Velocity Reviews > Ruby > hello! first post to clr. I'm asking about an attempt at a lazy rubysolution to computing fibonacci numbers for a project euler problem. seems tobe a bug in lazy ruby...

# hello! first post to clr. I'm asking about an attempt at a lazy rubysolution to computing fibonacci numbers for a project euler problem. seems tobe a bug in lazy ruby...

tphyahoo
Guest
Posts: n/a

 08-06-2008
Hi, first post to clr here.

I have a guess a haskell bias as that's the last language I learned,
and now I'm learning ruby.

I was trying to solve project euler problem 2 as an exercise. This is,
sum all the even fibonacci numbers less than 4 million.

There is a solution that works at

http://www.absorbeo.net/2008/01/10/p...ler-problem-2/

however I didn't quite like it because the test for evenness is in the

puts a.inject(0) {|s,v|
if v > 1_000_000 then
break s
elsif v % 2 == 0
s+v
else
s
end
}

t = ( putStrLn . show . sum . takeWhile (<=4000000) . filter even )
fibs2
fibs2 = 1 : 2 : zipWith (+) fibs2 (tail fibs2)

but it relies on laziness and who knows what other magic.

I decided to see if I could find a ruby solution that was more
haskellish. It turns I can... almost.

There is a ruby library for laziness

# http://lazylist.rubyforge.org/

and it would seem to do what I want. But it hangs for some reason.

I have poor sense of ruby culture. Should I report this as a bug? Is
it even a bug? Do people ever use laziness, or is this crazy
experimentation?

Code below:

thartman@thartman-laptop:~/thomashartman-learning/ruby/katas/euler>cat
2.rb
require 'rubygems'
require 'lazylist' # http://lazylist.rubyforge.org/

# Solve Project Euler problem 2: sum fibonacci numbers <= 4_000_000,
which are even.
fibs = ( LazyList.tabulate(0) { |x| x < 2 ? 1 : fibs[x-2] +
fibs[x-1] } )

fibsUnder4M = ( fibs.partition{ |x| x <= 4_000_000 })[0]
evenFibsUnder4M = fibsUnder4M.select{ |x| x %2 == 0}

# problem seems to be with partition
# evenFibsLessThanFourMil = ( evenFibs.partition{ |x| x <= 4_000_000 })
[0]

# works
# puts evenFibsLessThanFourMil.take(1)

# hangs
puts evenFibsUnder4M.take(10) # this is ok
puts evenFibsUnder4M.take(11) # this hangs, laptop gets hot, fan turns
on...
# exit

# this would be the answer to the euler problem, if only there wasn't
the bug...
result = evenFibsLessThanFourMil.inject(0){ | accum,val | accum + val}

main = puts result
thartman@thartman-laptop:~/thomashartman-learning/ruby/katas/
euler>ruby 2.rb
2
8
34
144
610
2584
10946
46368
196418
832040
..... hung...

Ryan Davis
Guest
Posts: n/a

 08-06-2008

On Aug 6, 2008, at 08:40 , tphyahoo wrote:

> t = ( putStrLn . show . sum . takeWhile (<=4000000) . filter even )
> fibs2
> fibs2 = 1 : 2 : zipWith (+) fibs2 (tail fibs2)

Gives me hives personally... I've never been able to handle the syntax

> but it relies on laziness and who knows what other magic.

The laziness I'm fine with... the other magic? not so much.

> I decided to see if I could find a ruby solution that was more
> haskellish. It turns I can... almost.

Ignoring my haskell bias for a bit... I'd recommend doing this, with
any pairing of languages. Find the idiomatic response appropriate for
the target language and you'll have a better time.

> There is a ruby library for laziness
>
> # http://lazylist.rubyforge.org/

I've not used this personally, I suspect using it is the problem.

> and it would seem to do what I want. But it hangs for some reason.
>
> I have poor sense of ruby culture. Should I report this as a bug? Is
> it even a bug? Do people ever use laziness, or is this crazy
> experimentation?

Assuming it is a bug, yes, report it. Again, not having used the
library, I can't really weigh in whether this behavior is a bug or not.

> require 'rubygems'
> require 'lazylist' # http://lazylist.rubyforge.org/
>
> # Solve Project Euler problem 2: sum fibonacci numbers <= 4_000_000,
> which are even.
> fibs = ( LazyList.tabulate(0) { |x| x < 2 ? 1 : fibs[x-2] +
> fibs[x-1] } )

so, according to the doco, tabulate is a generator function that
starts at 0 and yields the block on each successive succ.

(stop with all the parens)

> fibsUnder4M = ( fibs.partition{ |x| x <= 4_000_000 })[0]

partition is customized to return two lazy lists, so this _should_ be
fine.

> evenFibsUnder4M = fibsUnder4M.select{ |x| x %2 == 0}

likewise, select is customized to return a lazy list. again, fine.

> # hangs
> puts evenFibsUnder4M.take(10) # this is ok
> puts evenFibsUnder4M.take(11) # this hangs, laptop gets hot, fan turns
> on...

presumably the take actually executes and generates elements from the
lazy list, unwinding back through the select, the partition, and
finally the tabulate block. This all looks kosher to me on the surface
(albeit a bit more complicated than necessary because you're trying to

So, I would think that you've discovered a bug in the LazyList impl,

Here is my ruby-idiomatic solution to the problem:

> # Solve Project Euler problem 2:
> # sum fibonacci numbers <= 4_000_000, which are even.
>
> \$fib = {} # simple memoization
> def fib x
> \$fib[x] ||= x < 2 ? 1 : fib(x-2) + fib(x-1)
> end
>
> # fib 33 > 4m
> fibs = (1..32).map { |n| fib n }.reject { |n| n % 2 == 1 }
>
> sum = 0
> fibs.each do |n|
> sum += n
> end
>
> p sum
> # => 4613732

because of the memoization, it runs incredibly fast. If I wasn't
allowed to predetermine the upper bound of the calculation, I'd wrap
that up in a simple loop with a conditional... Nothing fancy.

