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String Calculator Kata

The following is a TDD Kata, an exercise in coding, refactoring and test-first, that you should apply daily for at least 15-30 minutes.

Before you start

  • Try not to read ahead.
  • Do one task at a time. The trick is to learn to work incrementally.
  • Make sure you only test for correct inputs. There is no need to test for invalid inputs for this kata.

The kata

Step 1: the simplest thing

Create a simple String calculator with a method int add(String numbers).

  • The string argument can contain 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0) for example "" or "1" or "1,2".
  • Start with the simplest test case of an empty string and move to 1 and two numbers.
  • Remember to solve things as simply as possible so that you force yourself to write tests you did not think about.
  • Remember to refactor after each passing test.

Step 2: handle an unknown amount of numbers

Allow the add() method to handle an unknown amount of numbers.

Step 3: handle new lines between numbers

Allow the add() method to handle new lines between numbers (instead of commas).

  • the following input is ok: "1\n2,3" (will equal 6)
  • the following input is NOT ok: "1,\n" (not need to prove it - just clarifying)

Step 4: support different delimiters

Support different delimiters: to change a delimiter, the beginning of the string will contain a separate line that looks like this:

"//[delimiter]\n[numbers...]"

For example "//;\n1;2" should return 3 where the default delimiter is ';'.

The first line is optional. All existing scenarios should still be supported.

Step 5: negative numbers

Calling add() with a negative number will throw an exception "negatives not allowed" - and the negative that was passed.

For example add("1,4,-1") should throw an exception with the message "negatives not allowed: -1".

If there are multiple negatives, show all of them in the exception message.

Step 6: ignore big numbers

Numbers bigger than 1000 should be ignored, so adding 2 + 1001 = 2

Part 2 - Go further

Add logging

Add Logging Abilities to your new String Calculator (to an ILogger.Write()) interface (you will need a mock). Every time you call Add(), the sum result will be logged to the logger.

When calling Add() and the logger throws an exception, the string calculator should notify an IWebservice of some kind that logging has failed with the message from the logger's exception (you will need a mock and a stub).

A More Difficult Variation of the Kata

Create a program (test first) that uses string calculator, which the user can invoke through the terminal / console by calling scalc '1,2,3' and will output the following line before exiting: "The result is 6". Note that scalc is the name of the app executable.

Then, instead of exiting after the first result, the program will ask the user for "another input please" and print the result of the new user input out as well, until the user gives no input and just press enter (i.e. empty string): in that case it will exit.

General requirements

  • Use whatever language and frameworks you want. Use something that you know well.
  • Provide a README with instructions on how to compile and run the application.

IMPORTANT: Implement the requirements focusing on writing the best code you can produce.

Thanks to XPeppers organization and our friend XPepper to have created this shiny README file.
Credits also to Roy Osherove for the original idea.