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?
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).
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?
I’m assuming a Unix/Linux environment…
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.
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.
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.
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.
it uses a constant amount of space (i.e., no recursion at all, just
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
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?)
On Wednesday 30 September 2009 04:17:31 pm Rob B. 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.
Probably would have.
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 Thu, 01 Oct 2009 04:47:28 +0900, Joshua M. wrote:
end
(#{result_h})"
find_next_match(p, h)
end
end
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.
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
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
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.
This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.
At least, this a useful piece of information, I don’t