Merge two python pandas data frames of different length but keep all rows in output data frame
Solution 1
You can read the documentation here: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.merge.html
What you are looking for is a left join. The default option is an inner join. You can change this behavior by passing a different how argument:
df1.merge(df2,how='left', left_on='Column1', right_on='ColumnA')
Solution 2
Looks like you're looking for something like a left-join. See if this example helps: http://pandas.pydata.org/pandas-docs/stable/comparison_with_sql.html#left-outer-join
You can basically pass a parameter to merge()
called how='left'
Solution 3
You can simply use merge with using on and list as well
result = df1.merge(df2, on=['Column1'])
For more information follow link
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sequence_hard
Recently started working in Bioinformatics, therefore learning python2 :)
Updated on July 09, 2022Comments
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sequence_hard almost 2 years
I have the following problem: I have two pandas data frames of different length containing some rows and columns that have common values and some that are different, like this:
df1: df2: Column1 Column2 Column3 ColumnA ColumnB ColumnC 0 a x x 0 c y y 1 c x x 1 e z z 2 e x x 2 a s s 3 d x x 3 d f f 4 h x x 5 k x x
What I want to do now is merging the two dataframes so that if ColumnA and Column1 have the same value the rows from df2 are appended to the corresponding row in df1, like this:
df1: Column1 Column2 Column3 ColumnB ColumnC 0 a x x s s 1 c x x y y 2 e x x z z 3 d x x f f 4 h x x NaN NaN 5 k x x NaN NaN
I know that the merge is doable through
df1.merge(df2,left_on='Column1', right_on='ColumnA')
but this command drops all rows that are not the same in Column1 and ColumnA in both files. Instead of that I want to keep these rows in df1 and just assign NaN to them in the columns where other rows have a value from df2, as shown above. Is there a smooth way to do this in pandas?
Thanks in advance!
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Mathiou over 8 yearsI think he is actually looking for
left
join :) -
sequence_hard over 8 yearsI will read the documentation, thank you for the fast answer! works fine.