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HP-12c Platinum
(feddit.uk)
HP's most accurate financial calculator, oddly enough, and despite only returning the ceiling of solve-for-n.
It is absolutely gorgeous, but I've always been confused by financial calculators. The buttons are all gibberish, and I have no idea what they do. I've got a TI-BA II Plus in my collection, but I think I've only turned it on four or five times 'cause I get scared.
I always thought of financial calculators as a sort of gelded single-function function machine and I didn't really get them. However, that would ignore the vast number of models available from HP, and that their second ever pocket calculator was a financial one. It turns out that solving the time value of money equation is non-trivial, and the work done on that probably paved the way for calculators with a solver.
The other thing I didn't appreciate until I had to use it, is that the interface of the 12c - with the 5 buttons in the top left for n, i, pv, pmt and fv is peak user interface. Press once to input data, but a second consecutive press of these buttons will trigger the solve and drop the result into x. It's perfect, and means you can solve and use all the calculator functions and stack continuously. Most modern methods use a table, which is hard to extract and input information from the calculator.
I'm definitely not saying that they are stupid machines, just that I'm too stupid to use them
I was too stupid to realise how hard they were to do right.