Why is my recursive function so slow in R?

Question

The following takes about 30 seconds to run whereas I would expect it to be nearly instant. Is there a problem with my code?

x <- fibonacci(35);

fibonac         


        

Answer 1

:-) because you use exponential algorithm!!! So for fibonacci number N it has to call the function 2^N times, which 2^35, which is heck of a number.... :-)

Use linear algorithm:

fib = function (x)
{
        if (x == 0)
                return (0)
        n1 = 0
        n2 = 1
        for (i in 1:(x-1)) {
                sum = n1 + n2
                n1 = n2
                n2 = sum
        }
        n2
}

Sorry, edit: the complexity of the exponential recursive algorithm is not O(2^N) but O(fib(N)), as Martinho Fernandes greatly joked :-) Really a good note :-)