Python Pandas : Set column values as number of occurrences of another column

Question

I have problems with a for loop on two columns of my dataframe. With this for loop I want to add a new column as I show you in this example:

I start with this dataframe:

     person       store_id
0    Rose         24256
1    Tim          24257
2    Tim          27896
3    Tim          29547
4    Jack         24258
5    Jack         27897

and the new column I would like to add should look like this. In words I want to add a column showing a sort of enumeration of the stores visited by each person.

     person       store_id      enum
0    Rose         24256         1
1    Tim          24257         1
2    Tim          27896         2
3    Tim          29547         3
4    Jack         24258         1
5    Jack         27897         2

This is the code I tried, but I do not get what I want.

for x in df['person']:
    i = 1
    for x in df['store_id']:
        df['enum'] = i
        i = i + 1

I have also tried as follows, getting again a wrong result:

for x,y in zip(df['person'], df['store_id']):
    i = 1
    df['enum'] = i
    i = i + 1

What am I doing wrong? Thank you for your help!

Answer 1

No need to iterate over multiple columns; iterate with iterrows and use a dictionary to track visited store id position "enum", then you set the enum column:

import pandas as pd
import numpy as np

df = pd.DataFrame(data={"person":[ 'Rose', 'Tim', 'Tim', 'Tim', 'Jack', 'Jack' ],
                    "store_id": [1580407549, 1580557630, 1580571344, 1580647900, 1580755371, 1580911127]})

store_index = {}
enums = []
for index, row in df.iterrows():
    sid = row['person']
    if sid not in store_index:
        store_index[sid] = 0
    store_index[sid] += 1
    enums.append(store_index[sid])
df['enum'] = enums
print(df)

Output:

  person    store_id  enum
0   Rose  1580407549     1
1    Tim  1580557630     1
2    Tim  1580571344     2
3    Tim  1580647900     3
4   Jack  1580755371     1
5   Jack  1580911127     2