[已解决]想问问大佬们一个错误提示的问题

四四四四四秒

第20讲的动动手第二题：
请用已学过的知识编写程序，找出小甲鱼藏在下边这个长字符串中的密码，密码的埋藏点符合以下规律：
    a) 每位密码为单个小写字母
    b) 每位密码的左右两边均有且只有三个大写字母

我为它编写了这样一个代码：

a = """ACFlCTLIQlAIVMTqHFkswqbDDHtpgcWaXSSglUYKE
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qKWGc"""
s = 0
def fun1():
    global a
    global s
    str1 = ""
    for i in a:
        if a[s].islower() and a[s-1].isupper() and a[s-2].isupper() and a[s-3].isupper() and a[s+1].isupper() and a[s+2].isupper() and a[s+3].isupper():
            str1 = str1 + i
        s += 1
    print(str1)
fun1()

结果运行之后出现了IndexError: string index out of range（索引器错误：字符串索引超出范围）这样的提示。我写出的代码无法完成题目的要求，但我不明白为什么会报错，理论上来说，它应该是可以运行的，只是会得出错误的结果。
我试着删除了末尾的一个字母，或者为它添加了一个新的字母后，却又能运行了。想问问各位大佬，为什么会出现这样的错误提示？

最佳答案

月排行榜 / 总排行榜

疾风怪盗

2020-9-12 11:20:28

if a[s].islower() and a[s-1].isupper() and a[s-2].isupper() and a[s-3].isupper() and a[s+1].isupper() and a[s+2].isupper() and a[s+3].isupper():

a的长度位8307位

当最后一位8306位读取时，a[s]=c,符合这句判断的第一个条件，然后就往后，符合第二个、第三个、第四个条件，但是到了第五个，a[s+1]没有这个值，所以就报错了

当删了最后一位时，最后一位8305位读取，a[s]=W，不符合判断的第一个条件，就直接退出判断了，后面的不用考虑，所以没有报错
当加了一位c后，最后一位8307位读取，a[s]=c，符合判断的第一个条件，然后就往后，符合第二个、第三个、第四个条件，但是到了第五个，但是a[s+1]=c，不符合条件就直接退出判断了，后面的不用考虑，即时没有a[s+2]也没有报错
同样的，如果补最后一位为W，也会报错，需要补三位WWW才不会报错

跳转到最佳答案楼层

heidern0612

报错都告诉你了，索引超过范围。也就是你的a【s】这一部分出问题了。

仔细看下你的for循环，不管条件如何，s都+=1.

也就是说，假定你的字符串长度为100，当你i循环到最后的时候,也就是i为99的时候。

你的if条件里a[s+3]是需要a至少有102个字符的，显然超出了a字符串100的长度，所以报错了。

疾风怪盗

if a[s].islower() and a[s-1].isupper() and a[s-2].isupper() and a[s-3].isupper() and a[s+1].isupper() and a[s+2].isupper() and a[s+3].isupper():

a的长度位8307位

当最后一位8306位读取时，a[s]=c,符合这句判断的第一个条件，然后就往后，符合第二个、第三个、第四个条件，但是到了第五个，a[s+1]没有这个值，所以就报错了

当删了最后一位时，最后一位8305位读取，a[s]=W，不符合判断的第一个条件，就直接退出判断了，后面的不用考虑，所以没有报错
当加了一位c后，最后一位8307位读取，a[s]=c，符合判断的第一个条件，然后就往后，符合第二个、第三个、第四个条件，但是到了第五个，但是a[s+1]=c，不符合条件就直接退出判断了，后面的不用考虑，即时没有a[s+2]也没有报错
同样的，如果补最后一位为W，也会报错，需要补三位WWW才不会报错

疾风怪盗

def fun1():
    str1 = ""
    for s in range(len(a)-3):
        print(s)
        print(a[s])
        if a[s].islower() and a[s-1].isupper() and a[s-2].isupper() and a[s-3].isupper() and a[s+1].isupper() and a[s+2].isupper() and a[s+3].isupper():
            str1 = str1 + a[s]
    print(str1)
fun1()

最后三位肯定不会符合条件的，干脆就不要循环到了

四四四四四秒

heidern0612 发表于 2020-9-12 08:22
报错都告诉你了，索引超过范围。也就是你的a【s】这一部分出问题了。

仔细看下你的for循环，不管条件如 ...

谢谢解答！

四四四四四秒

疾风怪盗 发表于 2020-9-12 11:20

感谢解答！

四四四四四秒

疾风怪盗 发表于 2020-9-12 11:25
最后三位肯定不会符合条件的，干脆就不要循环到了

已经明白了，十分感谢！！！