当我改变我正在迭代的列表时，如何避免代码中的列表错误功能？

Question

I am looking to use pop and remove functions to remove numbers from the list 2 to 100 in order to get a list of prime numbers. The main problem is that k always ends up causing an error. Also, when a put a print function after k , it shows up only even numbers, not sure why that is happening.

x=[] 
for i in range(2,100): 
    x.append(i)

primes=[]

count=0

while count < 99:
    k = x[count]
    print(k)
    primes.append(k)
    """for j in range(2,100):
        if k % j ==0:
            x.remove(j)"""
    x.pop(count)
    count = count + 1

print(x)

Answer 1

Errors can happen because you are removing elements from a list while iterating over it.

Consider a list [x, y, z] and you're at position 0. If you decide to remove the element at position 0 then Python will move on to check position 1 in the next iteration of the loop. But position then refers to element z (because position 1 in the list [y, z] is z , not y ).

Answer 2

Your out of range:

x len is 98, and the while loop counts for 48 times....

You can fix it easily like that (Just fixed the While condition to count < 48 ):

    x=[]
    for i in range(2,100):
        x.append(i)

    primes=[]

    print len(x)

    count=0

    while count < 48:
        k = x[count]
        print(k)
        primes.append(k)
        """for j in range(2,100):
            if k % j ==0:
                x.remove(j)"""
        x.pop(count)
        count = count + 1

    print(x)

Answer 3

You aren't testing for prime numbers at all. You're getting only even numbers cause you're removing all odd ones. From the logic point there's no need to pre-polulate the list to further remove unwanted elements.

Here's what you're looking for:

def is_prime(n):
    if n < 2 or (n % 2) == 0:
        return n == 2

    f = 3

    while (f * f) <= n:
        if (n % f) == 0:
            return False

        f += 2

    return True

primes = [n for n in range(2, 100) if is_prime(n)]
print(primes)