Python：如何将数字映射到列中的唯一项（枚举唯一对象）？

Question

I need to assign numbers to unique values from a specific column in a dataframe, and make it as a new column...

For example:

ex_list=['12-B', '10002', '3A', '4', 'DCX', '3A']
df=pd.DataFrame(ex_list, columns=['Items'])

This is the expected output :

Please note that I need to do this for a dataframe with over 1M rows, so definitely would appreciate an efficient approach!

Answer 1

You can do it like this.

import pandas as pd
ex_list=['12-B', '10002', '3A', '4', 'DCX', '3A']
df=pd.DataFrame(ex_list, columns=['Items'])
df['Num_Items']=pd.factorize(df['Items'].tolist())[0]

Output

    Items   Num_Items
0   12-B    0
1   10002   1
2   3A      2
3   4       3
4   DCX     4
5   3A      2

Answer 2

ex_list=['12-B', '10002', '3A', '4', 'DCX', '3A']
df=pd.DataFrame(ex_list, columns=['Items'])

Get the unique values from your column, create a mapping between those unique items and integers and then apply that mapping along the Items column.

mapping = {item:i for i, item in enumerate(df["Items"].unique())}
df["Num_Items"] = df["Items"].apply(lambda x: mapping[x])
df