16
Jul
12

Finding Primes: Part II – A Python Implementation

As it is easy to get started I first wrote a prime finding algorithm in Python. I used a very basic algorithm for this. I store a list of prime numbers, and I check the numbers less than the square root of the possible prime, if any are a factor of the number I’m checking then it’s a composite otherwise I append it to the list of prime numbers.

The code for this is on my Box account here.

This way I only check for prime factors and I check the least possible numbers as if there are any factors then some will be less than the square root.

Here is the code for the _test function that contains this algorithm:

    def _test(self, possible_prime):
        """Test if any elements of primes divides possible_prime."""
        prime = True
        for i in self.primes:
            if (i > math.sqrt(possible_prime)):
                break
            if (possible_prime % i) == 0:
                prime = False
                break
        if (prime):
            self.primes.append(possible_prime)

There are two other methods in my Prime class; primes_below(self, limit) and primes_number_of(self, limit). These are methods to get either all primes below a number or a specific number of primes. Here is the code for both of these:

    def primes_below(self, limit):
        """Return a list of primes below the input argument."""
        i = self.primes[len(self.primes) - 1]
        if (i == 2):
            i += 1
        else:
            i +=2
        while i <= limit:
            self._test(i)
            i += 2  
        return self.primes

In this I get the last member of the list of primes and increment either to two or to the next odd number. This is because self.primes is initialised to [2]. Then I check only odd numbers to cut down on time and use the _test function I have described earlier.

The other function runs in much the same way except the loop runs until the length of self.primes is equal to limit.

If this file is run then it asks what mode you want (looping till you give it a satisfactory answer) then runs the method you wanted and gives you the output. Here is that code:

if __name__ == "__main__":
    type = -1
    while (type != 0 and type != 1):
        question = "Do you want number of primes [0]"
        question += ", primes below a number[1]?\n"
        type = input(question)
    limit = 1
    print("")
    if (type == 0):
        limit = input("How many primes do you want?\n")
    else:
        limit = input("Primes (inclusivly) below what number?\n")
    t_start = time.time()
    calculator = Prime()
    if (type == 0):
        primes = calculator.primes_number_of(limit)
        end_calc = time.time()
        msg = "\n"
        if (limit < 1000):
            msg += "The first " + str(limit) + " primes are:\n" + str(primes) + "\n"
        msg += "The " + str(limit) + "th primes is: " + str(primes[-1])
        print(msg)
    else:
        if (type == 1):
            primes = calculator.primes_below(limit)
        else:
            assert(False)
        end_calc = time.time()
        msg = "\n"
        if (limit < 1000):
            msg += "The primes below " + str(limit) + " are:\n" + str(primes) + "\n"
        msg += "There are " + str(len(primes)) + " primes below " + str(limit)
        msg += "\nThe largest of which is: " + str(primes[-1])
        print(msg)
    print("\nCalculation time:"),
    print(str(end_calc-t_start))

You’ll notice that when this is run it will return the calculation time. This is because I was interested in a comparison between Python and C++ for this task. The difference in speed (which I will publish in a later post) was such that for large computational tasks I will only use Python for an indication of the result or if it’s something I can just run overnight. The next part: C++ and the Sieve of Eratosthenes.

The full code for this is on my Box account here.

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