paulharrington.rb

class Polynomial
  def initialize ary
    raise ArgumentError, "Need at least 2 coefficients" unless ary.size > 1
    @coeffs = ary.reverse # low order terms first
    @coeffs.pop while @coeffs.last == 0
  end

  def to_s
    return "0" if @coeffs.all? {|c| c == 0} # short circuit if all terms are 0
    str = ""
    @coeffs.each_with_index do |coeff, exp|
      unless coeff == 0                                    # skip terms that are 0
        op = if coeff == @coeffs.last && coeff > 0         # don't display the sign of the last term if positive
          :""
        else
          coeff > 0 ? :+ : :-
        end
        coeff_str = coeff.abs == 1 ? :"" : coeff.abs.to_s  # don't display the coefficient if it's 1
        exp_str = exp > 1 ? "^#{exp}" : :""                # don't display the exponent if it's 1
        var = exp > 0 ? :x : :""                           # the first term is a constant; do not display a variable
        str.insert 0, "" << op << coeff_str << var << exp_str
      end
    end
    str
  end
end