Permutations & Combinations for large inputs

[Werner]

(05-26-2015 05:14 PM)Dieter Wrote:  Since the program calculates n!/(n–r)! first (which always is an integer) the results should be exact here.

Only for n<16 I'm afraid.
eg. PERM(16,5) = 16!/(11!) = 524160.0001. There's 110 cases like this for n<70.
These can be corrected with 0.1 + INT.
Worst case is PERM(64,5) = 914941440.4

Then there are a few cases with a wrong result of exactly 10 significant digits:
PERM(31,9) = 7.315688014e12 (exact 7.315688016e12)
PERM(35,8) = 8.489642628e11 (exact 8.489642624e11)
PERM(38,7) = 6.360609025e10 (exact 6.360609024e10)
39 7
45 7
52 7
54 7
59 6
61 6
64 6
68 6

All of these are larger than 1e10 though, so you're at least able to deliver correct results less than 1e10