In Pandas, how can I replace a subset of the dataframe with another subset of the dataframe?

Suppose I have a dataframe, where every 5th row is filled with NAs, like this:

I want to replace the NA values you can find in the 5th row with the values in the 3rd row.

What I’ve tried so far:

df.loc[df['Number'] % 5 == 0, ['A','B','C']] = df.loc[df['Number'] % 3 == 0, ['A','B','C']]

This approach does not throws any error, but it does nothing. Howewer, if I replace the right side of the assignment with a constant, it works fine, like this:

df.loc[df['Number'] % 5 == 0, ['A','B','C']] = 3

Any idea on how to make it work?

EDIT:

I did not want to give the full original task, howewer this dummy example is a little bit wrong. So let’s assume that the right side has as many rows as the left side. The main objective is still, to replace every 5th row’s values with a subset of the dataframe based on conditions. I hope it is a little bit more understandable, and sorry for the miscralification.

>Solution :

Your indexes are misaligned that’s why it doesn’t work:

# Left hand side
>>> df.loc[df['Number'] % 5 == 0, ['A','B','C']]
     A    B    C
1  NA   NA   NaN
3   3    5   2.0

# Right hand side
>>> df.loc[df['Number'] % 3 == 0, ['A','B','C']]
    A   B    C
2  5   6   7.0

Pandas can’t align index between (1, 3) and 2. The solution is to convert the right hand side to numpy array with .values or .to_numpy(). However if you do this, you also change the fourth row.

Update:

So let’s assume that the right side has as many rows as the left side

In this case, use:

df.loc[df['Number'] % 5 == 0, ['A','B','C']] = \
    df.loc[df['Number'] % 3 == 0, ['A','B','C']].values

# Before (I slightly modified your example)
>>> df
   Number    A    B    C
0       3  2.0  1.0  0.0
1       5  NaN  NaN  NaN
2       6  5.0  6.0  7.0
3     100  NaN  NaN  NaN

# After
>>> df
   Number    A    B    C
0       3  2.0  1.0  0.0
1       5  2.0  1.0  0.0
2       6  5.0  6.0  7.0
3     100  5.0  6.0  7.0