Iterative double factorial

Write an iterative function that returns the double factorial @n@ $!!$ for a natural $n$ .

Recall that $n!! = n \times (n - 2) \times (n - 4) \times \dots$ . For instance, $9!! = 9 \times 7 \times 5 \times 3 \times 1 = 945$ and $8!! = 8 \times 6 \times 4 \times 2 = 384$ . By definition, $0!! = 1!! = 1$ .

Interface

C++,C	`int double_factorial(int x);`
Java	`public static int doubleFactorial(int x);`
Python	`double_factorial(x) # returns int`
	`double_factorial(x: int) -> int`

Precondition

Assume $0\le$ @n@ $\le 19$ .

Observation

You only need to submit the required procedure; your main program will be ignored.

Problem information

Author: Unknown
Translator: Salvador Roura

Generation: 2026-01-25T10:19:33.423Z