python - Pandas: How to use apply function to multiple columns -

i have problems pandas apply function, when using multiple columns following dataframe

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

and following function

def my_test(a, b):     return % b 

when try apply function :

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

i error message:

nameerror: ("global name 'a' not defined", u'occurred @ index 0') 

i not understand message, defined name properly.

i highly appreciate on issue

update

thanks help. made indeed syntax mistakes code, index should put ''. have still same issue using more complex function such as:

def my_test(a):     cum_diff = 0     ix in df.index():         cum_diff = cum_diff + (a - df['a'][ix])     return cum_diff  

thank you

seems forgot '' of string.

in [43]: df['value'] = df.apply(lambda row: my_test(row['a'], row['c']), axis=1)  in [44]: df out[44]:                        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 opinion, following way more elegant:

in [53]: def my_test2(row): ....:     return row['a'] % row['c'] ....:       in [54]: df['value'] = df.apply(my_test2, axis=1)