# Calculator with reducer

> Refactor the calculator from the previous useState implementation to useReducer, maintaining all functionality and edge case handling.

- Difficulty: intermediate
- Tags: useState, useReducer
- Estimated time: 30 minutes
- Tests: 16
- Requires subscription: no

## Overview

Refactor the calculator from the previous useState implementation to useReducer, maintaining all functionality and edge case handling.


## Requirements

- Display the current expression as the user types.
- Evaluate mathematical expressions correctly when the equals button is pressed.
- Support the four basic operations: addition, subtraction, multiplication, and division.
- Allow decimal numbers and negative numbers.
- Prevent invalid inputs or sequences:
  - The expression cannot start with `+`, `*`, or `/`.
  - The expression can start with `-` to represent a negative number.
  - Disallow consecutive operators except when using `-` to indicate a negative number.
  - Avoid multiple dots within the same number.
  - Prevent evaluation when the last element is an operator or when the expression is empty.
- Handle negative numbers properly after any operator.
- Prevent more than two consecutive `-` signs.
- Replace leading zeros when typing new numbers (e.g., `03` → `3`).
- Allow chaining of operations and continuous usage after evaluating a result:
  - If a result is displayed and the user presses an operator, continue from that result.
  - If a result is displayed and the user presses a number, start a new expression.
- Clear the display and reset the calculator when pressing `C`.


## Notes

- Implemented using useReducer instead of useState. Maintain all previous functionality and edge case handling from the prior calculator challenge.
- The display should show either the full expression or the result, depending on context.
- Expressions must update immediately as buttons are pressed.
- Consecutive operators must be handled gracefully (e.g., replacing the previous one if needed).
- Negative numbers after operators (e.g., `5*-2`) must be valid.
- The calculator should handle decimal precision correctly (round results to two decimals if needed).
- Clearing should reset both the expression and the result.


## Tests

01. basic sum operation
02. more complex sum operation
03. does not allow '+' as the first value
04. does not allow '*' as the first value
05. does not allow '/' as the first value
06. allows '-' as the first value (negative numbers)
07. allow negative numbers after any operator
08. prevents more than two consecutive '-' and allows negative numbers correctly
09. handle leading zeros correctly
10. if there is a result and a number or operator is pressed
11. prevent multiple dots in the same number
12. prevent consecutive operators
13. concatenate numbers/dots if the last element is a number
14. prevent evaluating if the last element is an operator
15. prevent evaluating if is empty
16. clear