Class: Expression

Inherits:
Object
  • Object
show all
Defined in:
lib/calc_dongqs/expression.rb

Constant Summary collapse

PRECEDENCE =

shunting-yard algorithm

{
  '+' => 0,
  '-' => 0,
  '*' => 1,
  '/' => 1,
}
OPERATORS = 
PRECEDENCE.keys
FUNCTIONS = 
['sqrt']

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(exp) ⇒ Expression

Returns a new instance of Expression.



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# File 'lib/calc_dongqs/expression.rb', line 5

def initialize exp

  @exp = exp
  @tokens = self.class.tokenize @exp
  @rpn = self.class.shunt @tokens
  @result = self.class.evaluate @rpn
end

Instance Attribute Details

#exp ⇒ Object

Returns the value of attribute exp.



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# File 'lib/calc_dongqs/expression.rb', line 3

def exp
  @exp
end

#result ⇒ Object

Returns the value of attribute result.



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# File 'lib/calc_dongqs/expression.rb', line 3

def result
  @result
end

#rpn ⇒ Object

Returns the value of attribute rpn.



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# File 'lib/calc_dongqs/expression.rb', line 3

def rpn
  @rpn
end

#tokens ⇒ Object

Returns the value of attribute tokens.



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# File 'lib/calc_dongqs/expression.rb', line 3

def tokens
  @tokens
end

Class Method Details

.evaluate(tokens) ⇒ Object

evaluate



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# File 'lib/calc_dongqs/expression.rb', line 66

def self.evaluate tokens
  stack = []
  while !tokens.empty?
    token = tokens.shift
    case token
    when 'sqrt'
      stack.push Math.sqrt stack.pop
    when *OPERATORS
      b = stack.pop
      a = stack.pop
      case token
      when '+'
        c = a + b
      when '-'
        c = a - b
      when '*'
        c = a * b
      when '/'
        c = a / b
      end
      stack.push c
    else
      if token.to_i.to_s == token
        stack.push token.to_i
      else
        stack.push token.to_f
      end
    end
  end
  if stack.length == 1
    stack.last
  else
    raise
  end
end

.shunt(tokens) ⇒ Object 



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# File 'lib/calc_dongqs/expression.rb', line 28

def self.shunt tokens
  stack = []
  output = []

  while !tokens.empty?
    token = tokens.shift
    case token
    when *FUNCTIONS
      stack.push token
    when *OPERATORS
      while OPERATORS.include?(stack.last) and PRECEDENCE[token] <= PRECEDENCE[stack.last]
        output.push stack.pop
      end
      stack.push token
    when '('
      stack.push token
    when ')'
      while !stack.empty? and stack.last != '('
        output.push stack.pop
      end
      raise 'mismatched parentheses' if stack.empty?
      stack.pop
      output.push stack.pop if FUNCTIONS.include? stack.last
    else
      output.push token
    end
  end

  while !stack.empty?
    token = stack.pop
    raise 'mismatched parentheses' if ['(', ')'].include? token
    output.push token
  end

  output
end

.tokenize(expression) ⇒ Object

tokenize by regex, can NOT process negative numbers



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# File 'lib/calc_dongqs/expression.rb', line 14

def self.tokenize expression
  expression.scan /\d+\.\d+|\d+\.|\.\d+|\d+|\+|-|\*|\/|\(|\)|sqrt/
end