Polynomial equation solver in Ruby

Question

I've written a small function for solving simple quadratic equations:

class EquationSolver
  def solve(x, *args)
    args.reverse.map.with_index { |coefficient, index| coefficient * x ** index }.reduce { |result, element| result + element }
  end
end

To calculate f(3) for f(x)=3x3−2x2−x+5, one would write:

puts EquationSolver.new.solve(3, 3, -2, -1, 5)

However, is there a more elegant version of my function, more like reduce.with_index or something similar?

Answer 1

Some notes:

• If you use a OOP approach, it seems more logical to use methods.
• Use arrays to group values that go together.
• I don't think solve is the correct term, that's when you are finding the roots of a function, here you're just evaluating it at a given point.

I'd write:

class Polynomial
  attr_accessor :coefficients 

  def initialize(coefficients)
    self.coefficients = coefficients.reverse
  end

  def evaluate(x)
    coefficients.map.with_index { |k, power| k * (x**power) }.reduce(0, :+)
  end
end

polynomial = Polynomial.new([3, -2, -1, 5])
puts polynomial.evaluate(3) #=> 65

Answer 2

Yes, there is a more elegant version. Here it is:

class EquationSolver
  def solve(x, *args)
    args.reverse.each_with_index.reduce(0) { |result, (coefficient, index)| result + coefficient * x ** index }
  end
end

It's possible to do this because iterator methods like each_with_index, when called without a block, return an Enumerator object on which you can call all the methods of the Enumerable.

In fact, an even shorter solution is obtained by using a variation of reduce to use a symbolic operator:

class EquationSolver
  def solve(x, *args)
    args.reverse.map.with_index { |coefficient, index| coefficient * x ** index }.inject(:+)
  end
end