Re: Help with nchoosek

[Julien Bect]

Le 31/08/2014 05:43, Rik a écrit :
> nchoosek ([4 5], 8)

With Matlab R2012a:

-----------------------------------------------------------------
>> nchoosek (5, 8)

Error using nchoosek (line 48)
K must be an integer between 0 and N.

>> nchoosek ([4 5], 8)

ans =

   Empty matrix: 0-by-8
-----------------------------------------------------------------

I find this behaviour of Matlab rather unfortunate. Actually, I would 
have defined the output to be 0 and [0 0].

Having nchoosek (n, k) be 0 when k > n is a very useful convention that 
make lots of combinatorial formulas easier to write (and prove).

This is for instance the behaviour of R:

-----------------------------------------------------------------
> choose (5, 8)
[1] 0
> choose (c(4, 5), 8)
[1] 0 0
-----------------------------------------------------------------

and Python + SciPy:

-----------------------------------------------------------------
>>> from scipy.special import binom
>>> binom(5, 8)
0.0
>>> binom((4, 5), 8)
array([ 0.,  0.])
-----------------------------------------------------------------

@++
Julien