如何检查 Python 列表中是否存在第 n 个元素？
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How to check if the n-th element exists in a Python list?
提问by A B
I have a list in python
我在 python 中有一个列表
x = ['a','b','c']
with 3 elements. I want to check if a 4th element exists without receiving an error message.
有 3 个元素。我想在没有收到错误消息的情况下检查第四个元素是否存在。
How would I do that?
我该怎么做？
回答by Martijn Pieters
You check for the length:
您检查长度：
len(x) >= 4
or you catch the IndexErrorexception:
或者您捕获IndexError异常：
try:
value = x[3]
except IndexError:
value = None # no 4th index
What you use depends on how often you can expectthere to be a 4th value. If it is usually there, use the exception handler (better to ask forgiveness); if you mostly do nothave a 4th value, test for the length (look before you leap).
您使用什么取决于您期望出现第四个值的频率。如果它通常在那里，请使用异常处理程序（最好请求原谅）；如果您大多没有第 4 个值，请测试长度（在跳跃之前查看）。
回答by Silas Ray
You want to check if the list is 4 or more elements long?
您想检查列表是否有 4 个或更多元素？
len(x) >= 4
You want to check if what would be the fourth element in a series is in a list?
您想检查一个系列中的第四个元素是否在列表中？
'd' in x
