#19Calculator

Overview

Build a simple calculator that supports basic arithmetic operations, decimal numbers, and negative values, while handling edge cases in user input.

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

• 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).
• Avoid any syntax errors or invalid evaluations when using eval or equivalent.
• Clearing should reset both the expression and the result.

Tests

1. basic sum operation
2. more complex sum operation
3. does not allow '+' as the first value
4. does not allow '*' as the first value
5. does not allow '/' as the first value
6. allows '-' as the first value (negative numbers)
7. allow negative numbers after any operator
8. prevents more than two consecutive '-' and allows negative numbers correctly
9. 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