Implementing operator precedence in my calculator interpreter - ruby

Question

As part of learning Ruby am trying to implement a basic interpreter which reads input and do basic arithmetic calculations. So far basic arithmetic operations are working but having problem in operator precedence. Which is not handled yet. This is the code. Am at a beginner level. Any mistakes in this code are due to my lack of knowledge. How this code can be modified to handle operator precedence.
Sample output
2+2+2 = 6 #correct
10+10/2 = 10 # incorrect as in irb answer must be 15
Github Repo of this interpreter
=begin
Basic calculator Interpreter
can add, substract, multiply , divide with any number of operands at a time
Drawback : Lacks operator precedence
=end
class Interpreter
attr_accessor :input
def initialize
#input = gets.chomp
end
def intepret
first_operand = []
f = []
operator = '+'
array = Array.new
lc = 0
#input.split.join.split("").each_with_index.map do |i, index|
if i.is_number?
first_operand.push(i)
if index == #input.length-1
array.push(first_operand.join("").to_i)
end
elsif i.is_plus?
f = first_operand
first_operand = nil
first_operand = []
array.push(f.join("").to_i)
array.push("+")
elsif i.is_minus?
f = first_operand
first_operand = nil
first_operand = []
operator = '-'
array.push(f.join("").to_i)
array.push("-")
elsif i.is_multi?
f = first_operand
first_operand = nil
first_operand = []
operator = '*'
array.push(f.join("").to_i)
array.push("*")
elsif i.is_divide?
f = first_operand
first_operand = nil
first_operand = []
operator = '/'
array.push(f.join("").to_i)
array.push("/")
else
puts "Illegal input exiting.."
exit
end
lc = lc+1
end
#apply the appropriate operation on the inputs based on the operand
#puts "=======TOKENS======"
#puts array.inspect
result = 0
array.each_with_index.map do |x, key|
result = x if key == 0
if x == '+'
if key == 0
result = add(result, array[key+1])
else
result = add(result, array [key+1])
end
elsif x == '-'
if key == 0
result = minus(result, array[key+1])
else
result = minus(result, array [key+1])
end
elsif x == '*'
if key == 0
result = multi(result, array[key+1])
else
result = multi(result, array [key+1])
end
elsif x == '/'
begin
if key == 0
result = divide(result, array[key+1])
else
result = divide(result, array [key+1])
end
rescue
puts "Zero Divsion error"
exit
end
end
end
puts "Result is: "+result.to_s
end
def print_token(type, value)
puts type + ' '+ value
end
def add(f,s)
return f.to_i + s.to_i
end
def minus(f,s)
return f.to_i - s.to_i
end
def multi(f,s)
return f.to_i * s.to_i
end
def divide(f,s)
return f.to_i / s.to_i
end
end
# Override the string class, to directly use methods like obj.is_number? rather than is_number?(obj)
class String
def is_number?
true if Float(self) rescue false
end
def is_plus?
true if self == '+' rescue false
end
def is_minus?
true if self == '-' rescue false
end
def is_multi?
true if self == '*' rescue false
end
def is_divide?
true if self == '/' rescue false
end
end
#continue accepting inputs until exit CTRL + D
while true
print 'pck>:'
i_obj = Interpreter.new
i_obj.intepret
end

Answer 1

First, process the input using the Shunting-yard algorithm. This should give a list of tokens in Reverse Polish notation (RPN). Then you can evaluate the RPN expression.