**Double factorial** of a non-negative integer n, is the product of all the integers from 1 to n that have the same parity (odd or even) as n. It is also called as **semifactorial** of a number and is denoted by **!!**. For example, double factorial of 9 is 9*7*5*3*1 which is 945. Note that, a consequence of this definition is 0!! = 1.

**Examples:**

Input:6Output:48 Note that 6*4*2 = 48Input:7Output:105 Note that 7*5*3 = 105

For even n, the double factorial is:

For odd n, the double factorial is:

**Recursive Solution:**

Double factorial can be calculated using following recursive formula.

n!! = n * (n-2)!! n!! = 1 if n = 0 or n = 1

Following is the implementation of double factorial.

## C++

`#include<stdio.h> ` ` ` `// function to find double factorial of given number ` `unsigned ` `int` `doublefactorial(unsigned ` `int` `n) ` `{ ` ` ` `if` `(n == 0 || n==1) ` ` ` `return` `1; ` ` ` `return` `n*doublefactorial(n-2); ` `} ` ` ` `int` `main() ` `{ ` ` ` `printf` `(` `"Double factorial is %d"` `, doublefactorial(5)); ` ` ` `return` `0; ` `} ` |

## Java

`import` `java.io.*; ` ` ` `class` `GFG { ` ` ` ` ` `// function to find double factorial ` ` ` `// of given number ` ` ` `static` `long` `doublefactorial(` `long` `n) ` ` ` `{ ` ` ` `if` `(n == ` `0` `|| n==` `1` `) ` ` ` `return` `1` `; ` ` ` ` ` `return` `n * doublefactorial(n - ` `2` `); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `static` `public` `void` `main (String[] args) ` ` ` `{ ` ` ` `System.out.println(` `"Double factorial"` ` ` `+ ` `" is "` `+ doublefactorial(` `5` `)); ` ` ` `} ` `} ` ` ` `// This code is contributed by anuj_67. ` |

## Python3

`# function to find double ` `# factorial of given number ` `def` `doublefactorial(n): ` ` ` ` ` `if` `(n ` `=` `=` `0` `or` `n ` `=` `=` `1` `): ` ` ` `return` `1` `; ` ` ` `return` `n ` `*` `doublefactorial(n ` `-` `2` `); ` ` ` `# Driver Code ` `print` `(` `"Double factorial is"` `, ` ` ` `doublefactorial(` `5` `)); ` ` ` `# This code is contributed ` `# by Smitha ` |

## C#

`using` `System; ` ` ` `class` `GFG { ` ` ` ` ` `// function to find double factorial ` ` ` `// of given number ` ` ` `static` `uint` `doublefactorial(` `uint` `n) ` ` ` `{ ` ` ` `if` `(n == 0 || n==1) ` ` ` `return` `1; ` ` ` ` ` `return` `n * doublefactorial(n - 2); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `static` `public` `void` `Main () ` ` ` `{ ` ` ` `Console.WriteLine(` `"Double factorial"` ` ` `+ ` `" is "` `+ doublefactorial(5)); ` ` ` `} ` `} ` ` ` `// This code is contributed by anuj_67. ` |

## PHP

`<?php ` `// PHP code for ` `// Double factorial ` ` ` `// function return ` `// double factorial ` `function` `doublefactorial(` `$n` `) ` `{ ` ` ` `if` `(` `$n` `== 0 || ` `$n` `==1) ` ` ` `return` `1; ` ` ` `return` `$n` `* doublefactorial(` `$n` `- 2); ` `} ` ` ` ` ` `// Driver Code ` ` ` `echo` `"Double factorial is "` `, ` ` ` `doublefactorial(5); ` ` ` `// This code is contributed by Ajit. ` `?> ` |

Output:

Double factorial is 15

**Iterative Solution:**

Double factorial can also be calculated iteratively as recursion can be costly for large numbers.

## C++

`#include<stdio.h> ` ` ` `// function to find double factorial of given number ` `unsigned ` `int` `doublefactorial(unsigned ` `int` `n) ` `{ ` ` ` `int` `res = 1; ` ` ` `for` `(` `int` `i=n; i>=0; i=i-2) ` ` ` `{ ` ` ` `if` `(i==0 || i==1) ` ` ` `return` `res; ` ` ` `else` ` ` `res *= i; ` ` ` `} ` `} ` ` ` `int` `main() ` `{ ` ` ` `printf` `(` `"Double factorial is %d"` `, doublefactorial(5)); ` ` ` `return` `0; ` `} ` |

## Java

`// Java Program to find double factorial ` `// of given number ` `import` `java .io.*; ` ` ` `class` `GFG { ` ` ` ` ` `// function to find double factorial ` ` ` `// of given number ` ` ` `static` `int` `doublefactorial(` `int` `n) ` ` ` `{ ` ` ` `int` `res = ` `1` `; ` ` ` `for` `(` `int` `i = n; i >= ` `0` `; i = i-` `2` `) ` ` ` `{ ` ` ` `if` `(i == ` `0` `|| i == ` `1` `) ` ` ` `return` `res; ` ` ` `else` ` ` `res *= i; ` ` ` `} ` ` ` ` ` `return` `res; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main(String[] args) ` ` ` `{ ` ` ` `System.out.println(` `"Double factorial"` ` ` `+ ` `" is "` `+ doublefactorial(` `5` `)); ` ` ` `} ` `} ` ` ` `// This code is Contributed by Anuj_67 ` |

## C#

`// C# Program to find double factorial ` `// of given number ` `using` `System; ` ` ` `class` `GFG { ` ` ` ` ` `// function to find double factorial ` ` ` `// of given number ` ` ` `static` `int` `doublefactorial(` `int` `n) ` ` ` `{ ` ` ` `int` `res = 1; ` ` ` `for` `(` `int` `i = n; i >= 0; i = i-2) ` ` ` `{ ` ` ` `if` `(i == 0 || i == 1) ` ` ` `return` `res; ` ` ` `else` ` ` `res *= i; ` ` ` `} ` ` ` ` ` `return` `res; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `static` `void` `Main() ` ` ` `{ ` ` ` `Console.Write(` `"Double factorial"` ` ` `+ ` `" is "` `+ doublefactorial(5)); ` ` ` `} ` `} ` ` ` `// This code is Contributed by Anuj_67 ` |

## PHP

`<?php ` ` ` `// function to find double ` `// factorial of given number ` `function` `doublefactorial( ` `$n` `) ` `{ ` ` ` `$res` `= 1; ` ` ` `for` `(` `$i` `= ` `$n` `; ` `$i` `>= 0; ` `$i` `= ` `$i` `- 2) ` ` ` `{ ` ` ` `if` `(` `$i` `== 0 ` `or` `$i` `== 1) ` ` ` `return` `$res` `; ` ` ` `else` ` ` `$res` `*= ` `$i` `; ` ` ` `} ` `} ` ` ` ` ` `// Driver Code ` ` ` `echo` `"Double factorial is "` `, doublefactorial(5); ` ` ` `// This code is contributed by anuj_67. ` `?> ` |

Output:

Double factorial is 15

Time complexity of the above solutions is O(n).

**Important Points :**

- Double factorial and factorial are related using below formula.
**Note :**n!! means double factorial. If n is even, i.e., n = 2k n!! = 2^{k}k! Else (n = 2k + 1) n!! = (2k)! / 2^{k}k! - Double factorial is frequently used in combinatorics. Refer wiki for list of applications. An example application is count of perfect matchings of a complete graph K
_{n+1}for odd n.

**References:**

https://en.wikipedia.org/wiki/Double_factorial

This article is contributed by **Rahul Agrawal**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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