prefix.rb

=begin
PREFIX EXPRESSIONS
CHALLENGE DESCRIPTION:
You are given a prefix expression. Write a program which evaluates it.
INPUT SAMPLE:
Your program should accept a file as its first argument. The file contains one
prefix expression per line.
For example:
* + 2 3 4
Your program should read this file and insert it into any data structure you
like. Traverse this data structure and evaluate the prefix expression. Each
token is delimited by a whitespace. You may assume that sum ‘+’,
multiplication ‘*’ and division ‘/’ are the only valid operators appearing in
the test data.
OUTPUT SAMPLE:
Print to stdout the output of the prefix expression, one per line.
For example:
20
CONSTRAINTS:
The evaluation result will always be an integer ≥ 0.
The number of the test cases is ≤ 40.
=end

lines = File.readlines(ARGV[0])

OPERATORS = %w[+ * /]

operators = []
accumulator_empty = true
accumulator = nil

lines.each do |line|
  characters = line.split(" ")
  next if characters == []
  characters.each do |character|
    if OPERATORS.include?(character)
      operators << character
    elsif accumulator_empty
      accumulator = character.to_f
      accumulator_empty = false
    else
      operator = operators.pop
      case operator
        when "+"
          accumulator += character.to_i
        when "*"
          accumulator *= character.to_i
        when "/"
          accumulator /= character.to_i
      end
    end
  end
  puts accumulator.to_i
  accumulator_empty = true
end