Listing Prime Factors in Python

Finding Prime Factors in Python

Finding prime factors is all about selecting those factors of a number that are prime.
The prime factors of 36 as an example, are:

2 X 2 X 3 X 3.

Prime Factor code

We'll go about the code in a simple way:

Step 1:

Starting with 2, find the first factor of the number of interest - this factor will definitely be a prime.
Store this factor away.

Step 2:

Divide number in question by found factor.

Step 3:

Using result from Step 2 as the new number in question (number whose prime factors we are trying to find), repeat Step 1.

Step 4:

Continue recursively until we arrive at 1.

Step 5:

We'll use the square-root of number range.

Create a new module file; File, New File.
Call it listPrimeFactors.py.

Type out the adjoining Python code for finding prime factors.

All those steps in a few lines of code;
Now functions come in handy, don't they?

Note: You can comment out the Python code for the Arithmetic class from the previous lesson if you have been following.

Python code for listPrimeFactors Module File.

from math import ceil, sqrt

# A class
class PrimeFactors:
    def __init__(self, val):
        self.find_my_factors = val
        self.found_prime_factors = []

    # takes one argument, the candidate for factorisation
    # returns an array reference of the prime factors
    def onlyPrimeFactors(self):
        self.temp_limit = ceil(sqrt(self.find_my_factors))

        # STEP 1:
        self.i = 2
        while self.i <= self.temp_limit:
            # STEP 4:
            # avoid an infinite loop with the i != 1 check.
            if self.i != 1 and self.find_my_factors % self.i == 0:
                self.found_prime_factors.append(self.i)
                # STEP 2:
                self.find_my_factors = int(self.find_my_factors / self.i)
                # STEP 3:
                return self.onlyPrimeFactors()

            self.i += 1

        self.found_prime_factors.append(self.find_my_factors)
        return self.found_prime_factors

Main class code.

#!/usr/bin/python
from ListPrimeFactors import PrimeFactors

# Use the list prime factors module/class
test_guy = 48
prime_factors = PrimeFactors(test_guy)
print("Prime factorising", test_guy, "gives:\n", str(prime_factors.onlyPrimeFactors()))

print("\n\n")

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