Question

As an exercise of learning scala and functional programming, I implemented the following non tail-recursive def that calculates the pascal’s number at any location. The program itself serves as the definition of pascal’s triangle. It looks pictorially as follows

      1
    1  1
   1  2  1
  1 3   3 1
 1 4  6  4 1
1 5 10 10 5 1
...

def pascal(c: Int, r: Int): Int =
  if (c == 0 || c == r) 1 else pascal(c - 1, r - 1) + pascal(c, r - 1)

However when trying to run for pascal(25,50) on Mac OS X 10.6.8 (2.53 GHz Intel Core 2 Duo) it still hasn’t finished running after 20 min.

Just to compare with erlang, I installed R15B02 and wrote equivalent program as follows:

-module(pascal).
-export([calc_pascal/2]).

calc_pascal(0,_) -> 1;
calc_pascal(C,R) when C==R -> 1;
calc_pascal(C,R) when C<R  -> calc_pascal(C-1,R-1) + calc_pascal(C-1,R).

pascal:calc_pascal(25,50) finishes in ~4sec.

Why might be the reason for such a huge performance difference? Is jvm not as advanced as erlang runtime for recursive programs?

Answer 1

If I make the same mistake in the Scala program that you made in the Erlang version, it runs very fast. Might this be the reason?