String Calculator
Level: Intermediate 30–60 minConcepts: Strings
Solutions: C# | TypeScript | Python
Create a simple String calculator with a method that takes a string of numbers and returns their sum.
Requirements
- The method can take 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0).
- Allow the Add method to handle an unknown amount of numbers.
- Allow the Add method to handle new lines between numbers (instead of commas).
- Support different delimiters by starting the string with ”//[delimiter]\n[numbers…]”.
- Calling Add with a negative number will throw an exception “negatives not allowed” - and the negative that was passed.
- Numbers bigger than 1000 should be ignored.
- Delimiters can be of any length with the following format: ”//[delimiter]\n”.
- Allow multiple delimiters like this: ”//[delim1][delim2]\n”.
- Make sure you can also handle multiple delimiters with length longer than one char.
Test Cases
| Input | Result | Notes |
|---|---|---|
| "" | 0 | Empty string returns 0 |
| ”1” | 1 | Single number returns itself |
| ”1,2” | 3 | Two numbers separated by comma |
| ”1,2,3” | 6 | Multiple numbers separated by comma |
| ”1\n2,3” | 6 | New line as delimiter |
| ”//;\n1;2” | 3 | Custom delimiter |
| ”-1,2” | Exception: “negatives not allowed: -1” | Negative numbers not allowed |
| ”2,1001” | 2 | Numbers > 1000 are ignored |
| ”//[]\n12***3” | 6 | Delimiter of any length |
| ”//[][%]\n12%3” | 6 | Multiple delimiters |
Tips
- Start with the simplest test case first (empty string)
- Add one requirement at a time
- Refactor after each passing test
- Consider edge cases like invalid input
- Think about how to handle the delimiter parsing
Reference Walkthrough
Full C#, TypeScript, and Python implementations live at tddbuddy-reference-katas/string-calculator. This is a Pedagogy mode kata: the commit log is the teaching, stepping through the TDD cycle commit-by-commit — red, green, refactor, reflect — with the gear visibly shifting from low (fake-it, triangulate) to middle (one scenario per cycle) to high (obvious implementation; later scenarios pass on arrival).
- C# (.NET 8, xUnit, FluentAssertions) — walkthrough
- TypeScript (Node 20, Vitest, strict types) — walkthrough
- Python (3.11, pytest) — walkthrough
Unlike Bank Account (which ships in middle gear, one commit per language), String Calculator ships as the TDD arc itself: roughly twenty commits per language, each labeled with its cycle phase and gear. See the repo’s Gears section for why the gear shift is the real teaching point.