From: Robert Klemme <shortcutter@...>
Date: 2007-11-15T00:33:50+09:00
Subject: Re: efficiency of recursion (stack level too deep)

2007/11/14, Chet Nichols III <chet.nichols@gmail.com>:
> hi there-
>
> so i started running into a problem using recursion with ruby- it seemed a
> little too soon for me to be getting a stack overflow ('stack level too deep
> (SystemStackError)'), so i wrote up a quick simple script to do some
> recursion:
>
> -----
> #!/usr/local/bin/ruby
>
> def print_depth num
>  puts num
>  print_depth(num+1)
> end
>
> print_depth 0
> -----
>
> and sure enough, once im at a recursive depth of about 4,360, it errors out.
> i tried making num a global $num instead so it wasnt allocating memory on
> the stack each time for building a new 'num' object, but that only got me to
> about 4,500 until the overflow. to me, it seems a little low, especially for
> something this simple. my stack size is 10240. and, of course, when i up it
> to something like 20480, i get about double the recursive depth.. but when i
> try the same thing in C:
>
> -----
> #include <stdio.h>
>
> void print_depth(int num);
>
> int main() {
>     print_depth(0);
> }
>
> void print_depth(int num) {
>     printf("%d\n",num);
>     print_depth(++num);
> }
> -----
>
> i get a recursive depth of about 350,000 before it bombs out (which is what
> i'd expect more or less). i know C is super efficient and all, and ruby is
> truly object oriented, so each thing pushed on the stack is an object with
> its own set of methods, etc, so its by design going to take up more space,
> but i was wondering if there are any plans to do anything to help the
> efficiency of this at all?
>
> to be honest, i've only been using ruby for about a week now, and i totally
> love it. so, this e-mail really isn't a complaint at all, just more of a
> question. if there's already an answer (which there probably is), feel free
> to shout it out- i love everything i've seen and done so far with the
> language, so the more i know about it, the better! hope to hear from someone
> soon- thanks!

You'll find it in the archives. This issue comes up frequently.
Basically Ruby is not good at recursion for exactly the reason you
discovered.

Cheers

robert

-- 
use.inject do |as, often| as.you_can - without end