I want to merge df1 and df2, both have different lengths. The intersection on key column needs to be such that the output table has the values from df2 for every corresponding key as the values within the key column are repeating.
df1
key
value
1
5
1
5
2
9
3
11
4
14
4
14
df2
key
value
1
a
2
b
3
c
4
d
output
key
value
value
1
5
a
1
5
a
2
9
b
3
11
c
4
14
d
4
14
d
I'm trying
output = pd.merge(df1, df2, left_on = 'key', right_on = 'key')
But it's creating extra rows.
Thanks for your help in advance.
Try:
pd.merge(df1,df2,on='KEY', how='left').rename(columns={'VALUE_x':'VALUE','VALUE_y':'VALUE'})
Prints:
KEY VALUE VALUE
0 1 5 a
1 1 5 a
2 2 9 b
3 3 11 c
4 4 14 d
5 4 14 d
Related
I have a dataframe df1 looks like this:
id A B
0 1 10 5
1 1 11 6
2 2 10 7
3 2 11 8
And another dataframe df2:
id A
0 1 3
1 2 4
Now I want to replace A column in df1 with the value of A in df2 based on id, so the result should look like this:
id A B
0 1 3 5
1 1 3 6
2 2 4 7
3 2 4 8
There's a way that I can drop column A in df1 first and merge df2 to df1 on id like df1 = df1.drop(['A'], axis=1).merge(df2, how='left', on='id'), but if there're like 10 columns in df2, it will be pretty hard. Is there a more elegant way to do so?
here is one way to do it, by making use of pd.update. However, it requires to set the index on the id, so it can match the two df
df.set_index('id', inplace=True)
df2.set_index('id', inplace=True)
df.update(df2)
df['A'] = df['A'].astype(int) # value by default was of type float
df.reset_index()
id A B
0 1 3 5
1 1 3 6
2 2 4 7
3 2 4 8
Merge just the id column from df to df2, and then combine_first it to the original DataFrame:
df = df[['id']].merge(df2).combine_first(df)
print(df)
Output:
A B id
0 3 5 1
1 3 6 1
2 4 7 2
3 4 8 2
Having two data frames:
df1 = pd.DataFrame({'a':[1,2,3],'b':[4,5,6]})
a b
0 1 4
1 2 5
2 3 6
df2 = pd.DataFrame({'c':[7],'d':[8]})
c d
0 7 8
The goal is to add all df2 column values to df1, repeated and create the following result. It is assumed that both data frames do not share any column names.
a b c d
0 1 4 7 8
1 2 5 7 8
2 3 6 7 8
If there are strings columns names is possible use DataFrame.assign with unpack Series created by selecing first row of df2:
df = df1.assign(**df2.iloc[0])
print (df)
a b c d
0 1 4 7 8
1 2 5 7 8
2 3 6 7 8
Another idea is repeat values by df1.index with DataFrame.reindex and use DataFrame.join (here first index value of df2 is same like first index value of df1.index):
df = df1.join(df2.reindex(df1.index, method='ffill'))
print (df)
a b c d
0 1 4 7 8
1 2 5 7 8
2 3 6 7 8
If no missing values in original df is possible use forward filling missing values in last step, but also are types changed to floats, thanks #Dishin H Goyan:
df = df1.join(df2).ffill()
print (df)
a b c d
0 1 4 7.0 8.0
1 2 5 7.0 8.0
2 3 6 7.0 8.0
I am trying to select a subset of a DataFrame based on the columns of another DataFrame.
The DataFrames look like this:
a b c d
0 0 1 2 3
1 4 5 6 7
2 8 9 10 11
3 12 13 14 15
a b
0 0 1
1 2 3
2 4 5
3 6 7
4 8 9
I want to get all rows of the first Dataframe for the columns which are included in both DataFrames. My result should look like this:
a b
0 0 1
1 4 5
2 8 9
3 12 13
You can use pd.Index.intersection or its syntactic sugar &:
intersection_cols = df1.columns & df2.columns
res = df1[intersection_cols]
import pandas as pd
data1=[[0,1,2,3,],[4,5,6,7],[8,9,10,11],[12,13,14,15]]
data2=[[0,1],[2,3],[4,5],[6,7],[8,9]]
df1 = pd.DataFrame(data=data1,columns=['a','b','c','d'])
df2 = pd.DataFrame(data=data2,columns=['a','b'])
df1[(df1.columns) & (df2.columns)]
I need to remove all rows from a pandas.DataFrame, which satisfy an unusual condition.
