Extra Credit $($try if you have extra time$)$

Develop a calculator APP that can perform four arithmetic operations: addition $+,$

subtraction $-,$ multiplication $*,$ and division $/.$ The GUI should look the same as the example

below.

A user can enter floating numbers and operations by clicking the

Buttons with corresponding digits, period, or operations. The

expression will show$/$update at the top Label of the APP after

each button is clicked.

When the Button $"="$ is clicked, the expression will be evaluated

to show the result.

The Button "CLEAR" will clear the display.

In addition, your program should be able to handle following error conditions:

$(1)$ when a user input starts with an operator, your code should add a zero before it$.$ For

example, the user starts with an empty display, then clicks $+,$ the display should show:

$0+$

$(2)$ when a user enters several operators, only the last one is shown and used. For example,

the user enters $5,$ then $*,$ then $+,$ the display should show: $5+$

$(3)$ when a user finishes the expression with an operator, and then clicks $"=",$ the last

operand will be repeated. For example, the user enters $"5\mathrm{.}1+{4\mathrm{.}3}^{**}=",$ the expression will

be changed to $"5\mathrm{.}1+4\mathrm{.}3**4\mathrm{.}3="$ for evaluation.

HINT: you can try the function eval$()$ to evaluate an arithmetic expression, e$.$g$.$ If the display

shows the string STR$=\text{'}2\mathrm{.}1+3\mathrm{.}4-1\mathrm{.}33\text{'},$ calling eval$($STR$)$ will evaluate the expression and

give the result of $4\mathrm{.}17.$