---
id: 6a0dcd03ee4e68698080ef6c
title: "Challenge 308: Credit Card Validator"
challengeType: 29
dashedName: challenge-308
---

# --description--

Given a string of digits for a credit card number, determine if it's a valid card number using the following method:

- Starting from the second-to-last digit, double every other digit moving left.
- If doubling a digit results in a number greater than 9, subtract 9.
- Sum all the digits (doubled and undoubled).
- If the total is divisible by 10, the number is valid.

# --hints--

`is_valid_card("4532015112830366")` should return `True`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("4532015112830366"), True)`)
}})
```

`is_valid_card("5425233430109903")` should return `True`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("5425233430109903"), True)`)
}})
```

`is_valid_card("371449635398431")` should return `True`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("371449635398431"), True)`)
}})
```

`is_valid_card("6011111111111117")` should return `True`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("6011111111111117"), True)`)
}})
```

`is_valid_card("4532015112830367")` should return `False`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("4532015112830367"), False)`)
}})
```

`is_valid_card("1234567890123456")` should return `False`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("1234567890123456"), False)`)
}})
```

`is_valid_card("4532015112830368")` should return `False`.

```js
({test: () => { runPython(`
from unittest import TestCase
TestCase().assertIs(is_valid_card("4532015112830368"), False)`)
}})
```

# --seed--

## --seed-contents--

```py
def is_valid_card(number):

    return number
```

# --solutions--

```py
def is_valid_card(number):
    total = 0
    for i, digit in enumerate(reversed(number)):
        d = int(digit)
        if i % 2 == 1:
            d *= 2
            if d > 9:
                d -= 9
        total += d
    return total % 10 == 0
```