根据另一个 DF 列中值的第一位数字更改 DF 列中的值

Question

I have the following dataframe and for my purposes, when the first digit in the Subref is X, the Centre should be X and when the fist letter of the Subref is Y, the Centre value should be Y.

So how do I go from this df

 Subref     Det      Centre
0   C12345  JOHNEL      1
1    C3245    BVDI      3
2  X035769  JOHNEL      0
3  Y038450    BVDI      0

To this df

 Subref     Det      Centre
0   C12345  JOHNEL      1
1    C3245    BVDI      3
2  X035769  JOHNEL      X
3  Y038450    BVDI      Y

Answer 1

You can use str[0] to get the first character, then where or loc or np.where to replace

first_chars = df['Subref'].str[0]

df['Centre'] = df['Centre'].where(first_chars=='C', first_chars)

# or 
# df['Centre'] = np.where(first_char=='C', df['Center'], first_chars)

# or
# df.loc[first_chars!='C', 'Centre'] = first_chars

Output:

    Subref     Det Centre
0   C12345  JOHNEL      1
1    C3245    BVDI      3
2  X035769  JOHNEL      X
3  Y038450    BVDI      Y

Answer 2

Try like this:

df.loc[df['Subref'].str[0].isin(['X','Y']),'Centre'] = df['Subref'].str[0]
print(df)