How to replace values in selected rows columns with an array in dataframe?

Question

I have a df

df = pd.DataFrame(data={'A': [1,2,3,4,5,6,7,8],
                       'B': [10,20,30,40,50,60,70,80]})
    A   B
0   1   10
1   2   20
2   3   30
3   4   40
4   5   50
5   6   60
6   7   70
7   8   80

which I selected a few rows from. Then I have a dictionary containig values that I should insert in B column if key matches with value in A column of df

my_dict = {2: 39622884,
           4: 82709546,
           5: 28166511,
           7: 89465652}

When I use the following assignment

df.loc[df['A'].isin(my_dict.keys())]['B'] = list(my_dict.values())

I get the error: ValueError: Length of values does not match length of index

The desirable output is

    A   B
0   1   10
1   2   39622884
2   3   30
3   4   82709546
4   5   50
5   6   28166511
6   7   89465652
7   8   80

What is the correct way to implement this procedure?

@QuangHoang solution is the way I would have done it. But, I think this fixes your problem. df.loc[df['A'].isin(my_dict.keys()), 'B'] = list(my_dict.values()) — Scott Boston
– Scott Boston, Commented Feb 18, 2020 at 15:48

Quang Hoang · Accepted Answer · 2020-02-18 15:42:37Z

3

You can make do with map and fillna:

df['B'] = df['A'].map(my_dict).fillna(df['B'])

Output:

   A           B
0  1        10.0
1  2  39622884.0
2  3        30.0
3  4  82709546.0
4  5  28166511.0
5  6        60.0
6  7  89465652.0
7  8        80.0

answered Feb 18, 2020 at 15:42

Quang Hoang

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