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Ruby Quiz #84: Pascal’s Triangle

I’ve started getting interested in Ruby Quiz as a way to keep my Ruby-fu sharp, and I think I’m gonna make it a regular feature here.

Today’s quiz question revolves around Pascal’s Triangle.

Here’s my answer, after the fold.

class PascalsTriangle
  # Instantiates an n-row Pascal's Triangle.
  def initialize(number_of_rows = 5)
    @rows = [[1]]
    2.upto(number_of_rows) do |i|
      row = [1]
      1.upto(i-2) do |j|
        row << @rows.last[(j-1) % @rows.last.size] + @rows.last[j % @rows.last.size]
      end
      @rows << (row << 1)
    end
  end

  # Converts the triangle into a centered string display.
  def to_s
    longest_digit = @rows.flatten.inject(0){ |max, num| num > max ? num : max }.to_s.size
    padded_rows = @rows.map{ |row| row.map{ |num| num.to_s.center(longest_digit) }.join(’   ‘) }
    longest_row = padded_rows.last.size
    padded_rows.map{ |row| row.center(longest_row) }.join(”\\n”)
  end
end

puts PascalsTriangle.new(ARGV[0].to_i)

Witness the fitness:

$ ruby pp_pascal.rb 10
                            1
                         1     1
                      1     2     1
                   1     3     3     1
                1     4     6     4     1
             1     5    10    10     5     1
          1     6    15    20    15     6     1
       1     7    21    35    35    21     7     1
    1     8    28    56    70    56    28     8     1
 1     9    36    84    126   126   84    36     9     1

Yay! Numbers!

This entry was posted on Saturday, June 24th, 2006 at 2:52am and is filed under Programming, Ruby Quiz, Ruby and Rails. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.