Question

So been studying for my past exam and come across this question which asks what will the return value be if num = 7? Plugging it into BlueJ tells me 16, what does the func1 do to make it 16? How can the method declared be used again within the method? I searched but hard to find this exact example as it all comes up with just using methods normally.

Thanks,

public int func1(int num) 
{
  if ( num <= 2 ) return 1;
  return func1(num – 2) + num;
}

Answer 1

That is a recursive call to the same function you are using it in.

So, func1(num – 2) will invoke the same function – public int func1(int num) with num = num - 2, until num >= 2

So, you recursion goes like this: –

func(n)
   calls func(n-2)
      calls func(n-4)
         .. so on
         calls func(1)
            returns 1
         returns 1 + 3 + ... + (n - 4)
      returns 1 + 3 + ... + (n - 4) + (n - 2) 
   returns 1 + 3 + ... + (n - 4) + (n - 2) + n

UPDATE: – Generalized the above recursion, to let you figure out how it works.

You can go through: – Recursion – Wiki Page