查找以列表形式返回整数的素因数的 prime_factors

Question

def prime_factors(n):
    i = 2
    lst = []
    while i <= n:
        if isprime(i):
            lst.append(i)
        i += 1
    return lst
def isprime(n):
    n = isdivisor(n)
    i = 2
    lst = []
    for j in n:
        while i <= j/2:
            if j % i != 0:
                return True
            else:
                return False
            i += 1
def isdivisor(n):
    i = 2
    lst = []
    while i <= n/2:
        if n % i == 0:
            lst.append(i)
        i += 1
    return lst
print prime_factors(15)

First, I tried to find divisors of integer n and checked the divisors whether they are prime or not.However the output is somehow [10, 14, 15] instead of [3,5] . Is it because I can't do the assignment n = isdivisor(n) or something else ?

Answer 1

I just wrote a pice of code to do It, without using libs. I hope It can help you.

---

my code:

def get_square_root(n):
  """Return square root of n."""
  return n ** (1.0 / 2)


def is_divisor(n, d):
  """Return True if n divides d, False otherwise."""
  return n % d == 0


def is_prime(n):
  """Return True if n is prime, False otherwise."""
  limit = int(get_square_root(n))
  for i in range(2, limit+1):
    if is_divisor(n, i):
      return False
  return True


def get_prime_factors(n):
  """Return a list of the prime factors of n."""
  prime_factors = []
  for i in range(2, n):
    if is_divisor(n, i) and is_prime(i):
      prime_factors.append(i)
  return prime_factors


if __name__ == '__main__':
  n = 15  # Change n to test 
  prime_factors = get_prime_factors(n)
  print(prime_factors)

output:

[3, 5]

Answer 2

There are some logical errors in your code. You are overcomplicating things somewhat by returning lists of divisors and primes. You already have a loop in the prime_factors() to go through all the numbers from 2 to n . So you can simplify your isdivisor() and isprime() functions:

def isdivisor(i, n):
    return n % i == 0

def isprime(n):
    i = 2
    while i < n:
        if n % i == 0:
            return False
        i += 1
    return True

Then change your main loop in prime_factors() like this:

def prime_factors(n):
    i = 2
    lst = []
    while i < n:
        if isdivisor(i, n) and isprime(i):
            lst.append(i)
        i += 1
    return lst

Output:

>>>prime_factors(15)
[3,5]