Pandas python - matching values

Question

I currently have two dataframes that have two matching columns. For example :

Data frame 1 with columns : A,B,C

Data frame 2 with column : A

I want to keep all lines in the first dataframe that have the values that the A contains. For example if df2 and df1 are:

So in this case, I want to only keep the second and third line of df1. I tried doing it like this, but it didnt work since both dataframes are pretty big:

for index, row in df1.iterrows():
    counter = 0
    for index2,row2 in df2.iterrows():
        if row["A"] == row2["A"]:
            counter = counter + 1
    if counter == 0:
        df2.drop(index, inplace=True)

EdChum · Accepted Answer · 2015-12-09 16:43:16Z

6

Use isin to test for membership:

In [176]:
df1[df1['A'].isin(df2['A'])]

Out[176]:
   A  B  C
1  4  2  5
2  6  3  1

answered Dec 9, 2015 at 16:43

EdChum

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Comments

dlm · Accepted Answer · 2015-12-09 18:15:29Z

3

Or use the merge method:

df1= pandas.DataFrame([[0,1,3],[4,2,5],[6,3,1],[8,0,0],[2,1,1]], columns = ['A', 'B', 'C'])
df2= pandas.DataFrame([4,6,1], columns = ['A'])
df2.merge(df1, on = 'A')

answered Dec 9, 2015 at 18:15

dlm

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1 Comment

piRSquared Over a year ago

Interesting thing is that when you merge, pandas creates a new dataframe in which the index is unrelated to either either index from left or right side of merge. This is to accommodate possible duplicates on either side. Using isin is an exercise in filtering one dataframe with values in another. That said, this is useful!

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