Credit card vlidator problem

Question

I wrote this code for cc validator challange but It doesn't work for all cases.
Can anyone help me with this please?

Chalange tasks :

● Number should be exactly 16 digits
● You need to verify if the given credit card number is valid. For that you need to use the Luhn test.

Here is the Luhn formula:
1. Reverse the number.
2. Multiple every second digit by 2.
3. Subtract 9 from all numbers higher than 9.
4. Add all the digits together.
5. Modulo 10 of that sum should be equal to 0.

https://code.sololearn.com/cc8uK71oBtDv/?ref=app

Answer 1

Use for n in range(16) instead of num, with that, change all the n to num[n]. Change if num.index(n) % 2 == 0 to n % 2 == 1 because what it checks is every second digit.

Answer 2

num = list(input()[::-1])

for i in range(1, len(num)+1, 2):
num[i] = str((int(num[i]))*2)

num_2 = num
for i in range(0, len(num_2)):
if int(num_2[i]) > 9:
num_2[i] = str(int(num_2[i])-9)

print('valid' if len(num_2) == 16 and sum([int(i) for i in num_2 ]) % 2 == 0 else 'not valid')

Answer 3

CarrieForle thanks alot for your time.
I did as you said and I understood my problem.
Thank you so much for the help 🙏🙏

Answer 4

My solution:
xcreditcardnum = input()
new_x_c_n = list(xcreditcardnum).reverse().join()
newstp1 = new_x_c_n
for k in new_x_c_n:
if (newstp1.index(k,0,15) + 1)%2 == 0:
newstp1[newstp1.index(k,0,15)]*=2
for i in newstp1:
if i > 9:
newstp1[newstp1.index(i,0,15)] -= 9
suM = sum(newstp1[:])
chck = suM%10
if chck == 0:
print('valid')
else:
print('invalid')
The only problem is I need to join. I can't can anybody help?