pandas ValueError：DataFrame 的真值不明确
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ValueError: The truth value of a DataFrame is ambiguous
提问by Ken Wallace
I have a dataframe that looks like this:
我有一个看起来像这样的数据框：
total downloaded avg_rating
id
1 2 2 5.0
2 12 12 4.5
3 1 1 5.0
4 1 1 4.0
5 0 0 0.0
I'm trying to add a new column with the percent difference of two of these columns, but only for columns that do not have a 0 in 'downloaded'.
我正在尝试添加一个新列，其中两个列的百分比差异，但仅适用于“已下载”中没有 0 的列。
I'm trying to use a function for this that looks like:
我正在尝试为此使用一个函数，如下所示：
def diff(ratings):
if ratings[ratings.downloaded > 0]:
val = (ratings['total'] - ratings['downloaded']) / ratings['downloaded']
else:
val = 0
return val
ratings['Pct Diff'] = diff(ratings)
I'm getting an error:
我收到一个错误：
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-129-729c09bf14e8> in <module>()
6 return val
7
----> 8 ratings['Pct Diff'] = diff(ratings)
<ipython-input-129-729c09bf14e8> in diff(ratings)
1 def diff(ratings):
----> 2 if ratings[ratings.downloaded > 0]:
3 val = (ratings['total'] - ratings['downloaded']) /
ratings['downloaded']
4 else:
5 val = 0
~\Anaconda3\lib\site-packages\pandas\core\generic.py in __nonzero__(self)
953 raise ValueError("The truth value of a {0} is ambiguous. "
954 "Use a.empty, a.bool(), a.item(), a.any() or
a.all()."
--> 955 .format(self.__class__.__name__))
956
957 __bool__ = __nonzero__
ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().
Can someone please help me understand what this error means?
有人可以帮我理解这个错误是什么意思吗？
Also, would this be a good application for an apply function? Can I use conditions in an apply? How would I use it in this case?
另外，这是否是应用功能的好应用？我可以在申请中使用条件吗？在这种情况下我将如何使用它？
采纳答案by jpp
The reason for your error is you are attempting to do a row-wise (vectorised calculation), but in fact in your function diff()ratings[ratings.downloaded > 0]returns a subset of the dataframe and preceding it by ifis ambiguous. The error message reflect this.
出现错误的原因是您正在尝试按行进行（向量化计算），但实际上在您的函数中diff()ratings[ratings.downloaded > 0]返回数据帧的一个子集，并且在其前面if是不明确的。错误消息反映了这一点。
You may wish to review Indexing and Selecting Data. The below solution sets the default value 0 by setting it at the beginning.
您可能希望查看索引和选择数据。下面的解决方案通过在开始时设置它来设置默认值 0。
import pandas as pd
df = pd.DataFrame([[2, 2, 5.0], [12, 12, 4.5], [10, 5, 3.0],
[20, 2, 3.5], [3, 0, 0.0], [0, 0, 0.0]],
columns=['total', 'downloaded', 'avg_rating'])
df['Pct Diff'] = 0
df.loc[df['downloaded'] > 0, 'Pct Diff'] = (df['total'] - df['downloaded']) / df['total']
# total downloaded avg_rating Pct Diff
# 0 2 2 5.0 0.0
# 1 12 12 4.5 0.0
# 2 10 5 3.0 0.5
# 3 20 2 3.5 0.9
# 4 3 0 0.0 0.0
# 5 0 0 0.0 0.0
