为什么会出现"list index out of range“的错误？

Question

我已经写了一个程序，它应该打印出从0到100的所有合成数，但我一直收到错误"list index out of range“。

我能做些什么来修复它呢？

def isPrime(x):
    if x==0:
        return False
    if x==1 or x==2:
        return True
    for i in range(2, x):
        if x%i==0:
            return False
            break
        elif x%i==1:
            return True
        else:
            print("error")

i=0

num = list(range(100))

while i<100:
    if isPrime(num[i]==True):           #I get the error here
        del (num[i])
        i += 1
    else:
        i += 1

print(num)

Answer 1

因为当你在列表上循环时，你正在编辑它。在索引74处，该索引不再存在，因为列表的长度是73。请注意，您的主函数似乎没有按照您的预期工作。

from math import sqrt
from itertools import count, islice

def isPrime(n): # Taken from https://stackoverflow.com/questions/4114167/checking-if-a-number-is-a-prime-number-in-python/27946768#27946768
    if n < 2:
        return False

    for number in islice(count(2), int(sqrt(n) - 1)):
        if n % number == 0:
            return False

    return True


num = list(range(100))
to_delete = []

for i in num:
    if isPrime(i):           #I get the error here
        to_delete.append(i)

final = [n for n in num if n not in to_delete]

print(final)

正确的输出：

[0, 1, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99]

Answer 2

因为如果您的条件为真，您将删除一个元素，但仍在增加i。将您的代码更改为以下代码：

def isPrime(x):
    if x==0:
        return False
    if x==1 or x==2:
        return True
    for i in range(2, x):
        if x%i==0:
            return False
    return True

i=0
lim = 100
num = list(range(lim))

while i<lim:
    if isPrime(num[i])==True:           #I get the error here
        del (num[i])
        lim -=1
    else:
        i += 1

print(num)

顺便说一句，你的代码包含了一个错误的if isPrime(num[i]==True)，我已经修改过了。

输出：

[0, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99]

因为你的isPrime算法也不正确，所以我已经纠正了它。