# Efficient program to print all prime factors of a given number

Given a number n, write an efficient function to print all prime factors of n. For example, if the input number is 12, then output should be “2 2 3”. And if the input number is 315, then output should be “3 3 5 7”.

Following are the steps to find all prime factors. **1)** While n is divisible by 2, print 2 and divide n by 2. **2)** After step 1, n must be odd. Now start a loop from i = 3 to square root of n. While i divides n, print i and divide n by i. After i fails to divide n, increment i by 2 and continue. **3)** If n is a prime number and is greater than 2, then n will not become 1 by above two steps. So print n if it is greater than 2.

## C++

`// C++ program to print all prime factors` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// A function to print all prime` `// factors of a given number n` `void` `primeFactors(` `int` `n)` `{` ` ` `// Print the number of 2s that divide n` ` ` `while` `(n % 2 == 0)` ` ` `{` ` ` `cout << 2 << ` `" "` `;` ` ` `n = n/2;` ` ` `}` ` ` `// n must be odd at this point. So we can skip` ` ` `// one element (Note i = i +2)` ` ` `for` `(` `int` `i = 3; i <= ` `sqrt` `(n); i = i + 2)` ` ` `{` ` ` `// While i divides n, print i and divide n` ` ` `while` `(n % i == 0)` ` ` `{` ` ` `cout << i << ` `" "` `;` ` ` `n = n/i;` ` ` `}` ` ` `}` ` ` `// This condition is to handle the case when n` ` ` `// is a prime number greater than 2` ` ` `if` `(n > 2)` ` ` `cout << n << ` `" "` `;` `}` `/* Driver code */` `int` `main()` `{` ` ` `int` `n = 315;` ` ` `primeFactors(n);` ` ` `return` `0;` `}` `// This is code is contributed by rathbhupendra` |

## C

`// Program to print all prime factors` `# include <stdio.h>` `# include <math.h>` `// A function to print all prime factors of a given number n` `void` `primeFactors(` `int` `n)` `{` ` ` `// Print the number of 2s that divide n` ` ` `while` `(n%2 == 0)` ` ` `{` ` ` `printf` `(` `"%d "` `, 2);` ` ` `n = n/2;` ` ` `}` ` ` `// n must be odd at this point. So we can skip` ` ` `// one element (Note i = i +2)` ` ` `for` `(` `int` `i = 3; i <= ` `sqrt` `(n); i = i+2)` ` ` `{` ` ` `// While i divides n, print i and divide n` ` ` `while` `(n%i == 0)` ` ` `{` ` ` `printf` `(` `"%d "` `, i);` ` ` `n = n/i;` ` ` `}` ` ` `}` ` ` `// This condition is to handle the case when n` ` ` `// is a prime number greater than 2` ` ` `if` `(n > 2)` ` ` `printf` `(` `"%d "` `, n);` `}` `/* Driver program to test above function */` `int` `main()` `{` ` ` `int` `n = 315;` ` ` `primeFactors(n);` ` ` `return` `0;` `}` |

## Java

`// Program to print all prime factors` `import` `java.io.*;` `import` `java.lang.Math;` `class` `GFG` `{` ` ` `// A function to print all prime factors` ` ` `// of a given number n` ` ` `public` `static` `void` `primeFactors(` `int` `n)` ` ` `{` ` ` `// Print the number of 2s that divide n` ` ` `while` `(n%` `2` `==` `0` `)` ` ` `{` ` ` `System.out.print(` `2` `+ ` `" "` `);` ` ` `n /= ` `2` `;` ` ` `}` ` ` `// n must be odd at this point. So we can` ` ` `// skip one element (Note i = i +2)` ` ` `for` `(` `int` `i = ` `3` `; i <= Math.sqrt(n); i+= ` `2` `)` ` ` `{` ` ` `// While i divides n, print i and divide n` ` ` `while` `(n%i == ` `0` `)` ` ` `{` ` ` `System.out.print(i + ` `" "` `);` ` ` `n /= i;` ` ` `}` ` ` `}` ` ` `// This condition is to handle the case whien` ` ` `// n is a prime number greater than 2` ` ` `if` `(n > ` `2` `)` ` ` `System.out.print(n);` ` ` `}` ` ` `public` `static` `void` `main (String[] args)` ` ` `{` ` ` `int` `n = ` `315` `;` ` ` `primeFactors(n);` ` ` `}` `}` |

## Python

`# Python program to print prime factors` `import` `math` `# A function to print all prime factors of` `# a given number n` `def` `primeFactors(n):` ` ` ` ` `# Print the number of two's that divide n` ` ` `while` `n ` `%` `2` `=` `=` `0` `:` ` ` `print` `2` `,` ` ` `n ` `=` `n ` `/` `2` ` ` ` ` `# n must be odd at this point` ` ` `# so a skip of 2 ( i = i + 2) can be used` ` ` `for` `i ` `in` `range` `(` `3` `,` `int` `(math.sqrt(n))` `+` `1` `,` `2` `):` ` ` ` ` `# while i divides n , print i ad divide n` ` ` `while` `n ` `%` `i` `=` `=` `0` `:` ` ` `print` `i,` ` ` `n ` `=` `n ` `/` `i` ` ` ` ` `# Condition if n is a prime` ` ` `# number greater than 2` ` ` `if` `n > ` `2` `:` ` ` `print` `n` ` ` `# Driver Program to test above function` `n ` `=` `315` `primeFactors(n)` `# This code is contributed by Harshit Agrawal` |

