HP 50g Double factorial

[DavidM]

(01-27-2024 05:51 PM)FLISZT Wrote:  <<
{ 2 − } { 1 > } LWHL
1 + @ To insert the missing 1 in the list.
LPROD
>>

… "et voilà" as (some) English speakers say (sometimes).

0 returns 1
1 returns 1
2 returns 2
3 returns 3
4 returns 8
5 returns 15
…
14 yields 645 120
15 yields 2 027 025
etc.

In this case, you don't actually need the "1 +" after the LWHL command, since multiplying by 1 has no impact on the final result. It doesn't hurt anything for it to be there, of course. But it's not needed.

Note that this method isn't particularly good with large n, since it requires enough free memory for the entire list of multiplicands to be created en masse, and also the program essentially has to iterate the large list twice behind the scenes (once when building, then a second time for computing the product).

John Keith's example covers most everything you could want here, and is much more efficient. I just wanted to point out the use of LWHL and LPROD in this context, which shortens the code significantly. As is frequently the case, though, large lists take lots of memory and can be slow to process.