为什么我收到错误“列表索引超出范围”？

Question

I have written a program that should print out all the composite numbers from 0 to 100, but I keep getting the error "list index out of range"

What could I do to fix it?

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

Because you are deleting an element if your condition is true, but still increasing i . Change your code to the following:

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)

Btw, your code contained an error if isPrime(num[i]==True) which I have changed.

Output:

[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]

As your isPrime algorithm wasn't correct, too, I have corrected it.

Answer 2

Because you're editing the list while you're looping over it. At index 74 this index does not exist anymore because the length of the list is 73. Note that your prime function does not seem to work as you intend it to.

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)

Correct output:

[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]