# 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:

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

## Errors management

Introduce an internal add function returning a number instead of a String, and test many solutions for the error messages.

• Exception