Python 在字符串中反向查找
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Python Reverse Find in String
提问by
I have a string and an arbitrary index into the string. I want find the first occurrence of a substring before the index.
我有一个字符串和字符串中的任意索引。我想在索引之前找到第一次出现的子字符串。
An example: I want to find the index of the 2nd I by using the index and str.rfind()
一个例子：我想通过使用索引找到第二个 I 的索引，然后 str.rfind()
s = "Hello, I am 12! I like plankton but I don't like Baseball."
index = 34 #points to the 't' in 'but'
index_of_2nd_I = s.rfind('I', index)
#returns = 36 and not 16
Now I would expect rfind() to return the index of the 2nd I (16) but it returns 36. after looking it up in the docs I found out rfind does not stand for reverse find.
现在我希望 rfind() 返回第二个 I (16) 的索引，但它返回 36。在文档中查找后我发现 rfind 不代表反向查找。
I'm totally new to Python so is there a built in solution to reverse find? Like reversing the string with some python [::-1] magic and using find, etc? Or will I have to reverse iterate char by char through the string?
我对 Python 完全陌生，所以是否有内置的反向查找解决方案？喜欢用一些 python [::-1] 魔法反转字符串并使用 find 等？或者我是否必须通过字符串一个字符一个字符地反向迭代？
采纳答案by Blair Conrad
Your call tell rfind to start lookingat index 34. You want to use the rfind overloadthat takes a string, a start and an end. Tell it to start at the beginning of the string (0) and stop looking at index:
您的调用告诉 rfind开始查看索引 34。您想使用rfind 重载，它接受一个字符串、一个开始和一个结束。告诉它从字符串 ( 0)的开头开始并停止查看index：
>>> s = "Hello, I am 12! I like plankton but I don't like Baseball."
>>> index = 34 #points to the 't' in 'but'
>>> index_of_2nd_I = s.rfind('I', 0, index)
>>>
>>> index_of_2nd_I
16
回答by Tony Veijalainen
I became curious how to implement looking n times for string from end by rpartition and did this nth rpartition loop:
我开始好奇如何通过 rpartition 实现对字符串的 n 次查找，并执行了第 n 个 rpartition 循环：
orig = s = "Hello, I am 12! I like plankton but I don't like Baseball."
found = tail = ''
nthlast = 2
lookfor = 'I'
for i in range(nthlast):
tail = found+tail
s,found,end = s.rpartition(lookfor)
if not found:
print "Only %i (less than %i) %r in \n%r" % (i, nthlast, lookfor, orig)
break
tail = end + tail
else:
print(s,found,tail)
