Apply Python function to multiple Pandas columns

Question

I am attempting to write a function and apply it to multiple fields in a pandas dataframe. The function takes column colA1, and assigns a value to a new column, colB2 based on conditional statements. This function works if a single column is given, e.g. colA1, but how could I write it to iterate through a list of columns, returning a corresponding number of new columns?

The following function works on a single column:

dict = {'colA1':[2,6,8,28,5], 
    'colA2': [38,6,14,63,3], 
    'colA3':[90,40,80,98,3]} 

df = pd.DataFrame(dict) 

def function(x):
    if x <= 10:
        return '<= 10'
    elif x > 10:
        return '> 10' 

df['colB1']=df['colA1'].apply(function)

df['colB1']

This returns:

I attempted to apply it to multiple columns as shown here: Update Multiple Columns using Pandas Apply Function

df[['colB1', 'colB2', 'colB3']]=df[['colA1', 'colA2', 'colA3']].apply(function)

But this returns: ValueError: ('The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().', 'occurred at index colA1')

anky · Accepted Answer · 2019-06-14 10:08:16Z

5

If this actually what you want to do, a faster alternative is np.select():

cond=[df<= 10,df > 10]
choice=['<= 10','> 10' ]
df[:]=np.select(cond,choice)
print(df)

   colA1  colA2  colA3
0  <= 10   > 10   > 10
1  <= 10  <= 10   > 10
2  <= 10   > 10   > 10
3   > 10   > 10   > 10
4  <= 10  <= 10  <= 10

You can also try with df.applymap() for your function:

df[['colA1','colA2','colA3']].applymap(function)
#df.applymap(function)

   colA1  colA2  colA3
0  <= 10   > 10   > 10
1  <= 10  <= 10   > 10
2  <= 10   > 10   > 10
3   > 10   > 10   > 10
4  <= 10  <= 10  <= 10

edited Jun 14, 2019 at 10:08

answered Jun 14, 2019 at 9:56

anky

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1 Comment

anky Over a year ago

Also don't use dict as dictionary names since it overwrites the builtin python dict func.

iamklaus · Accepted Answer · 2019-06-14 10:01:13Z

1

this should do it

df.apply(lambda x: pd.Series([function(x['colA1']),function(x['colA2']),function(x['colA3'])]), axis=1).rename({0:'colA1',1:'colA2',2:'colA3'}, axis=1)

Output

   colA1  colA2  colA3
0  <= 10   > 10   > 10
1  <= 10  <= 10   > 10
2  <= 10   > 10   > 10
3   > 10   > 10   > 10
4  <= 10  <= 10  <= 10

edited Jun 14, 2019 at 10:01

answered Jun 14, 2019 at 9:54

iamklaus

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