pandas create one column equal to another if condition is satisfied

Question

I have two columns as below:

id, colA, colB
0, a, 13
1, a, 52
2, b, 16
3, a, 34
4, b, 946
etc...

I am trying to create a third column, colC, that is colB if colA == a, otherwise 0.

This is what I was thinking, but it does not work:

data[data['colA']=='a']['colC'] = data[data['colA']=='a']['colB']

I was also thinking about using np.where(), but I don't think that would work here.

Any thoughts?

Answer 1

Use loc with a mask to assign:

In [300]:
df.loc[df['colA'] == 'a', 'colC'] = df['colB']
df['colC'] = df['colC'].fillna(0)
df

Out[300]:
   id colA  colB  colC
0   0    a    13    13
1   1    a    52    52
2   2    b    16     0
3   3    a    34    34
4   4    b   946     0

EDIT

or use np.where:

In [296]:
df['colC'] = np.where(df['colA'] == 'a', df['colC'],0)
df

Out[296]:
   id colA  colB  colC
0   0    a    13    13
1   1    a    52    52
2   2    b    16     0
3   3    a    34    34
4   4    b   946     0