calculate sum of a column based on another col-CodePudding

My df looks like this:

value    type
12       x
34       z
54       x
14       y

I want to create a new column df.sum where I want to do a sum of the value col but only where the type == x. The remaining rows should be empty. So for example, the output should be like this:

value    type    sum
12       x       86
34       z
54       x       86
14       y

CodePudding user response：

If you want to handle a single type (only x):

mask = df['type'].eq('x')
df.loc[mask, 'sum'] = df.loc[mask, 'value'].sum()

if you might need to handle several:

types = ['x'] # add others, e.g.: types = ['x', 'y']
df.loc[df['type'].isin(types), 'sum'] = (df.groupby('type')['value']
                                           .transform('sum')
                                         )

output:

   value type   sum
0     12    x  66.0
1     34    z   NaN
2     54    x  66.0
3     14    y   NaN

CodePudding user response：

We can use boolean indexing with loc:

m = df['type'].eq('x')
df.loc[m, 'sum'] = df.loc[m, 'value'].sum()