the most sophisticated of the voyager series, the 15c came out in 1982. i was so impressed, i bought this one as soon as it was available and generally regard it as one of hp's finest machines.

i recently replaced the 3 G13 batteries and now use it again as my everyday machine and still find it by far way quicker and easier to operate that any other calculator of this complexity, size and convenience.

unlike the 41c which had limited built-in functionality but vast expandability, the 15c has vast built-in functionality, competent programmability but no expandability.

the most impressive thing about this machine is that it comfortably handles complex numbers, matrices and even complex matrices just like they were another number on display; nothing else comes close, even today.

keyboard functions operate on any data type in a meaningful way. eg chs changes the sign of a number, conjugates a complex number or chs a matrix. even the 4 basic operators work happily on matrices as they would ordinary numbers, multiplying a matrix by a scalar is also no problem. dividing by a matrix is defined to invert and premultiply - just like you would want.

a bonus was the implementation of factorial as the gamma function, x! = gamma(x+1).

a full set of programming features are available, loops (dsz, isz), conditional jumps (lots with a handy reminder table on the back label), subroutines and labels. labels A thru E are on the keyboard itself allowing quick execution.

this model was one pioneering the implementation of solve and integrate numerically, i think the first was the hp34. the implementations used several tricks to make these operations stable. for example, using a modified romberg method for quadrature in order to track the error and avoid sampling the function at its endpoints. error tracking meant that the machine knew if its answer was on track or just garbage.

its biggest drawback was performance. programs ran too slow. this hampered the integrate and solve features because 30 seconds might only allow something like 15 samples which makes quadrature accuracy tricky. despite this, a lot could be achieved. the manual and the advanced functions handbook (well worth a read) showed how the user could manually rearrange harder problems so that the machine could get a quicker answer. for example by splitting up the range of definite integration or by variable substitution.

later, brute force took over and a simple implementation of simpson's rule can often suffice (see fx-3900pv and the torture test). even some later hp models take this route (eg hp20s).

example program

here is an example program for the 15c. routine `A' tries to determine if the currently displayed number (ie in X) is prime. it returns 1 if prime, 0 if composite.

lblA fix0 sto1 2 x>y? gto3 x=y? gto4 0 sto6 sto8 rcl1 1 - sto7 lbl5 rcl7 rcl7 2
/ int sto9 2 * x<>y?  gto6 1 sto+6 rcl9 sto7 gto5 lbl6 rcl6 x=0? gto3 1 sto-6
lbl7 2 sto+8 rcl8 7 x=y? gto4 3 - x=y? dse8 rcl8 sto2 rcl1 x=y? gto4 1 sto3
rcl7 sto9 lbl8 rcl9 rcl9 2 / int sto9 2 * x<>y? gsbB rcl9 x=0? gto.0 rcl3 sto0
rcl2 sto3 gsbB sto2 rcl0 sto3 gto8 lbl.0 rcl3 1 x=y? gto7 chs rcl1 + x=y? gto7
rcl6 sto9 lbl.1 rcl9 x=0? gto3 rcl3 sto2 gsbB rcl1 1 - x=y? gto7 1 sto-9 gto.1
lbl4 1 rtn lbl3 0 rtn
lblB rcl2 rcl3 * rcl1 / frac rcl1 * rnd sto3 rtn

this is an attempt to implement the rabin-miller strong pseudoprime test. using bases 2, 3 and 5. the first strong pseudoprime concurrent to all three bases is 25,326,001. the program should be modified to reject this special case. however subroutine `B' limits the possible input range to x < 10^6 because it requires computing x*y (mod m) and so the given exception is out of range.

unfortunately x*y (mod m) is a tricky problem when x, y & m can be your machine precision. see the secret life of the mod function. it is possible, however, by writing your own multiply, thus,

lblB rcl2 sto5 0 sto4 lbl1 rcl3 rcl3 2 / int sto3 2 * x==y?  gto.3 rcl5 sto+4
rcl4 rcl1 x<y?  sto-4 lbl.3 rcl3 x==0? gto2 2 sto*5 rcl5 rcl1 x<y? sto-5 gto1
lbl2 rcl4 sto3 rtn

this has the same effect as the original lblB routine above, but the multiply is written out so as to affect the modulo as it goes and thus prevent overflow.

the bad news. its too slow for the 15c. in theory it should cope with larger numbers, the practice is that it gets too slow to be useful as the numbers increase in magnitude.