I'm trying to create check digits and append them after the original UPCs. Here's the sample data

Because there are leading 0's, I have to read the data as strings first:

import pandas as pd                                                                 
upc = pd.read_csv("/Users/lee/Desktop/upc.csv", dtype = str)

Here's an example of the check digit algorithm:
If upc is 003459409000
step (1) 0 + 3*0 + 3 + 3*4 + 5 + 3*9 + 4 + 3*0 + 9 + 3*0 + 0 + 3*0 = 60
step (2) 60 mod 10 = 0
step (3) check digit = 0 (if it's not 0, then check digit = 10 - number in step 2)

Based on the algorithm, here's the code:

def add_check_digit(upc_str):  upc_str = str(upc_str)if len(upc_str) != 12: raise Exception("Invalid length")odd_sum = 0even_sum = 0 for i, char in enumerate(upc_str): j = i+1 if j % 2 == 0: even_sum += int(char) else:odd_sum += int(char) total_sum = (even_sum * 3) + odd_sum mod = total_sum % 10 check_digit = 10 - mod if check_digit == 10: check_digit = 0 return upc_str + str(check_digit) 

If I run this code, it gives correct check digit and appends this result to the end of the original UPC. For the example above, if I type:

add_check_digit('003459409000')

The output gives 13-digit UPC 0034594090000.

Now my questions are:

1. This function works only for a single upc, i.e., I have to copy/paste each single upc and get the check digit. How do I create a function that works for a list of UPSs in a dataframe? Each result should return a 13-digit UPC with the check digits appended after the original UPC.

2. The UPCs are read as strings. How do I apply the function to the UPCs? I suppose I should convert the strings to numbers somehow.

3. After I get the new UPCs, how do I save the result in a csv file?

data set up for me as I don't have CSV file, below step is the same as your

df = pd.read_csv("/Users/lee/Desktop/upc.csv", dtype = str)

data setup

import pandas as pd
df=pd.DataFrame({"upc_in_file":['003459409000','003459409001','003459409002']})

def add_check_digit(upc_str):  upc_str = str(upc_str)if len(upc_str) != 12: raise Exception("Invalid length")odd_sum = 0even_sum = 0 for i, char in enumerate(upc_str): j = i+1 if j % 2 == 0: even_sum += int(char) else:odd_sum += int(char) total_sum = (even_sum * 3) + odd_sum mod = total_sum % 10 check_digit = 10 - mod if check_digit == 10: check_digit = 0 return upc_str + str(check_digit) 

apply the above function to the upc column(the one which was read from file)

df['new_upc']=df['upc_in_file'].apply(add_check_digit)

now save the file!

df.to_csv("my_updated_upc.csv")

this will look like