In case there is an exactly the same row, except for it has Nan value in column "C", I want to remove this row.
Given a table:
A B C D
1 2 NaN 3
1 2 50 3
10 20 NaN 30
5 6 7 8
I need to remove the first row, since it has Nan in column C, but there is absolutely same row (second) with real value in column C.
However, 3rd row must stay, because there're no rows with same A, B and D values as it has.
How do you perform this using pandas? Thank you!
You can achieve in using drop_duplicates.
Initial DataFrame:
df=pd.DataFrame(columns=['a','b','c','d'], data=[[1,2,None,3],[1,2,50,3],[10,20,None,30],[5,6,7,8]])
df
a b c d
0 1 2 NaN 3
1 1 2 50 3
2 10 20 NaN 30
3 5 6 7 8
Then you can sort DataFrame by column C. This will drop NaNs to the bottom of column:
df = df.sort_values(['c'])
df
a b c d
3 5 6 7 8
1 1 2 50 3
0 1 2 NaN 3
2 10 20 NaN 30
And then remove duplicates selecting taken into account columns ignoring C and keeping first catched row:
df1 = df.drop_duplicates(['a','b','d'], keep='first')
a b c d
3 5 6 7 8
1 1 2 50 3
2 10 20 NaN 30
But it will be valid only if NaNs are in column C.
You can try fillna along with drop_duplicates
df.bfill().ffill().drop_duplicates(subset=['A', 'B', 'D'], keep = 'last')
This will handle the scenario such as A, B and D values are same but C has non-NaN values in both the rows.
You get
A B C D
1 1 2 50 3
2 10 20 Nan 30
3 5 6 7 8
This feels right to me
notdups = ~df.duplicated(df.columns.difference(['C']), keep=False)
notnans = df.C.notnull()
df[notdups | notnans]
A B C D
1 1 2 50.0 3
2 10 20 NaN 30
3 5 6 7.0 8
Let's say I create a DataFrame:
import pandas as pd
df = pd.DataFrame({"a": [1,2,3,13,15], "b": [4,5,6,6,6], "c": ["wish", "you","were", "here", "here"]})
Like so:
a b c
0 1 4 wish
1 2 5 you
2 3 6 were
3 13 6 here
4 15 6 here
... and then group and aggregate by a couple columns ...
gb = df.groupby(['b','c']).agg({"a": lambda x: x.nunique()})
Yielding the following result:
a
b c
4 wish 1
5 you 1
6 here 2
were 1
Is it possible to merge df with the newly aggregated table gb such that I create a new column in df, containing the corresponding values from gb? Like this:
a b c nc
0 1 4 wish 1
1 2 5 you 1
2 3 6 were 1
3 13 6 here 2
4 15 6 here 2
I tried doing the simplest thing:
df.merge(gb, on=['b','c'])
But this gives the error:
KeyError: 'b'
Which makes sense because the grouped table has a Multi-index and b is not a column. So my question is two-fold:
Can I transform the multi-index of the gb DataFrame back into columns (so that it has the b and c column)?
Can I merge df with gb on the column names?
Whenever you want to add some aggregated column from groupby operation back to the df you should be using transform, this produces a Series with its index aligned with your orig df:
In [4]:
df['nc'] = df.groupby(['b','c'])['a'].transform(pd.Series.nunique)
df
Out[4]:
a b c nc
0 1 4 wish 1
1 2 5 you 1
2 3 6 were 1
3 13 6 here 2
4 15 6 here 2
There is no need to reset the index or perform an additional merge.
There's a simple way of doing this using reset_index().
df.merge(gb.reset_index(), on=['b','c'])
gives you
a_x b c a_y
0 1 4 wish 1
1 2 5 you 1
2 3 6 were 1
3 13 6 here 2
4 15 6 here 2