Ruby Math (nPr, nCr)

[Ari Brown]

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.

 

[Dan Zwell]

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]

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]

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

Class methods.

 

[Brian Candler]

# 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.