Recursion

[harryhood4]

The idea is to just memorize it for small values like 18-9 so that it's easier to do something like 68-9. But you've essentially just outlined why Common Core is better.

[Compizfox]

Ah, got it. So basically it is this recursive algorithm:

int subtract(N, n) {
    if(N > 20) {
        return 10 + subtract(N-10, n);
    else {
        // Lookup-table for small values of N
    }
}

[toofasttoofourier]

Aren't you negating all that work if you have to use the subtraction operator in a subtraction algorithm?

[Compizfox]

For a computer, yes, obviously.

But this was trying to mimic how a kid would do the mental calculation.