试图通过 python 中的递归来解决 Project Euler（问题 3）中的最大素因子问题

Question

I have been trying to solve Problem #3 in ProjectEuler in python. I have tried using recursion to get factors of my factors. But I keep on running into a recursive limit reached error for some reason. Can anyone help me out as to why its happening?

def getPrimeFactors(y):
  AllFactors = [[x, int(y/x)] for x in range(1, (int(math.sqrt(y)+1))) if y%x==0]
  Flattened_AF = [j for i in AllFactors for j in i]
  print(AllFactors)
  print(Flattened_AF)

  if len(Flattened_AF)==2:
    print(Flattened_AF)
    return Flattened_AF
  else:
    PrimeFactors = [x for x in Flattened_AF if len(getPrimeFactors(x))==2]
    print (f'loop in else - {PrimeFactors}')
    print(max(PrimeFactors)

getPrimeFactors(4)

This is the problem as quoted in the website.

I am sorry if the code readability quality is a bit low but I had been trying to debug for a long time in vain.

Answer 1

When you define AllFactors for in input y , you iterate from 1, so the same number y is always contained within AllFactors. As such, when you call getPrimeFactors on the input, that same input y is passed, so this becomes an infinite loop.

Iterating from 2 instead of 1 stops the recursion error.

Also, just a tip, typically in python variables names begin with a lower case and classes begin with uppercase. There is more about naming conventions here: https://visualgit.readthedocs.io/en/latest/pages/naming_convention.html .