Permutations & Combinations for large inputs

[Ángel Martin]

(05-27-2015 12:27 PM)Werner Wrote:  

(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

The NCR and NPR functions on the SandMath can be a good reference. Since they use internal 13-digit accuracy the results are pretty much right on, always integers (no, there isn't any rounding done by the function in case you wonder).

But I'm positive Dieter will find a counter-example to this rule as soon as he starts using the SandMath on V41 ;-)

Cheers,
'AM