pascal triangle out to out.txt and length received by console

gistfile1.rb

  def pascal(n)
    file = File.open("out.txt", "w")
    p=[1]
    while(p.length<n)
      file.write(p.join(" ")+"\n")
      p=Array.new(p.length+1) do |i|
        a=i<p.length ? p[i] : 0
        b=i>0 ? p[i-1] : 0
        a+b 
      end
    end 
  end
  pascal(ARGV.first.to_i)