macOS C: Arbitrary-precision can't correctly subtract

[jsmwoolf]

You then take one away to get 781, the 9's-complement. But why didn't you just calculate the 9's-complement directly, like the example shows? It's as if you read a simple example and can't simply follow it as given. You add complexity that simply isn't there in the original.

Well, I did understand it, but I just wanted to find a way how. Beside just finding the 10's complement and subtracting 1 isn't really that complex, just a little more work.

Actually, I used the ~ and added ten to each digit since a ~digit tends to equal a negative number.

 

[chown33]

Well, I did understand it, but I just wanted to find a way how. Beside just finding the 10's complement and subtracting 1 isn't really that complex, just a little more work.

Actually, I used the ~ and added ten to each digit since a ~digit tends to equal a negative number.

"The complement of a decimal digit in the nines' complement system is the number that must be added to it to produce 9." (from the wikipedia article)

Expressed algebraically, where D is any decimal digit, and C is its 9's-complement:

D + C = 9

Now, using basic algebra to solve for C:

C = 9 - D

Ultimately, the test is whether the code works or not. So did you code it, and does it work? Does it handle negative numbers or not?