puts "find next match for p=#{p} (#{result_p}) and h=#{h}
(#{result_h})"
find_next_match(p, h)
end
end
The task is to find the next pair of p and h that result in the same
number. The first pair is 165 and 143, so I run it with these arguments:
puts find_next_match(165, 143)
Sadly, after some seconds, Ruby throwns a SystemStackError:
SystemStackError: stack level too deep
method p in uebung-2-1.rb at line 2
method find_next_match in uebung-2-1.rb at line 10
method find_next_match in uebung-2-1.rb at line 18
at top level in uebung-2-1.rb at line 22
copy output
Program exited with code #1 after 8.29 seconds.
But I'd like to run it and run it and run it without this limitation. Is
there a workaround for this, so I can change the stack level maximum?
puts "find next match for p=#{p} (#{result_p}) and h=#{h}
(#{result_h})"
find_next_match(p, h)
end
end
The task is to find the next pair of p and h that result in the same
number. The first pair is 165 and 143, so I run it with these arguments:
puts find_next_match(165, 143)
Sadly, after some seconds, Ruby throwns a SystemStackError:
SystemStackError: stack level too deep
method p in uebung-2-1.rb at line 2
method find_next_match in uebung-2-1.rb at line 10
method find_next_match in uebung-2-1.rb at line 18
at top level in uebung-2-1.rb at line 22
copy output
Program exited with code #1 after 8.29 seconds.
But I'd like to run it and run it and run it without this limitation. Is
there a workaround for this, so I can change the stack level maximum?
The stack limitation is enforced by the OS. You'll have to change
the limits in your shell so that when the Ruby interpreter is started
it is allowed to grow a larger stack.
The code you have posted is tail recursive, which means that the original
function call does nothing after it makes the recursive call, and returns
the recursive call's return value. Many compilers can optimize the code
to set up new values for the parameters, and then jump back to the
beginning of the function (with something like a goto), reusing the
current stack frame. (This is called tail call optimization.)
Ruby does not perform tail call optimization, because it prefers to keep
track of the exact call stack in case you raise an exception. So you need
to invent the loop to do this yourself.
Something like
def find_next_match(p,h)
loop do #do stuff #wherever you would have a recursive call
# just update the values of p and h respectively
end
end
Alternatively, it appears you can enable tail call recursion in Ruby 1.9
changing a #define in the source code and recompiling. See http://
redmine.ruby-lang.org/issues/show/1256
You can also accomplish tail call optimization in Ruby 1.9 without
recompiling the interpreter. You can include the algorithm as a string,
compile it to an instruction sequence with a dynamic option to enable
tail call optimization, and eval that instruction sequence. The
instructions are given at the above link.
···
On Thu, 01 Oct 2009 04:47:28 +0900, Joshua Muheim wrote:
Hi all
I have a recursive algorith that must run until it finds the result:
puts "find next match for p=#{p} (#{result_p}) and h=#{h}
(#{result_h})"
find_next_match(p, h)
end
end
--
Chanoch (Ken) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology. http://www.iit.edu/~kbloom1/
And my solution (in Ruby) could even be made more efficient. However, it uses a constant amount of space (i.e., no recursion at all, just some nested loops).
[Note: parts of this message were removed to make it a legal post.]
Rewrite it to not be recursive.
That advice is about as useful as "Rewrite it in a language other than Ruby"
But it's the only answer there is. Unrecognized tail recursion will kill
every stack.
def find_next_match(p, h)
loop do
result_p = p(p + 1)
result_h = h(h + 1)
if result_p == result_h # result found!
return result_p
if result_p < result_h
p += 1
else
h += 1
end
puts "find next match for p=#{p} (#{result_p}) and h=#{h} (#{result_h})"
end
end
mfg, simon .... not tried
···
On Wed, Sep 30, 2009 at 1:55 PM, Jason Roelofs <jameskilton@gmail.com>wrote:
And my solution (in Ruby) could even be made more efficient. However,
it uses a constant amount of space (i.e., no recursion at all, just
some nested loops).
Yeah, it's from Project Euler (our professor is just too lazy to create
his own tasks...).
Your solution sounds interesting; would you mind posting it here? I
won't steal it; we don't have to submit perfect solutions, it's more
sort a thinking-task where we're told to deliver possible solutions.
* Tony Arcieri <tony@medioh.com> (05:25) schrieb:
> [Note: parts of this message were removed to make it a legal post.]
>
>> Rewrite it to not be recursive.
>
> That advice is about as useful as "Rewrite it in a language other than
> Ruby"
Somehow, I think rewriting a tail-recursive algorithm to not be tail-recursive
is much, much faster than translating the algorithm to another language,
especially if it's part of a larger program.
It is a bit like, when you go from C to Ruby, you learn to use each loops
instead of for loops. An argument could be made that tail-recursion isn't
idiomatic Ruby.
But it's the only answer there is. Unrecognized tail recursion will kill
every stack.
