Question

I need to write a recursive function that can add two numbers (x, y), assuming y is not negative. I need to do it using two functions which return x-1 and x+1, and I can’t use + or – anywhere in the code. I have no idea how to start, any hints?

Answer 1

Lets say that

succ(x)=x+1
pre(x)=x-1

Then, (in pseudocode)

add(x,y) = {
    If(y==0) Return x;
    Return add(succ(x),pre(y))
}

Observe, this only works for non-negative y.