问题
If I have a dataframe that has columns that include the same name, is there a way to combine the columns that have the same name with some sort of function (i.e. sum)?
For instance with:
In [186]:
df["NY-WEB01"].head()
Out[186]:
NY-WEB01 NY-WEB01
DateTime
2012-10-18 16:00:00 5.6 2.8
2012-10-18 17:00:00 18.6 12.0
2012-10-18 18:00:00 18.4 12.0
2012-10-18 19:00:00 18.2 12.0
2012-10-18 20:00:00 19.2 12.0
How might I collapse the NY-WEB01 columns (there are a bunch of duplicate columns, not just NY-WEB01) by summing each row where the column name is the same?
回答1:
I believe this does what you are after:
df.groupby(lambda x:x, axis=1).sum()
Alternatively, between 3% and 15% faster depending on the length of the df:
df.groupby(df.columns, axis=1).sum()
EDIT: To extend this beyond sums, use .agg() (short for .aggregate()):
df.groupby(df.columns, axis=1).agg(numpy.max)
回答2:
v0.20+ Answer: GroupBy with level and axis argument
You don't need a lambda here, nor do you explicitly have to query df.columns; groupby accepts a level argument you can specify in conjunction with the axis argument. This is cleaner, IMO.
# Setup
np.random.seed(0)
df = pd.DataFrame(np.random.choice(50, (5, 5)), columns=list('AABBB'))
df
A A B B B
0 44 47 0 3 3
1 39 9 19 21 36
2 23 6 24 24 12
3 1 38 39 23 46
4 24 17 37 25 13
df.groupby(level=0, axis=1).sum()
A B
0 91 6
1 48 76
2 29 60
3 39 108
4 41 75
Handling MultiIndex columns
Another case to consider is when dealing with MultiIndex columns. Consider
df.columns = pd.MultiIndex.from_arrays([['one']*3 + ['two']*2, df.columns])
df
one two
A A B B B
0 44 47 0 3 3
1 39 9 19 21 36
2 23 6 24 24 12
3 1 38 39 23 46
4 24 17 37 25 13
To perform aggregation across the upper levels, use
df.groupby(level=1, axis=1).sum()
A B
0 91 6
1 48 76
2 29 60
3 39 108
4 41 75
or, if aggregating per upper level only, use
df.groupby(level=[0, 1], axis=1).sum()
one two
A B B
0 91 0 6
1 48 19 57
2 29 24 36
3 39 39 69
4 41 37 38
Alternate Interpretation: Dropping Duplicate Columns
If you came here looking to find out how to simply drop duplicate columns (without performing any aggregation), use Index.duplicated:
df.loc[:,~df.columns.duplicated()]
A B
0 44 0
1 39 19
2 23 24
3 1 39
4 24 37
Or, to keep the last ones, specify keep='last' (default is 'first'),
df.loc[:,~df.columns.duplicated(keep='last')]
A B
0 47 3
1 9 36
2 6 12
3 38 46
4 17 13
The groupby alternatives for the two solutions above are df.groupby(level=0, axis=1).first(), and ... .last(), respectively.
回答3:
Here is possible simplier solution for common aggregation functions like sum, mean, median, max, min, std - only use parameters axis=1 for working with columns and level:
#coldspeed samples
np.random.seed(0)
df = pd.DataFrame(np.random.choice(50, (5, 5)), columns=list('AABBB'))
print (df)
print (df.sum(axis=1, level=0))
A B
0 91 6
1 48 76
2 29 60
3 39 108
4 41 75
df.columns = pd.MultiIndex.from_arrays([['one']*3 + ['two']*2, df.columns])
print (df.sum(axis=1, level=1))
A B
0 91 6
1 48 76
2 29 60
3 39 108
4 41 75
print (df.sum(axis=1, level=[0,1]))
one two
A B B
0 91 0 6
1 48 19 57
2 29 24 36
3 39 39 69
4 41 37 38
Similar it working for index, then use axis=0 instead axis=1:
np.random.seed(0)
df = pd.DataFrame(np.random.choice(50, (5, 5)), columns=list('ABCDE'), index=list('aabbc'))
print (df)
A B C D E
a 44 47 0 3 3
a 39 9 19 21 36
b 23 6 24 24 12
b 1 38 39 23 46
c 24 17 37 25 13
print (df.min(axis=0, level=0))
A B C D E
a 39 9 0 3 3
b 1 6 24 23 12
c 24 17 37 25 13
df.index = pd.MultiIndex.from_arrays([['bar']*3 + ['foo']*2, df.index])
print (df.mean(axis=0, level=1))
A B C D E
a 41.5 28.0 9.5 12.0 19.5
b 12.0 22.0 31.5 23.5 29.0
c 24.0 17.0 37.0 25.0 13.0
print (df.max(axis=0, level=[0,1]))
A B C D E
bar a 44 47 19 21 36
b 23 6 24 24 12
foo b 1 38 39 23 46
c 24 17 37 25 13
If need use another functions like first, last, size, count is necessary use coldspeed answer
来源：https://stackoverflow.com/questions/13078751/combine-duplicated-columns-within-a-dataframe