## C#

`// C# Program to print all prime factors` `using` `System;` `namespace` `prime` `{` `public` `class` `GFG` `{ ` ` ` ` ` `// A function to print all prime` ` ` `// factors of a given number n` ` ` `public` `static` `void` `primeFactors(` `int` `n)` ` ` `{` ` ` `// Print the number of 2s that divide n` ` ` `while` `(n % 2 == 0)` ` ` `{` ` ` `Console.Write(2 + ` `" "` `);` ` ` `n /= 2;` ` ` `}` ` ` `// n must be odd at this point. So we can` ` ` `// skip one element (Note i = i +2)` ` ` `for` `(` `int` `i = 3; i <= Math.Sqrt(n); i+= 2)` ` ` `{` ` ` `// While i divides n, print i and divide n` ` ` `while` `(n % i == 0)` ` ` `{` ` ` `Console.Write(i + ` `" "` `);` ` ` `n /= i;` ` ` `}` ` ` `}` ` ` `// This condition is to handle the case whien` ` ` `// n is a prime number greater than 2` ` ` `if` `(n > 2)` ` ` `Console.Write(n);` ` ` `}` ` ` ` ` `// Driver Code` ` ` `public` `static` `void` `Main()` ` ` `{` ` ` `int` `n = 315;` ` ` `primeFactors(n);` ` ` `}` `}` `}` `// This code is contributed by Sam007` |

## PHP

`<?php` `// PHP Efficient program to print all` `// prime factors of a given number` `// function to print all prime` `// factors of a given number n` `function` `primeFactors(` `$n` `)` `{` ` ` ` ` `// Print the number of` ` ` `// 2s that divide n` ` ` `while` `(` `$n` `% 2 == 0)` ` ` `{` ` ` `echo` `2,` `" "` `;` ` ` `$n` `= ` `$n` `/ 2;` ` ` `}` ` ` `// n must be odd at this` ` ` `// point. So we can skip` ` ` `// one element (Note i = i +2)` ` ` `for` `(` `$i` `= 3; ` `$i` `<= sqrt(` `$n` `);` ` ` `$i` `= ` `$i` `+ 2)` ` ` `{` ` ` ` ` `// While i divides n,` ` ` `// print i and divide n` ` ` `while` `(` `$n` `% ` `$i` `== 0)` ` ` `{` ` ` `echo` `$i` `,` `" "` `;` ` ` `$n` `= ` `$n` `/ ` `$i` `;` ` ` `}` ` ` `}` ` ` `// This condition is to` ` ` `// handle the case when n` ` ` `// is a prime number greater` ` ` `// than 2` ` ` `if` `(` `$n` `> 2)` ` ` `echo` `$n` `,` `" "` `;` `}` ` ` `// Driver Code` ` ` `$n` `= 315;` ` ` `primeFactors(` `$n` `);` `// This code is contributed by aj_36` `?>` |

## Javascript

`<script>` `// JavaScript program to print all prime factors` `// A function to print all prime` `// factors of a given number n` `function` `primeFactors(n)` `{` ` ` ` ` `// Print the number of 2s that divide n` ` ` `while` `(n % 2 == 0)` ` ` `{` ` ` `document.write(2 + ` `" "` `);` ` ` `n = Math.floor(n / 2);` ` ` `}` ` ` `// n must be odd at this point.` ` ` `// So we can skip one element` ` ` `// (Note i = i +2)` ` ` `for` `(let i = 3;` ` ` `i <= Math.floor(Math.sqrt(n));` ` ` `i = i + 2)` ` ` `{` ` ` ` ` `// While i divides n, print i` ` ` `// and divide n` ` ` `while` `(n % i == 0)` ` ` `{` ` ` `document.write(i + ` `" "` `);` ` ` `n = Math.floor(n / i);` ` ` `}` ` ` `}` ` ` `// This condition is to handle the` ` ` `// case when n is a prime number` ` ` `// greater than 2` ` ` `if` `(n > 2)` ` ` `document.write(n + ` `" "` `);` `}` `// Driver code` `let n = 315;` `primeFactors(n);` `// This code is contributed by Surbhi Tyagi.` `</script>` |

Output:

3 3 5 7

**How does this work?**

The steps 1 and 2 take care of composite numbers and step 3 takes care of prime numbers. To prove that the complete algorithm works, we need to prove that steps 1 and 2 actually take care of composite numbers. This is clear that step 1 takes care of even numbers. And after step 1, all remaining prime factor must be odd (difference of two prime factors must be at least 2), this explains why i is incremented by 2.

Now the main part is, the loop runs till square root of n not till n. To prove that this optimization works, let us consider the following property of composite numbers.

*Every composite number has at least one prime factor less than or equal to square root of itself.*

This property can be proved using counter statement. Let a and b be two factors of n such that a*b = n. If both are greater than √n, then a.b > √n, * √n, which contradicts the expression “a * b = n”.

In step 2 of the above algorithm, we run a loop and do following in loop

a) Find the least prime factor i (must be less than √n,)

b) Remove all occurrences i from n by repeatedly dividing n by i.

c) Repeat steps a and b for divided n and i = i + 2. The steps a and b are repeated till n becomes either 1 or a prime number.

**Related Article : **

Prime Factorization using Sieve O(log n) for multiple queries

Thanks to **Vishwas Garg** for suggesting the above algorithm. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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