nPk ( n k -- nPk )

Vocabulary

math.combinatorics

Inputs

n	a non-negative integer
k	a non-negative integer

Outputs

nPk	an integer

Word description

Outputs the total number of unique permutations of size k (order does matter) that can be taken from a set of size n.

Examples

USING: math.combinatorics prettyprint ; 10 4 nPk .
5040

Definition

USING: kernel math math.combinatorics.private ranges sequences ;

IN: math.combinatorics

: nPk ( n k -- nPk )
    2dup possible? [ dupd - [a..b) product ] [ 2drop 0 ] if ;