A means of specifying an exact number without having to append zeroes. (It would be a nusiance in "significant digits" mode to see and utilize the full calculated value of 2*pi without having to enter '2.000000000000'.)
The calculator must track the number of significant digits of every result, imposing some extra overhead on the machine. Users, however, generally want it to provide the full-accuracy calculation, without analyzing results to determine what it signifies. In practice, if the operands don't have a decent number of significant figures (at least 4-5), the results often won't be very informative.
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To run a program, type gA, or gB, ... , gF. (When in HEX mode that gets you a hex numeral, so you have to type "g" twice to get the program when in HEX mode.)
To write a program, you have two options: 1. Simple programs can be recorded from the keyboard. To record program A, one would type g REC A, then keypress your program, then type g REC. 2. More complex programs can be written on a computer and input through a USB cable (sold separately).
Programmability (or entry of equations) is essential for your concept, due to SOLVE. Your proposed 7-segment display doesn't allow for equations.
Why not provide numerical integration along with rootfinding -- a pioneering design objective of the HP-34C of 1979 incorporated into the HP-15C of 1982?
If, e.g., "REC A" essentially does PRGM, then LBL A -- I see no particular means of editing a program once it is entered. Also not provided are the more-advanced HP-15C capabilities such as multiple entry points (via LBL), GOTO, subroutines, looping, conditional tests, and flags.
The capability of editing is essential for useful programming. That of the HP-12C and HP-10C is barely usable, due to the lack of insert/delete editing. I have a 1981 Casio whose built-in Simpson's Rule integration requires programmability. It offers 38 steps amongst two programs; steps are executed as they are entered and recorded (yielding possible math errors); programs are not displayable or editable -- a program must be deleted and re-entered if an error is made.
What would the format of USB-uploaded programs be?
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Complex numerals
More than just the ability to enter and calculate with complex numbers is needed. They must be displayable, and the germane utility functions -- or a means of doing them -- must be provided.
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If a seven segment display saves battery life, then I'm for it. Otherwise a good dot matrix display is better.
Dot-matrix displays aren't battery hogs -- time clocks, I/O, and peripherals are. Thin batteries such as the CR2032 also don't seem to last, I assume due to less volume available for chemicals.
Since you seem either to have, or have seen, an HP-15C, why not review this discussion of its functional, organized keyboard layout, and -- better yet -- read the fine Hewlett-Packard Journal article from May 1983, available on CD or DVD ROM from the MoHPC?
-- KS
Edited: 3 Oct 2009, 9:58 p.m. after one or more responses were posted