根据来自另一个 DataFrame 的值更新 pandas 列中的值

Question

Two DataFrames df1 and df2 both has Weight in lb columns if Weight in lb matches I need to update the corresponding BMI in df1 from df2.

DataFrame df1 =

Index	First Name	Age	Gender	Weight in lb	BMI
0	James	21	Male	167
1	John	25	Male	175
2	Patricia	23	Female	132
4	Kevin	22	Male	169
5	Alex	27	Male	169

DataFrame df2 =

Weight in lb	BMI	Height
165	16.5	180
166	17.0	180
167	17.3	180
168	17.4	180
169	17.9	180
170	18.4	180
171	18.7	180
172	18.9	180
173	19.2	180
174	19.3	180
175	19.6	180
176	19.9	180
177	20.0	180
178	20.2	180
179	21.6	180
180	21.9	180
181	22.1	180
182	22.3	180
183	22.5	180
184	22.8	180
185	22.9	180
186	23.0	180
187	22.1	180
188	22.1	180

I tried

df1['BMI'] = df2.loc[df2['Weight in lb'].isin(df1['Weight in lb'],'BMI']

But it is not working, I cannot do merge operation because the original dataframe is too complex for that

Desired Output df1 =

Index	First Name	Age	Gender	Weight in lb	BMI
0	James	21	Male	167	17.3
1	John	25	Male	175	19.6
2	Patricia	23	Female	132
4	Kevin	22	Male	169	17.9
5	Alex	27	Male	169	17.9

Answer 1

You can use df2 to make a mapping from weight to BMI and then use it on df1

weight_to_bmi = dict(df2.values)
df1['BMI'] = df1['Weight in lb'].map(weight_to_bmi)

Answer 2

Building off of mitoRibo's Answer

weight_to_bmi = pd.Series(df2['Weight in lb'].values,index=df2['BMI']).to_dict()
df1['BMI'] = df1['Weight in lb'].map(weight_to_bmi)