diff --git a/alex_calc.rb b/alex_calc.rb
new file mode 100644
index 0000000..c79ae98
--- /dev/null
+++ b/alex_calc.rb
@@ -0,0 +1,79 @@
+### Calculator program ###
+
+operators_list = %w(add + subtract - multiply * divide /)
+
+# methods
+def validate_operator(operator, operators_list)
+  while !(operators_list.include?(operator))
+    print "Please enter an operator word or symbol (i.e. add, +, *): "
+    operator = gets.chomp
+  end
+  return operator
+end
+
+def validate_num(num)
+  while num =~ /[[:alpha:]]/
+    print "Please enter a number (i.e. 1, 50, 100): "
+    num = gets.chomp
+  end
+  num = num.to_i
+  return num
+end
+
+def addition(x, y)
+  add_answer = x + y
+  puts "#{x} + #{y} = #{add_answer}"
+  return add_answer
+end
+
+def subtraction(x, y)
+  sub_answer = x - y
+  puts "#{x} - #{y} = #{sub_answer}"
+  return sub_answer
+end
+
+def multiplication(x, y)
+  mult_answer = x * y
+  puts "#{x} * #{y} = #{mult_answer}"
+  return mult_answer
+end
+
+def division(x, y)
+  div_answer = x / y
+  puts "#{x} / #{y} = #{div_answer}"
+  return div_answer
+end
+
+def operation(operator, first_num, second_num)
+  case operator
+  when "add", "+"
+    return addition(first_num, second_num)
+  when "subtract", "-"
+    return subtraction(first_num, second_num)
+  when "multiply", "*"
+    return multiplication(first_num, second_num)
+  when "divide", "/"
+    if second_num == 0
+      puts "Undefined: cannot divide by 0"
+    else
+      return division(first_num, second_num)
+    end
+  end
+end
+
+# prompts and inputs
+puts "This is a simple calculator program...\n"
+print "\nEnter an operator: "
+operator = gets.chomp
+operator = validate_operator(operator, operators_list)
+
+print "Enter a number: "
+first_num = gets.chomp
+first_num = validate_num(first_num)
+
+print "Enter another number: "
+second_num = gets.chomp
+second_num = validate_num(second_num)
+
+puts
+operation(operator, first_num, second_num)