Create dataframe based on index from list with different size

Question

What I'm trying to do is to create a dataframe based on a list as index and with a dataframe already determined:

df1:

index   flag   value
3       1      99
5       1      98
7       1      97

index list:

all_index = [1, 2, 3, 4, 5, 6, 7, 8, 9]

then get this output:

df2:

index   flag
1       NaN
2       NaN
3       1
4       NaN
5       1
6       NaN
7       1
8       NaN

I was trying to do it in this way but doesn't works:

df2 = df1.loc[all_index]['flag']

user_9177877 · Answer

use reindex

df.set_index('index').reindex(all_index)

user_9857631 · Answer

You just need reindex:

df2 = df1.reindex(all_index)["flag"]

user_15070697 · Answer

If you want to get the exact layout (as a dataframe instead of a series), you can use .reindex() in this way:

df2 = df1.reindex(all_index)['flag'].to_frame()

Or

df2 = df1.reindex(all_index)[['flag']]

Result:

print(df2)

   flag
1   NaN
2   NaN
3   1.0
4   NaN
5   1.0
6   NaN
7   1.0
8   NaN
9   NaN