Prime generator in python

Question

I am writing a prime generator, which is different from anyone in this link

generator in Python generating prime numbers

Here is my code

def sequence():
    i = 1 
    while True:
        i += 2
        yield i

def prime_generator(n):
    i = 2
   it = sequence()
    while i < n:
        it= filter(lambda x: x % i, it)
        i = next(it)
        yield i

when i run something like

for i in prime_generator(50):
    print(i)

It never dump 15, 33, sth like that for me. In a word, It gives me 2 and all odd numbers. what goes wrong here?

Answer 1

The problem is that i inside the lambda isn't "fixed"; when i changes in the outside scope, the previously created lambda functions all use the new value, and so they all do the same check: see if the current value from sequence() is divisible by the last found prime. Which they never are.

Wrapping it into another lambda and then calling it so that the value of i can be fixed works:

def prime_generator(n):
    i = 2
    it = sequence()
    while i < n:
        it = (lambda i: filter(lambda x: x % i, it))(i)
        i = next(it)
        yield i

Edit: also I don't believe your code (nor this) does yield 2, but that can be trivially fixed.