#66MediumFunctions· State and Program Design

Calculator Engine for a Tkinter GUI

A Tkinter calculator is two parts: buttons and a display (the GUI), and the engine that decides what the display shows after each button press. The GUI only forwards each press to the engine, so the engine is where the logic, and the bugs, live. Build the engine. Write `press_keys(keys)` that starts with the display `"0"` and returns the display after every key. Keys are the digits `"0"`–`"9"`, `"."`, `"+"`, `"-"`, `"*"`, `"/"`, `"="` and `"C"`: - **Digits** start a new number if the display shows a finished result, an operator was just pressed, or the display is `"Error"`. Otherwise they are appended, except that a lone `"0"` is replaced. - **`.`** starts `"0."` in the same situations a digit would start a new number; otherwise it is appended only if the number has no dot yet. - **Operators**: with no operator pending, the number on the display becomes the left operand. If an operator is pending and a second number has been entered, calculate first and show the result (so `2 + 3 *` shows `5`). Pressing operators one after another only replaces the pending operator. - **`=`** calculates when an operator is pending and a second number has been entered; otherwise it does nothing. - **`C`** resets everything to `"0"`. - Dividing by zero shows `"Error"` and clears the pending calculation. While `"Error"` is shown, operators and `=` are ignored. Show results without a trailing `.0` for whole numbers; otherwise round to at most 10 decimal places and remove trailing zeros (so 0.1 + 0.2 shows `0.3`).

Input format

`keys`: a list of single-character strings.

Output format

A list of display strings, one after each key.

Examples

Input: keys = ["1","2","+","3","*","2","="]

Output: ["1","12","12","3","15","2","30"]

`12 + 3` is calculated when `*` is pressed (15), then `15 * 2 = 30`.

Input: keys = ["7","/","0","=","5"]

Output: ["7","7","0","Error","5"]

Division by zero shows Error; the next digit starts a fresh number.

Constraints

0 <= len(keys) <= 1000
Results stay below 10^12 in absolute value

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Functions in Python

#python
#state-machine
#gui
#tkinter
#calculator