Pascal's Triangle in Ruby

Question

I am writing Pascal's triangle in Ruby, but keep getting the error message:

pascalsTriangle.rb:3:in 'triangle': undefined method `each' for 4:Fixnum (NoMethodError)
from pascalsTriangle.rb:18

def triangle(n)
  for r in n:
    lst=[1]
    term=1
    k=0
    (0..r+1).step(1){ |index|
      term=term*(r-k+1)/k
      lst.append(term)
      k+=1
    }
    print lst
  end
end

triangle(4)

score 4 · Answer 1 · answered Jan 18 '14 at 14:04

Why code C style in Ruby? :-)

Breaking the problem down would allow you to concentrate on one problem at a time and iterators would make the code more readable. I'm using the binomial theorem to calculate the values in the triangle. If you don't need a super large value from the triangle, this will be fast enough.

Calculating the 1000th line took 2.9 seconds on my virtual linux:

# factorial method
def fact(n)
  (1..n).reduce(:*)
end

# binomial theorem, n choose k
def binomial(n,k)
  return 1 if n-k <= 0
  return 1 if k <= 0
  fact(n) / ( fact(k) * fact( n - k ) )
end

def triangle(nth_line)
  (0..nth_line).map { |e| binomial(nth_line, e) }
end

p triangle(5)

mrd abd · Accepted Answer · 2013-10-06T07:58:36.990

the final source code:

def triangle(n)
    (0..n).each{|r|
            lst=[1]
            term=1
            k=1
            (0..r-1).step(1){|index|
                    term=term*(r-k+1)/k
                    lst.push term 
                    k+=1
            }
            p lst
    }
end
triangle(4)

changes:

you have syntax error on for r in n:.
a logical error on k=0 that causes Division by zero.
(0..r+1) is changed to (0..r-1)
there is no append method for array. changed to push
p is used instead of print

score 0 · Answer 3 · answered Jun 15 '14 at 02:44

Factorial(num), takes a number and return the factorial of it.
find_num(n, k), is the mathmatical formula of pascales triangle. !n/ !k * !(n - k) ---- '!' = factorial of number
Lastly pascale(num), this iterates a new row of the triangle by maping the index numbers or (k) for each row of (n).
If you want to truly understand how this works comment out the pascale, and simply run numbers through find_num((row number), (index number)). Then compare to a picture of the triangle to see the magic for your self

-

def find_num(n, k)
  result = factorial(n) / (factorial(k) * factorial(n - k))
end

def pascale(num)
i = 0
scale = 75
  while i <= num
    new_arr = []
   (0..i).map {|x| new_arr << find_num(i, x)}
    p new_arr.to_s.rjust(50 + scale)
    i += 1
    scale += 1
  end

def factorial(num)
 if num == 0
  return 1
 else
  num *= factorial(num - 1)
 end
end

end

pascale(12)

Pascal's Triangle in Ruby

3 Answers3