My suggestion: build a basic calculator then extend it... Add scientific and fixed formats. Add functions like log, sine, exp - especially if you create the

My suggestion: build a basic calculator then extend it... Add scientific and fixed formats. Add functions like log, sine, exp - especially if you create the math ops from scratch. Detect roundoff error and suppress it gracefully. Add binary, octal, and hex base numbers and binary, shift, rotate and bitwise operations. Add 2D graphs and 3D surfaces. Add control of illumination and stacking order for multiple plots. Add animation. Evaluate expressions. Add programmability. Or just invent your own private Emacs.