Technically, it's not -- Ruby _can_ do tail recursion. It's just not trivial
to turn on, not portable, and I believe disables some debugging features.
···
On Thursday 01 October 2009 05:05:13 am Simon Krahnke wrote:
> On Wed, Sep 30, 2009 at 1:55 PM, Jason Roelofs <jameskilton@gmail.com>wrote:
So I get put down for suggesting the very thing Rob did, who gets praise?
If you don't want long-running code crashing, don't do it recursive in a
language like Ruby. You will run out of memory, and you will crash. There
are very few, if any, algorithms normally written recursively that can't be
written iteratively.
Jason
···
On Wed, Sep 30, 2009 at 4:56 PM, Joshua Muheim <forum@josh.ch> wrote:
> Or a problem from Project Euler...
> http://projecteuler.net/index.php?section=problems&id=45
>
> real 0m0.639s
> user 0m0.623s
> sys 0m0.008s
>
> And my solution (in Ruby) could even be made more efficient. However,
> it uses a constant amount of space (i.e., no recursion at all, just
> some nested loops).
>
> -Rob
>
> Rob Biedenharn http://agileconsultingllc.com
> Rob@AgileConsultingLLC.com
Yeah, it's from Project Euler (our professor is just too lazy to create
his own tasks...).
Your solution sounds interesting; would you mind posting it here? I
won't steal it; we don't have to submit perfect solutions, it's more
sort a thinking-task where we're told to deliver possible solutions.
--
Posted via http://www.ruby-forum.com/\.
Which PE question is it? I've solved 140 of them, the computer that had the
majority of my solutions ended up dying, and a large number of them are
solved in Java, but there is no harm in checking to see if I have the
solution on this computer, in Ruby.
-Josh
···
On Wed, Sep 30, 2009 at 3:56 PM, Joshua Muheim <forum@josh.ch> wrote:
> Or a problem from Project Euler...
> http://projecteuler.net/index.php?section=problems&id=45
>
> real 0m0.639s
> user 0m0.623s
> sys 0m0.008s
>
> And my solution (in Ruby) could even be made more efficient. However,
> it uses a constant amount of space (i.e., no recursion at all, just
> some nested loops).
>
> -Rob
>
> Rob Biedenharn http://agileconsultingllc.com
> Rob@AgileConsultingLLC.com
Yeah, it's from Project Euler (our professor is just too lazy to create
his own tasks...).
Your solution sounds interesting; would you mind posting it here? I
won't steal it; we don't have to submit perfect solutions, it's more
sort a thinking-task where we're told to deliver possible solutions.
--
Posted via http://www.ruby-forum.com/\.
So I get put down for suggesting the very thing Rob did, who gets
praise?
If you don't want long-running code crashing, don't do it recursive in a
language like Ruby. You will run out of memory, and you will crash.
There
are very few, if any, algorithms normally written recursively that can't
be
written iteratively.
Jason
OK, thank you. At least, this a useful piece of information, I don't
have much experience in writing algorithms...
Anyway, I will hand in my solution above; in theory it works.
I'm on exactly the same page as Jason. The problem does not even need a recursive algorithm. (Well, except that it's not the Ruby language that is recursive, but the algorithm that you attempted to write *in* Ruby.)
Your attempt isn't really too far off, however. I'll point out that your algorithm is "tail recursive" and that's partly why you run out of stack space in a language like Ruby, but in a language optimized for tail recursion it might have worked. Think about what that means and how you could restructure your code to avoid making a new function call if the current pair is not a solution. (Hint: How would you search for the solution standing at a blackboard/whiteboard?)
And my solution (in Ruby) could even be made more efficient. However,
it uses a constant amount of space (i.e., no recursion at all, just
some nested loops).
Yeah, it's from Project Euler (our professor is just too lazy to create
his own tasks...).
Your solution sounds interesting; would you mind posting it here? I
won't steal it; we don't have to submit perfect solutions, it's more
sort a thinking-task where we're told to deliver possible solutions.
--
Posted via http://www.ruby-forum.com/\.
So I get put down for suggesting the very thing Rob did, who gets praise?
If you don't want long-running code crashing, don't do it recursive in a
language like Ruby. You will run out of memory, and you will crash. There
are very few, if any, algorithms normally written recursively that can't be
written iteratively.
The other thing is to note that the question the subject is asking should
never be asked. If you're in a language that optimizes tail-recursing, it's
probably about as efficient as a loop.
If you're in a language that doesn't optimize tail-recursing, the last thing
you want to do is increase the size of the stack. That would just give you the
same problem again later, and waste tons of RAM.
···
On Wednesday 30 September 2009 04:17:31 pm Rob Biedenharn wrote:
your algorithm is "tail recursive" and that's partly why you run out
of stack space in a language like Ruby, but in a language optimized
for tail recursion it might have worked.
At least, this a useful piece of information, I don't