polynomial.rb

class ArgumentError < StandardError; end

class Polynomial
  def initialize(coefficients)
    raise ArgumentError.new("Need at least 2 coefficients") if coefficients.size < 2
    @addends = []
    size = coefficients.size
    coefficients.each_with_index do |coef, index|
      unless coef == 0
        if index == size-1
          @addends << "#{coef}"
        elsif index == size -2
          @addends << "#{coef}x"          
        elsif coef == 1
          @addends << "x^#{size-index-1}"
        elsif coef == -1
          @addends << "-x^#{size-index-1}"
        else
          @addends << "#{coef}x^#{size-index-1}"
        end
      end
    end
  end

  def to_s
    @addends.empty? ? 0.to_s : @addends.join("+").gsub("+-", "-")
  end
end

polynomial_spec.rb

require 'polynomial'

describe Polynomial do
  before do
    @p1 = Polynomial.new([-3,-4,1,0,6])
    @p2 = Polynomial.new([1,0,2])
    @p3 = Polynomial.new([-1,-2,3,0])
    @p4 = Polynomial.new([0,0,0])
  end
 
  it "should test first negative" do
    @p1.to_s.should == "-3x^4-4x^3+x^2+6"
  end
 
  it "should test simple" do
    @p2.to_s.should == "x^2+2"
  end
 
  it "should test first minus one" do
    @p3.to_s.should == "-x^3-2x^2+3x"
  end
 
  it "should test all zero" do
    @p4.to_s.should == "0"
  end
 
  it "should test error" do
    lambda{ Polynomial.new([1])}.should raise_error(ArgumentError, /Need at least 2 coefficients/)
  end
end