How to iterate over pandas dataframe and create new column

Question

I have a pandas dataframe that has 2 columns. I want to loop through it\'s rows and based on a string from column 2 I would like to add a string in a newly created 3th colum

Answer 1

I think you can use double numpy.where, what is faster as loop:

df['Column3'] = np.where(df['Column2']==variable1, variable2, 
                np.where(df['Column2']==variable3, variable4))

And if need add variable if both conditions are False:

df['Column3'] = np.where(df['Column2']==variable1, variable2, 
                np.where(df['Column2']==variable3, variable4, variable5))

Sample:

df = pd.DataFrame({'Column2':[1,2,4,3]})
print (df)
   Column2
0        1
1        2
2        4
3        3

variable1 = 1
variable2 = 2
variable3 = 3
variable4 = 4
variable5 = 5

df['Column3'] = np.where(df['Column2']==variable1, variable2, 
                np.where(df['Column2']==variable3, variable4, variable5))

print (df)
   Column2  Column3
0        1        2
1        2        5
2        4        5
3        3        4

Another solution, thanks Jon Clements:

df['Column4'] = df.Column2.map({variable1: variable2, variable3:variable4}).fillna(variable5)
print (df)
   Column2  Column3  Column4
0        1        2      2.0
1        2        5      5.0
2        4        5      5.0
3        3        4      4.0