Basic Calculator

A basic calculator for the general public.
19 buttons are classed to four categories including digital, control, operations, and compute buttons, respectively.
(1) Digital buttons: 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, and ..
(2) Control buttons: (, ), and clear.
(3) Operational buttons: +, -, *, and /.
(4) Compute button: =.
It is noted that every single button is not always available. An available button is without transparency background and button color looked gray. Comparatively, an unavailable button is with transparency background and button color is nearly black.