added calculator as exercise 4

Author: perennialAutodidact

practice/unit_2/exercise_3.md

@@ -58,6 +58,6 @@ No [**solution**](solutions/exercise_3_solution.md) provided because this is a 1
 
 ---
 
-## [< Exercise 2](exercise_2.md) 
+## [< Exercise 2](exercise_2.md) | [Exercise 4 >](exercise_4.md) 
 
 ### [<< Back to Unit 2 Practice](/practice/unit_2/)

practice/unit_2/exercise_4.md

@@ -0,0 +1,53 @@
+# <a id="top"></a>Lab 2 - Simple Calculator
+
+[Back to Syllabus](https://github.com/PdxCodeGuild/Programming102#top)
+
+## 4.1
+
+---
+
+Let's write a simple REPL (read, evaluate, print, loop) calculator that can handle addition and subtraction. 
+
+Ask the user for an `operator` and each `operand`. Don't forget that `input` returns a `string`, which you can convert to a float using `float(user_input)` where `user_input` is the string you got from `input`. 
+
+Create a separate function for each operation which takes in both `operands` and `returns` the `result` of its operation. 
+
+Pass the user's `operands` to the appropriate function based on the user's selected `operator` and use the value the function returns as your `result`.
+
+Below is some sample input/output.
+
+```
+> what is the operation you would like to perform? +
+> what is the first number? 5
+> what is the second number? 7
+> 5 + 7 = 12
+> what is the operation you would like to perform? done
+> goodbye
+```
+---
+## 4.2
+
+Add functionality for multiplication `*` and division `/`
+
+---
+## 4.3
+
+Allow the user to have a running total, each command will perform an operation on that number.
+
+```
+> what is the starting number? 15
+> enter an operation: + 5
+> 20
+> enter an operation: * 3
+> 60
+> enter an operation: done
+> Your total is 60. Goodbye.
+```
+
+### Exercise 4 [solution](solutions/exercise_4_solution.md)
+
+---
+
+## [< Exercise 3](exercise_3.md) 
+
+### [<< Back to Unit 2 Practice](/practice/unit_2/)