pandas 列表理解 [英] list comprehension in pandas
问题描述
我正在举一个玩具示例,但它将帮助我了解我正在尝试做的其他事情的情况.假设我想在数据框optimal_fruit"中添加一个新列,即苹果 * 橙子 - 香蕉.
I'm giving a toy example but it will help me understand what's going on for something else I'm trying to do. Let's say I want a new column in a dataframe 'optimal_fruit' that is apples * orange - bananas.
我可以做这样的事情来得到它.
I can do something like this to get it.
df2['optimal_fruit'] = df2['apples'] * df2['oranges'] - df2['bananas']
apples oranges bananas optimal_fruit
1 6 11 -5
2 7 12 2
3 8 13 11
4 9 14 22
5 10 15 35
如果我尝试做这样的事情怎么办?而我该如何在列表理解中做到这一点?
What is happening if I try to do something like this? And how could I do this in a list comprehension?
df2['optimal_fruit'] = [x * y - z for x in df2['apples'] for y in df2['oranges'] for z in df2['bananas']]
我收到以下错误:
ValueError:值的长度与索引的长度不匹配
ValueError: Length of values does not match length of index
一如既往,非常感谢您的帮助!
As always, thank you all so much for your help!
推荐答案
基本上,您的列表理解语句是3个嵌套循环的集合.在代码中:
Essentially your list comprehension statement is a set of 3 nested loops. In code:
l = []
for x in df2['apples']:
for y in df2['oranges']:
for z in df2['bananas']:
l.extend([x * y - z])
结果列表的长度将是DataFrame长度的3倍.因此,错误.要解决此问题,您需要执行以下操作:
The length of your resultant list will be 3 times the length of your DataFrame. Hence the error. To fix, you need the equivalent of:
for x, y, z in zip(df2['apples'], df2['oranges'], df2['bananas']):
l.extend([x * y - z])
关于列表理解:
[x * y - z for x, y, z in zip(df2['apples'], df2['oranges'], df2['bananas'])]
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