Why isn't my Pandas 'apply' function referencing multiple columns working? [closed]

Question

Seems you forgot the '' of your string.

In [43]: df['Value'] = df.apply(lambda row: my_test(row['a'], row['c']), axis=1)

In [44]: df
Out[44]:
                    a    b         c     Value
          0 -1.674308  foo  0.343801  0.044698
          1 -2.163236  bar -2.046438 -0.116798
          2 -0.199115  foo -0.458050 -0.199115
          3  0.918646  bar -0.007185 -0.001006
          4  1.336830  foo  0.534292  0.268245
          5  0.976844  bar -0.773630 -0.570417

BTW, in my opinion, following way is more elegant:

In [53]: def my_test2(row):
....:     return row['a'] % row['c']
....:     

In [54]: df['Value'] = df.apply(my_test2, axis=1)

If you just want to compute (column a) % (column b), you don't need apply, just do it directly:

In [7]: df['a'] % df['c']                                                                                                                                                        
Out[7]: 
0   -1.132022                                                                                                                                                                    
1   -0.939493                                                                                                                                                                    
2    0.201931                                                                                                                                                                    
3    0.511374                                                                                                                                                                    
4   -0.694647                                                                                                                                                                    
5   -0.023486                                                                                                                                                                    
Name: a

Let's say we want to apply a function add5 to columns 'a' and 'b' of DataFrame df

def add5(x):
    return x+5

df[['a', 'b']].apply(add5)

All of the suggestions above work, but if you want your computations to by more efficient, you should take advantage of numpy vector operations (as pointed out here).

import pandas as pd
import numpy as np


df = pd.DataFrame ({'a' : np.random.randn(6),
             'b' : ['foo', 'bar'] * 3,
             'c' : np.random.randn(6)})

Example 1: looping with pandas.apply():

%%timeit
def my_test2(row):
    return row['a'] % row['c']

df['Value'] = df.apply(my_test2, axis=1)

The slowest run took 7.49 times longer than the fastest. This could mean that an intermediate result is being cached. 1000 loops, best of 3: 481 µs per loop

Example 2: vectorize using pandas.apply():

%%timeit
df['a'] % df['c']

The slowest run took 458.85 times longer than the fastest. This could mean that an intermediate result is being cached. 10000 loops, best of 3: 70.9 µs per loop

Example 3: vectorize using numpy arrays:

%%timeit
df['a'].values % df['c'].values

The slowest run took 7.98 times longer than the fastest. This could mean that an intermediate result is being cached. 100000 loops, best of 3: 6.39 µs per loop

So vectorizing using numpy arrays improved the speed by almost two orders of magnitude.