How do I assign a value to a pandas dataframe cell with a missing value with the value of another ce-CodePudding

I have a dataframe that has some missing values. I want to replace those missing values with a value from another cell in the dataframe based on a condition. So the dataframe looks like this:

x	a
xyz	A
lmn	B
None	A
xyz	A
qrs	C
None	B

What I want to do is set the value of the "None" cell to the value in column x when the values in column a match. So that it looks like this:

x	a
xyz	A
lmn	B
xyz	A
xyz	A
qrs	C
lmn	B

The index is just sequential numbers from 0 up and may change depending on the dataset so the index for the cells with the missing information will change.

CodePudding user response：

You can use ffill() to fill forward missing values:

df['x'] = df.replace('None', np.nan).groupby('a')['x'].ffill()
print(df)

# Output:
     x  a
0  xyz  A
1  lmn  B
2  xyz  A
3  xyz  A
4  qrs  C
5  lmn  B

CodePudding user response：

for i in range(len(df)):
    if df['a'][i] == 'A':
        df['x'][i] = 'xyz'

This worked for me, if you want to do all the other letters, just add an elif.