FactDouble
Description
This function is used to calculate the double factorial of a non-negative integer n, denoted by:
- \[n!! = \begin{cases} n*(n-2)...5*3*1, & \mbox{if }n \mbox{ is odd} \\ n*(n-2)...6*4*2, & \mbox{if }n \mbox{ is even} \\ 1, & n=0 \end{cases} \]
Please note that, to avoid overflow, the biggest integer can be specified is 299, if bigger than 299, this function will return missing value directly.
Syntax
double FactDouble(int n)
Parameters
n
- The non-negative number for calculating double factorial.
Return
Return the double factorial of the specified number.
Example
factdouble(5) = ; // 5*3*1=15 factdouble(6) = ; // 6*4*2=48 factdouble(0) = ; // 1 factdouble(1) = ; // 1