Maciej Tomaka
Guest
Posts: n/a

 08-07-2008
It is implementation failure, you are right.
LazyList::Enumerable has buggy select which works as follows:

So it search first true occurence and returns new object.

list = LazyList.tabulate(0) { |x| x }
=> [... ]
less_than_three = list.select { |x| x < 3 }
lest_than_three.take(0);lest_than_three.take(1);le st_than_three.take(2)
less_than_three.tail
=> [1, 2,... ]
less_than_three.tail.tail
=> [2,... ]
less_than_three.tail.tail.tail
# Hangs

So this should return lazy tail (which is empty, infinite loop occurs).

By the way, you are hunting ants with an rpg

Best Regards

Maciej
--
Posted via http://www.ruby-forum.com/.

Guest
Posts: n/a

 08-07-2008
Maciej Tomaka wrote:
> By the way, you are hunting ants with an rpg

He is hunting ants with a role playing game?
--
Posted via http://www.ruby-forum.com/.

Guest
Posts: n/a

 08-07-2008
There is an archive kept of threads and you can find some of this
discussed already by searching through it. As this is your first, post,
allow me to give it to as a freebie.

http://www.ruby-forum.com/forum/4?filter=fibonacci
--
Posted via http://www.ruby-forum.com/.

brabuhr@gmail.com
Guest
Posts: n/a

 08-07-2008
On Wed, Aug 6, 2008 at 3:51 PM, Ryan Davis <(E-Mail Removed)> wrote:
> On Aug 6, 2008, at 08:40 , tphyahoo wrote:
>
>> t = ( putStrLn . show . sum . takeWhile (<=4000000) . filter even )
>> fibs2
>> fibs2 = 1 : 2 : zipWith (+) fibs2 (tail fibs2)

>
> Gives me hives personally... I've never been able to handle the syntax of
>
>> but it relies on laziness and who knows what other magic.

>
> The laziness I'm fine with... the other magic? not so much.
>
>> I decided to see if I could find a ruby solution that was more
>> haskellish. It turns I can... almost.

>
> Here is my ruby-idiomatic solution to the problem:
>
>> # Solve Project Euler problem 2:
>> # sum fibonacci numbers <= 4_000_000, which are even.
>>
>> \$fib = {} # simple memoization
>> def fib x
>> \$fib[x] ||= x < 2 ? 1 : fib(x-2) + fib(x-1)
>> end
>>
>> # fib 33 > 4m
>> fibs = (1..32).map { |n| fib n }.reject { |n| n % 2 == 1 }
>>
>> sum = 0
>> fibs.each do |n|
>> sum += n
>> end
>>
>> p sum
>> # => 4613732

>
> because of the memoization, it runs incredibly fast. If I wasn't allowed to
> predetermine the upper bound of the calculation, I'd wrap that up in a
> simple loop with a conditional... Nothing fancy.

Personally, I like a Ruby memoizing solution, but seeing some other
lazy Ruby projects in a quick Google search, I whipped this up:

require 'lazy_stream'

even = lambda{|int| int[0] == 0}
under_4_million = lambda{|int| int <= 4_000_000 }

fibonacci = lambda{|array, index|
index < 2 ? 1 : array[index - 2] + array[index - 1]
}

# create a "lazy stream" that generates the fibonacci sequence
fibs = LazyStream.new(fibonacci)

# create a filtered lazy stream of even fibonacci numbers
even_fibs = LazyStream.filter(fibs, even)

# sum the even fibonacci numbers that are less than 4,000,000
sum = 0
even_fibs.each_while(under_4_million){|f| sum += f}
puts sum
# => 4613732

And the evil class that makes it "work":

# A *very* simple lazy generator based on Array
class LazyStream < Array

# When creating a LazyStream, supply a generator lambda.
# The generator should take as parameters the stream and the index.
def initialize generator
@generator = generator
end

# Override the basic array [] to generate as necessary.
def [](index)
if index.is_a? Range
index.map{|i| self[i]}
else
(self.fetch(index) rescue nil) ||
self[index] = @generator.call(self, index)
end
end

# Loop through the stream until the value evaluates the block to false.
def each_while filter
i = 0
loop do
break unless filter.call(self[i])
yield self[i]
i += 1
end
end

# Create a new stream that applies a filter to an existing stream.
def self.filter other_stream, filter
new_stream = new lambda{|s, index|
v = nil; @other_index ||= 0

index.times{|n| new_stream[n]} if index > new_stream.size

loop{
v = other_stream[@other_index]
@other_index += 1
break if filter.call(v)
}
v
}
end

private

# Move the basic array []= to private so people don't muck around with it
def []=(index, value)
super(index, value)
end
end

Maciej Tomaka
Guest
Posts: n/a

 08-08-2008
> Maciej Tomaka wrote:
>> By the way, you are hunting ants with an rpg

>
> He is hunting ants with a role playing game?

Have you ever seen Duke Nukem? RPG was a rocket launcher
--
Posted via http://www.ruby-forum.com/.