a quick challenge

[fliptw]

A UPC is a 12-digit number.
Typically its starts as a eleven digit number and a check digit is computed and stuck at the end.

The procedure to compute the check digit for UPC thusly follows:

1. sum the digits in the odd positions and multiply the result by 3
2. add to that the even digits
3. modulus the sum so far by 10
4. if the result isn't 0, subtract from 10

the challenge is not to compute the check digit without using any string manipulation.

[Sergeant Thorne]

I was doing good until you threw that double-negative at me in the last sentence. Screw this, this stuff is hard. ;P

the challenge is not to compute the check digit without using any string manipulation.

BTW I've already not computed it without using string manipulation, I'm just keeping it to myself because it wouldn't be unfair to everyone else.

[fliptw]

ahh I was in a rush when I wrote it.

the challenge is to compute the check digit without using any string manipulation.

[Jeff250]

Code: Select all

unsigned long long upc_check( unsigned long upc )
{
    unsigned long left = upc;
    int i, mod, sum = 0;
    for (i = 0; i < 11; ++i) {
        mod = left % 10;
        sum +=  mod + ((mod & ((i & 1) - 1)) << 1);
        left = left / 10;
    }
    return (unsigned long long) upc * 10 + (10 - (sum % 10)) % 10;
}

This C code should work as long as you have two's complement integers. It's a bit pedantic though, espcially if the C compiler partially or fully unrolls the loop.

[Isaac]

Darn it. I love string manipulation! I'll get back to this.

[Jeff250]

There still has yet to be a good solution.

[Isaac]

There still has yet to be a good solution.

I asked chatgpt. I was curious how it would fair. I tested it and I'm not sure if this is actually what the test is asking for.

Chatgpt:

Here's an implementation in Python of the algorithm to compute the check digit for a UPC without using any string manipulation:

Code: Select all

def compute_check_digit(upc):
    digits = [int(d) for d in str(upc)]
    odd_sum = sum(digits[-1::-2])
    even_sum = sum(digits[-2::-2])
    total_sum = odd_sum * 3 + even_sum
    if total_sum % 10 == 0:
        return 0
    else:
        return 10 - total_sum % 10

Here, upc is the 11-digit number that represents the UPC without the check digit. The function compute_check_digit takes upc as an argument and returns the check digit.

The first while loop extracts the individual digits of the UPC by repeatedly computing the remainder of upc divided by 10 (i.e., the last digit), appending it to a list of digits, and then integer dividing upc by 10 to remove the last digit. The digits are stored in reverse order in the list, so that the odd and even positions correspond to the correct digits in the UPC.

The odd and even sums are computed using list slicing and the sum function. The total sum is then computed according to the algorithm, and the check digit is computed by checking whether the total sum is already divisible by 10, and if not, subtracting it from 10 to get the digit that will make the sum divisible by 10.

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