Python Pandas:获取列匹配特定值的行的索引 [英] Python Pandas: Get index of rows which column matches certain value
问题描述
给定一个带有BoolCol列的DataFrame,我们想找到DataFrame的索引,其中BoolCol的值== True
Given a DataFrame with a column "BoolCol", we want to find the indexes of the DataFrame in which the values for "BoolCol" == True
我目前有迭代的方法,它完美地运作:
I currently have the iterating way to do it, which works perfectly:
for i in range(100,3000):
if df.iloc[i]['BoolCol']== True:
print i,df.iloc[i]['BoolCol']
但这不是正确的熊猫方式。
经过一些研究,我目前正在使用此代码:
But this is not the correct panda's way to do it. After some research, I am currently using this code:
df[df['BoolCol'] == True].index.tolist()
这个给我一个索引列表,但它们不匹配,当我检查它们时:
This one gives me a list of indexes, but they dont match, when I check them by doing:
df.iloc[i]['BoolCol']
结果实际上是假的!!
The result is actually False!!
这将是正确的熊猫方式这样做?
Which would be the correct Pandas way to do this?
推荐答案
df.iloc [i]
返回 ith
行 df
。 i
未引用索引标签, i
是基于0的索引。
df.iloc[i]
returns the ith
row of df
. i
does not refer to the index label, i
is a 0-based index.
相反,属性 index
返回实际索引标签,而不是数字行索引:
In contrast, the attribute index
returns actual index labels, not numeric row-indices:
df.index[df['BoolCol'] == True].tolist()
或等价,
df.index[df['BoolCol']].tolist()
通过使用DataFrame可以清楚地看到差异
异常指数:
You can see the difference quite clearly by playing with a DataFrame with an "unusual" index:
df = pd.DataFrame({'BoolCol': [True, False, False, True, True]},
index=[10,20,30,40,50])
In [53]: df
Out[53]:
BoolCol
10 True
20 False
30 False
40 True
50 True
[5 rows x 1 columns]
In [54]: df.index[df['BoolCol']].tolist()
Out[54]: [10, 40, 50]
如果你想使用t他指数,
In [56]: idx = df.index[df['BoolCol']]
In [57]: idx
Out[57]: Int64Index([10, 40, 50], dtype='int64')
然后您可以使用 loc
而不是 iloc选择行
:
then you can select the rows using loc
instead of iloc
:
In [58]: df.loc[idx]
Out[58]:
BoolCol
10 True
40 True
50 True
[3 rows x 1 columns]
请注意 loc
也可以接受布尔数组:
Note that loc
can also accept boolean arrays:
In [55]: df.loc[df['BoolCol']]
Out[55]:
BoolCol
10 True
40 True
50 True
[3 rows x 1 columns]
如果你有一个布尔数组, mask
,并且需要序数索引值,您可以使用 np.flatnonzero
计算它们:
If you have a boolean array, mask
, and need ordinal index values, you can compute them using np.flatnonzero
:
In [110]: np.flatnonzero(df['BoolCol'])
Out[112]: array([0, 3, 4])
使用 df.iloc
按顺序索引选择行:
Use df.iloc
to select rows by ordinal index:
In [113]: df.iloc[np.flatnonzero(df['BoolCol'])]
Out[113]:
BoolCol
10 True
40 True
50 True
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