找到特定號碼的主要因素

Question

我寫的代碼有什麼問題？ 即時通訊試圖解決歐拉項目中的一個問題。

謝謝:)

我加引號的代碼可以讓你瞭解我的一切都做了。它適用於13915號碼，但不適用於我需要的號碼。

問題是，即時檢查數字被分爲3，雖然它不是一個完整的數字，但它仍然說它是一個素數。

例如：

3 is a prime factor of 600851475143

new number: 200283825047.67

new largest prime: 3

<?php 
/** 
* Created by PhpStorm. 
* User: gabia_000 
* Date: 13/08/2015 
* Time: 23:32 
* 
* Project euler - Question 3 ; 
* The prime factors of 13195 are 5, 7, 13 and 29. 
* What is the largest prime factor of the number 600851475143 ? ; 
*/ 

    $num = 600851475143; // The number that contains the factors ; 
    $largest = 1; // the largest prime factor ; 
    $n = 3; // the count of number starting 3 to find the prime factor ; 
    echo '<b>number - '.$num.'</b><br /><br />'; // DISPLAY ; 
    while ($num > 1) { // as long as the number isnt 1 yet ; 
     $prime = 1; // defult true - the $n is prime; 
     $i = 2; // number that the prime is divide by ; 
     while($i < $n && $prime == 1) { // as long as $n is bigger then $i and prime is still true ; 
      if($n % $i == 0) { // if $n is divided in $i ; 
       $prime = 0; // prime false ; 
       echo "{$n} not a prime, divided by {$i}.<br />"; // DISPLAY ; 
       break 1; // break the while ; 
      } 
      $i = $i + 1; // increase the $i by 1 ; 
     } 
     if ($prime) { // if the prime is still true ; 
      if($num % $n == 0) { // if the prime number is a factor of the number ; 

       echo "{$n} is a prime factor of {$num}<br />"; // DISPLAY ; 
       $num = $num/$n; // set the new number ; 
       echo " - new number: {$num}<br />"; // DISPLAY ; 
       $largest = $n; // set the new largest prime factor ; 
       echo " - new largest prime: {$largest}<br />"; // DISPLAY ; 

      } 
      else { // if the prime number isnt a factor of the number ; 
       echo "{$n} is not a prime factor of {$num}<br />"; // DISPLAY ; 
      } 
     } 
     $n = $n + 1; // increase the $n by 1 ; 
     if($n > $num) { // if the $n is bigger then the number is there is no prime factor ; 
      break ; // break the while ; 
      echo "<br /><br /><b>no prime factors for </b>{$num}}"; // DISPLAY ; 
     } 
    } 
    // DISPLAY END RESULTS ; 
    echo " 
     latest \$n - <b>{$n}</b><br /> 
     largest prime - <b>{$largest}</b><br /> 
     number - {$num} 
    "; 

Answer 1

您不必檢查素性每一步。 也許更簡單：

<?php 
/** 
* Created by PhpStorm. 
* User: gabia_000 
* Date: 13/08/2015 
* Time: 23:32 
* 
* Project euler - Question 3 ; 
* The prime factors of 13195 are 5, 7, 13 and 29. 
* What is the largest prime factor of the number 600851475143 ? ; 
*/ 

    $num = 600851475143; // The number that contains the factors ; 
    $num_save = $num; 
    $largest = 1; // the largest prime factor ; 
    $n = 2; // the count of number starting 3 to find the prime factor ; 
    echo '<b>number - '.$num.'</b><br /><br />'; // DISPLAY ; 


    while ($num > 1) { // as long as the number isnt 1 yet ; 

     if($num % $n == 0) { 
      $largest = $n; 

      while($num % $n == 0) { 
       $num = $num/$n; 
      } 
     } 
     $n = $n + 1; 
    } 
    echo " 
     latest \$n - <b>{$n}</b><br /> 
     largest prime - <b>{$largest}</b><br /> 
     number - {$num_save} 
    "; 

?>