HP 15C Limited Edition RPN Programmable Scientific Calculator.

My dad asked what I wanted for Christmas, and I thought for a while.
What would I use on a daily basis in college, and enjoy doing so?

The HP 15C was my answer.

First made from 1982-1989 and then released as a limited edition in 2011, with only 10,000 made, this scientific calculator was a standard for scientists and engineers everywhere. It's a cousin of the famous HP 12C financial calculator, which my dad happens to use almost daily.

In operation, it uses HP's famed "RPN" input method: Reverse Polish Notation. Rather than inputting the operator, then the operation, then the operand, and then the equals button, you enter the operator, the operand, and then the operation. Example:

Problem: 2*3=?

RPN: 2 [enter] 3 [x]
Algebraic (normal) input: 2 [x] 3 [=] 6

This is advantageous because it affords fewer keystrokes, 4 vs. 5. The advantage is even more apparent when looking at more complex calculations.

Problem: (2+3)(5-1)=?

RPN: 2 [enter] 3 [+] 5 [enter] 1 [-] [x]
Algebraic: [(] 2 [+] 3 [)] [x] [(] 5 [-] 1 [)] [=]

9 vs. 12 keystrokes.

And it's just more fun to use!

This calculator can do numeric integration, complex calculations, matrix algebra, equation solving, and can execute programs. Overall a fun and useful tool.