How to make a new column in pandas using existing columns?

Question

I am pretty new to Python and I am trying to create a column in my pandas df using pre-exisitng columns (code below).

if ((df['a'] != -1) & (df['b'] == -1)):
    df['c'] = df['a']
elif ((df['a'] == -1 ) & (df['b'] != -1)):
    df['c'] = df['b']    
else: 
    df['c'] = 0

However, I keep getting the following error:

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

When I add '.any()', it still does not perform as needed. Am I totally off with this?

Use `np.where`. Understand that `db['a'] == -1` returns a series of booleans rather than a single one, which is what your code is set up to expect — ifly6, Oct 06 '20 at 13:49

mujjiga · Answer 1 · 2020-10-06T14:03:58.187

df = pd.DataFrame( {'a': [-1, 1, 1, -1], 'b': [1, 1, -1, -1]})

df['c'] = 0
df.loc[(df['a'] != -1) & 
       (df['b'] == -1), 'c'] = df[(df['a'] != -1) & (df['b'] == -1)]['a'] 

df.loc[(df['a'] == -1) & 
       (df['b'] != -1), 'c'] = df[(df['a'] == -1) & (df['b'] != -1)]['b'] 

print (df)

Output:

OR

df['c'] = df.apply(lambda r: 
                   r['a'] if r['a'] != -1 and r['b'] == -1 else (
                       r['b'] if r['a'] == -1 and r['b'] != -1 else 0), 
                   axis=1)

How to make a new column in pandas using existing columns?

1 Answers1