Explain this answer

Question

https://code.sololearn.com/cAgVOz9IOmuF/?ref=app

Answer 1

It has to do with the calls to fun. The lines 6 - 9 are being dismissed as they stack up on the "stack" and therefore the outcome of the original call is 33 when x = 0.

Answer 2

Very interesting, I do not know the answer. I would calculate Z to equal 4 on the 7th pass of fun.

changing a and b to the respective x, y
1st pass x = 13, y = 7 --> fun(x++-1, ++y+1)
2nd pass x = 12, y = 9 --> fun(x/2, y/2)
3rd pass x = 6, y = 4 --> fun(x/2, y/2)
4th pass x = 3, y = 2 --> fun(x++-1, ++y+1)
5th pass x = 2, y = 4 --> fun(x/2, y/2)
6th pass x = 1, y = 2 --> fun(x/2, y/2)
7th pass x = 0, y = 4 --> return y

for some reason when I ran it on GDB the 7th pass went to fun(x++-1, ++y+1) after the return b statement

Answer 3

William Owens Thanks for the clarification

Answer 4

Suresh Rajendiran this is an extremely inefficient function to calculate b+2*a.

In the middle of the function it performs unnecessary recursion without using the return value. And it modifies a and b without using them afterward. By cutting out those inefficiencies and reducing, we get:

int fun(int a, int b) {
return a? fun(a-1, b+2) : b;
}

Now it becomes clearer that a simply iterates down to 0 as b increments by 2. The result is to add 2 to b, a times; more simply, b+2*a. Try a=13, b=7: 7+2*13 = 7+26 = 33.

Answer 5

Brian - I think that was the point of the code someone wrote. To show how the fiction builds up on the stack during recuse. Not 100 percent sure though.

Answer 6

Hi William Owens. Somehow when I first viewed this question, and even refreshed a few times, there were no replies before I responded. But now I see there were a couple exchanges already that didn't appear until now.

Regarding the code, I agree that it may have an educational origin, not to be viewed as an example of good programming.

Because it does tail recursion, I had considered also pointing out how easy it would be to convert into a loop for more efficiency. Though when I realized that it was a simple expression implemented in an impractical way, that edified for me the notion that it was educational. So I decided to leave my final code in recursion form and let the OP study it in context of recursion.

When I am employed, I usually work with legacy code, and I do enjoy streamlining code like this after several generations of developers have added their tweaks without regard to the whole context.