计算Python中的素数总和-此代码行起什么作用？

Question

I have a short function that will calculate the sum of all prime numbers up to a limit. Here is the full code for context:

def primes_sum(limit):
    limitn = limit+1
    not_prime = [False] * limitn
    primes = []

    for i in range(2, limitn):
        if not_prime[i]:
            continue
        for f in xrange(i*2, limitn, i):
            not_prime[f] = True

        primes.append(i)

    return sum(primes)

Most of this makes sense to me, but I don't understand this part:

not_prime = [False] * limitn

What exactly is the purpose of this? How does it work and how does it find out whether or not a number is prime? I've tried looking it up, but I can't find the answer.

Thanks in advance.

Answer 1

You are simply creating a list of length limitn with all it's element initialized to False .

* is a repetition operator. When applied on a List with an integer value n as the right operand, it repeats the list n times.

You can use it over strings also:

>>> [False] * 2
[False, False]
>>> "rohit" * 2
rohitrohit

Answer 2

It initializes a list of limitn False values. eg:

>>> [False]*3
[False, False, False]

Basically it prevents you from getting an IndexError later on.

Answer 3

[False]*n initializes list with n elements which are all False. That way of initialization is often used as a way of optimization when you know how much elements will list contain in advance - it's much faster than adding elements with .append().