""
    pentagons = set(generate_pentagons(n))
    for pentagon1 in pentagons:
        for pentagon2 in pentagons:
            if pentagon1 + pentagon2 in pentagons and abs(pentagon1 - pentagon2) in pentagons:
                return pentagon1, pentagon2


if __name__ == '__main__':
    start_time = time()
    pent1, pent2 = get_pentagons(10000)
    print(abs(pent1 - pent2))
    print(f'Time: {time() - start_time} seconds.')
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  • 1
    \$\begingroup\$ Why do you ignore the repeated advice (1, 2) to tag Python questions correctly? \$\endgroup\$ – Martin R Jul 22 '19 at 5:17
  • \$\begingroup\$ I'm sorry if I seem to be ignoring your advice but in fact, 'python-3.x' as well as 'programming-challenge' are already tagged. I don't know what's wrong here. \$\endgroup\$ – user203258 Jul 22 '19 at 5:24
  • \$\begingroup\$ check under the code block, you must see them I guess. \$\endgroup\$ – user203258 Jul 22 '19 at 5:25
  • 1
    \$\begingroup\$ From the first referenced comment: “Please tag all Python questions with [python], ...” \$\endgroup\$ – Martin R Jul 22 '19 at 5:28
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    \$\begingroup\$ You're averaging 5 questions a day. Some of the downvotes might be from people who think you should take things a lot slower, waiting for answers to one question so that you can apply the same principles to your next program. \$\endgroup\$ – Peter Taylor Jul 22 '19 at 7:18
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This is an answer focused primarily about styling

  • Your parameter names can be renamed to be more specific, like generate_pentagons(n) can be rewritten to generate_pentagons(number_of_pentagons). This change can also be implemented in the get_pentagons method. While this may seem useless in this program, since n can be easily interpreted as the number of pentagons to generate, it's a good practice to follow when you start writing large programs that require more specific parameter names.
  • Parameter constants should be UPPERCASE.
  • You don't use i in your loop in generate_pentagons method. To avoid generating an unused variable, you can use an underscore. The _ immediately signals the reader that the value is not important.

Final Code

from time import time


def generate_pentagons(number_of_pentagons):
    """generates next n pentagons"""
    pentagons = (num * (3 * num - 1) // 2 for num in range(1, number_of_pentagons))
    for _ in range(number_of_pentagons - 1):
        yield next(pentagons)


def get_pentagons(number_of_pentagons):
    """Assumes n is a range > 0. Generates pentagons that obey to the + - rules."""
    pentagons = set(generate_pentagons(number_of_pentagons))
    for pentagon1 in pentagons:
        for pentagon2 in pentagons:
            if pentagon1 + pentagon2 in pentagons and abs(pentagon1 - pentagon2) in pentagons:
                return pentagon1, pentagon2

if __name__ == '__main__':
    START_TIME = time()
    PENTAGON_1, PENTAGON_2 = get_pentagons(10000)
    print(abs(PENTAGON_1 - PENTAGON_2))
    print(f"Time: {time() - START_TIME} seconds.")
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