create new column based on condition

[paul18fr]

you haven't gone to the code

[arvin]

Oops my bad.. I was looking at the previous post and trying to understand

[jefsummers]

Expansion and explanation on my code version. Here we use the dataframe "apply" function to apply a function to the dataframe

import pandas as pd

def my_transform(x): #function to be applied to the dataframe
    if x[0]>4: # refer to dataframe as "x". Test if first column is >4
        return x[0] # if >4, return that value
    else: #otherwise
        return x[1] #return the value from the second column

df = pd.DataFrame(data=[[1,2,3],[4,5,6],[7,8,9]]) #create a simple dataframe with 3 rows and 3 columns
df[3] = df.apply(my_transform, axis=1) #calls the function above, apply "by row" rather than by column (axis value)
df #display result

Output:
	0	1	2	3
0	1	2	3	2
1	4	5	6	5
2	7	8	9	7