Create new column to data frame

Question

import pandas as pd
import numpy as np

train = pd.read_csv("train.csv")

train["DayOfWeek_Monday"] = train.apply(lambda row: row["DayOfWeek"]=="Monday") 

What I want is to add new columns of 'DayOfWeek_Monday','DayOfWeek_Tuesday' and etc using existing column 'DayOfWeek'. How do I add new columns?

Answer 1

Use get_dummies with DataFrame.reindex for correct ordering and last DataFrame.join to original:

df = pd.DataFrame({'DayOfWeek':['Sunday','Monday','Wednesday','Thursday', 'Sunday']})

c = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
df1 = pd.get_dummies(df['DayOfWeek'], dtype=bool).reindex(c, axis=1, fill_value=False)
df = df.join(df1.add_prefix('DayOfWeek_'))
print (df)
  DayOfWeek  DayOfWeek_Monday  DayOfWeek_Tuesday  DayOfWeek_Wednesday  \
0     Sunday             False              False                False   
1     Monday              True              False                False   
2  Wednesday             False              False                 True   
3   Thursday             False              False                False   
4     Sunday             False              False                False   

   DayOfWeek_Thursday  DayOfWeek_Friday  DayOfWeek_Saturday  DayOfWeek_Sunday  
0               False             False               False              True  
1               False             False               False             False  
2               False             False               False             False  
3                True             False               False             False  
4               False             False               False              True