dataframe compare two columns to add a new column code example

Example 1: pandas create a new column based on condition of two columns

conditions = [
    df['gender'].eq('male') & df['pet1'].eq(df['pet2']),
    df['gender'].eq('female') & df['pet1'].isin(['cat', 'dog'])
]

choices = [5,5]

df['points'] = np.select(conditions, choices, default=0)

print(df)
     gender      pet1      pet2  points
0      male       dog       dog       5
1      male       cat       cat       5
2      male       dog       cat       0
3    female       cat  squirrel       5
4    female       dog       dog       5
5    female  squirrel       cat       0
6  squirrel       dog       cat       0

Example 2: pandas compare two columns

# Syntax:
# C = np.where(condition, A, B) 
# equal to A when condition true and B when false
import numpy as np
import pandas as pd

a = [['10', '1.2', '4.2'], ['15', '70', '0.03'], ['8', '5', '0']]
df = pd.DataFrame(a, columns=['one', 'two', 'three'])

df['que'] = np.where((df['one'] >= df['two']) & (df['one'] <= df['three'])
                     , df['one'], np.nan)

# If you have more than one condition, then you could use np.select 
# instead. For example, if you wish df['que'] to equal 
# df['two'] when df['one'] < df['two'], then
conditions = [
    (df['one'] >= df['two']) & (df['one'] <= df['three']), 
    df['one'] < df['two']]

choices = [df['one'], df['two']]

df['que'] = np.select(conditions, choices, default=np.nan)

# If we can assume that df['one'] >= df['two'] when 
# df['one'] < df['two'] is False, then the conditions and 
# choices could be simplified to
conditions = [
    df['one'] < df['two'],
    df['one'] <= df['three']]

choices = [df['two'], df['one']]

# Note that: 
a = [['10', '1.2', '4.2'], ['15', '70', '0.03'], ['8', '5', '0']]
df = pd.DataFrame(a, columns=['one', 'two', 'three'])

# defines a DataFrame with string values. Since they look numeric, 
# you might be better off converting those strings to floats:
df2 = df.astype(float)

# This changes the results, however, since strings compare 
# character-by-character, while floats are compared numerically.