Pandas apply but only for rows where a condition is met

I would like to use Pandas df.apply but only for certain rows

As an example, I want to do something like this, but my actual issue is a little more complicated:

import pandas as pd
import math
z = pd.DataFrame({'a':[4.0,5.0,6.0,7.0,8.0],'b':[6.0,0,5.0,0,1.0]})
z.where(z['b'] != 0, z['a'] / z['b'].apply(lambda l: math.log(l)), 0)

What I want in this example is the value in 'a' divided by the log of the value in 'b' for each row, and for rows where 'b' is 0, I simply want to return 0.

The other answers are excellent, but I thought I'd add one other approach that can be faster in some circumstances – using broadcasting and masking to achieve the same result:

import numpy as np

mask = (z['b'] != 0)
z_valid = z[mask]

z['c'] = 0
z.loc[mask, 'c'] = z_valid['a'] / np.log(z_valid['b'])

Especially with very large dataframes, this approach will generally be faster than solutions based on apply().

You can just use an if statement in a lambda function.

z['c'] = z.apply(lambda row: 0 if row['b'] in (0,1) else row['a'] / math.log(row['b']), axis=1)

I also excluded 1, because log(1) is zero.

Output:

   a  b         c
0  4  6  2.232443
1  5  0  0.000000
2  6  5  3.728010
3  7  0  0.000000
4  8  1  0.000000

Hope this helps. It is easy and readable

df['c']=df['b'].apply(lambda x: 0 if x ==0 else math.log(x))

You can use a lambda with a conditional to return 0 if the input value is 0 and skip the whole where clause:

z['c'] = z.apply(lambda x: math.log(x.b) if x.b > 0 else 0, axis=1)
                                       

You also have to assign the results to a new column (z['c']).

Use np.where() which divides a by the log of the value in b if the condition is met and returns 0 otherwise:

import numpy as np
z['c'] = np.where(z['b'] != 0, z['a'] / np.log(z['b']), 0)

Output:

     a    b         c
0  4.0  6.0  2.232443
1  5.0  0.0  0.000000
2  6.0  5.0  3.728010
3  7.0  0.0  0.000000
4  8.0  1.0       inf