[lnkForumImage]
TotalShareware - Download Free Software
Confronta i prezzi di migliaia di prodotti.
Asp Forum
 Home | Login | Register | Search 


 

Forums >

comp.lang.ruby

Ruby Math (nPr, nCr

Ari Brown

5/26/2007 1:57:00 AM

What's that? You want me to ask more questions? Sure!

So I'm writing a pascals triangle program. My goal is to write it in
fewer lines than those on the RubyQuiz site. My idea is to use more
math, and less programming.

So in order for my method to work, I sorta need to recreate nPr and
nCr functions. To be official and learn more, I am doing this through
a class. However, every time I run it, I get the following errors:

pascal.rb:29:in `pascal': undefined method `nCr' for 1:Fixnum
(NoMethodError)
from pascal.rb:24:in `upto'
from pascal.rb:24:in `pascal'
from pascal.rb:41

My code:

# Pascals triangle

class Integer

def self.nPr(r)
numerator = factorial(self)
denominator = factorial(self - r)

@permutations = numerator / denominator
end

def self.nCr(r)
numerator = factorial(self)
denominator = factorial(r) * factorial(self - r)

@combinations = numberator / denominator
end

end


def pascal max_row

0.upto(max_row) {|row_num|

holder = []
ticker = 0
while ticker != row_num
result = row_num.nCr(ticker)
ticker = ticker + 1
holder = result.push
end
puts holder.join(' ').center(80)

}

end

puts 'How many rows do you want?'
max_row = gets.chomp.to_i
pascal max_row


Any help?

Thanks,
Ari
--------------------------------------------|
If you're not living on the edge,
then you're just wasting space.
4 Answers

Dan Zwell

5/26/2007 2:45:00 AM

0

Ari Brown wrote:
> pascal.rb:29:in `pascal': undefined method `nCr' for 1:Fixnum
> (NoMethodError)
> from pascal.rb:24:in `upto'
> from pascal.rb:24:in `pascal'
> from pascal.rb:41
>
> My code:
> class Integer
>
> def self.nPr(r)
> numerator = factorial(self)
> denominator = factorial(self - r)
>
> @permutations = numerator / denominator
> end
> end

Ari,

It looks like you are defining--don't remember the ruby term--java calls
it a static method. You want an instance method. Just declare it as
def self(r)
and not
def self.nPr(r)
because that is called like:
Integer.nPr(something)
and you want:
my_number.nPr(r)

Dan

Dan Zwell

5/26/2007 3:57:00 AM

0

Dan Zwell wrote:
> You want an instance method. Just declare it as
> def self(r)
> and not
> def self.nPr(r)

Oops. I meant:

def nPr(r)

Good luck.
Dan

Sebastian Hungerecker

5/26/2007 8:03:00 AM

0

Dan Zwell wrote:
> It looks like you are defining--don't remember the ruby term--java calls
> it a static method.

Class methods.


--
Ist so, weil ist so
Bleibt so, weil war so

Brian Candler

5/26/2007 2:10:00 PM

0

On Sat, May 26, 2007 at 10:57:16AM +0900, Ari Brown wrote:
> # Pascals triangle
>
> class Integer
>
> def self.nPr(r)
> numerator = factorial(self)
> denominator = factorial(self - r)
>
> @permutations = numerator / denominator
> end
>
> def self.nCr(r)
> numerator = factorial(self)
> denominator = factorial(r) * factorial(self - r)
>
> @combinations = numberator / denominator
> end
>
> end

Of course Ruby has Bignums, but you could make the program more efficient by
not calculating such enormous intermediate values and then dividing them.

I suggest you cancel out the terms which are common to the numerator and
denominator.

That is, 9C4 is 9!/5!/!4!, but 9!/5! is 9*8*7*6, so the result is
9*8*7*6/4*3*2*1. Also, you can swap r and n-r to minimise the calculation.

Example:

def Math.nCr(n,r)
a, b = r, n-r
a, b = b, a if a < b # a is the larger
numer = (a+1..n).inject(1) { |t,v| t*v } # n!/r!
denom = (2..b).inject(1) { |t,v| t*v } # (n-r)!
numer/denom
end

(0..9).each { |r| puts Math.nCr(9,r) }

As for making these instance variables of Integer (without self.): that's
fine, but I'd suggest you make them class methods of Math or of your own
module. Whilst

Math.nCr(9,4)

is a bit more long-winded to type than 9.nCr(4), it avoids cluttering
Integer with more methods.

Regards,

Brian.