Simple Ruby Calculator

Clearly, you can directly use irb as a calculator. However, I'm learning Ruby, so I thought it would be fun to program a simple integer calculator. It's a great little example of a read-eval-print loop.

The calculator accepts input expressions of the form:

<non-negative integer> <op> <non-negative integer>

where op is one of:

+

addition

-

subtraction

*

multiplication

/

division

%

modulo

^

power

calc.rb

---

#!/usr/bin/env ruby

def evaluate(arg0, op, arg1)
  ans = case op
  when '+' 
    arg0 + arg1 
  when '-' 
    arg0 - arg21
  when '/' 
    arg0 / arg1
  when '*' 
    arg0 * arg1
  when '%' 
    arg0 % arg1
  when '^' 
    arg0 ** arg1
  else
    # should never happen
    "unknown op #{op}"
  end 
end

def parse_expression(line)
  regex = /\s*(\d+)\s*([+-\/*%^])\s*(\d+)\s*/
  expr = line.match(regex)
end

def eval_loop
  while true
    print 'calc> '
    $stdout.flush
    line = gets.chomp
    if line == 'exit'
      break
    end 
    expr = parse_expression(line)
    if expr
      arg0, op, arg1 = expr.captures
      puts evaluate(arg0.to_i, op, arg1.to_i)
    else
      puts 'invalid expression'
    end 
  end 
end

puts "enter 'exit' to quit the program'"
eval_loop

example

---

./calc.rb 
enter 'exit' to quit the program'
calc> 1 + 2
3
calc> 2 ^ 8
256
calc> 400 % 92
32
calc> exit