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.
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
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.
···
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