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String Calculator

This classic kata guides you step by step through the implementation of a calculator that receives a String as input. It is a good exercise on refactoring and incremental implementation. It is also a good candidate for practising TDD.

First step

Create a function add that takes a String and returns a String:

String add(String number)
  • The method can take 0, 1 or 2 numbers separated by comma, and returns their sum.
  • An empty string will return “0”.
  • Example of inputs: "", "1", "1.1,2.2".

Many numbers

Allow the add method to handle an unknow number of arguments.

Newline as separator

Allow the add method to handle newlines as separators:

  • "1\n2,3" should return "6".
  • "175.2,\n35" is invalid and should return the message "Number expected but '\n' found at position 6."

Missing number in last position

Don’t allow the input to end in a separator.

"1,3," is invalid and should return the message Number expected but EOF found.

Custom separators

Allow the add method to handle a different delimiter. To change the delimiter, the beginning of the input will contain a separate line that looks like this:

//[delimiter]\n[numbers]
  • "//;\n1;2" should return "3"
  • "//|\n1|2|3" should return "6"
  • "//sep\n2sep3" should return "5"
  • "//|\n1|2,3" is invalid and should return the message "'|' expected but ',' found at position 3."

All existing scenarios should work as before.

Negative numbers

Calling add with negative numbers will return the message "Negative not allowed : " listing all negative numbers that were in the list of numbers.

  • "-1,2" is invalid and should return the message "Negative not allowed : -1"
  • "2,-4,-5" is invalid and should return the message "Negative not allowed : -4, -5"

Multiple errors

Calling add with multiple errors will return all error messages separated by newlines.

"-1,,2" is invalid and return the message "Negative not allowed : -1\nNumber expected but ',' found at position 3."

Errors management

Introduce an internal add function returning a number instead of a String, and test many solutions for the error messages. - Exception - maybe and monad approch - POSIX return code with message managemement - tuple with error struct like in Go - etc.

Other operations

Write a function for multiply with same rules