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POProject Euler challenge 3: Finding the largest prime factor of a large number
 

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  1. POProject Euler challenge 3: Finding the largest prime factor of a large number
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    <p>Can't find the prime factor of 600851475143 for projecteuler. My code successfully computes the largest prime factor of the test number 13195 and every test number I throw at it, but somehow it degrades with the large prime number. Do you know why?</p> <pre><code>#include &lt;iostream&gt; #include &lt;queue&gt; using namespace std; int split(int split); int largestprimefactor(priority_queue&lt;int&gt; myints); int main() { int response = 2; do{ priority_queue&lt;int&gt; myints; int number; cout &lt;&lt; "Please enter a number: "; cin &gt;&gt; number; myints.push(number); int lcf = largestprimefactor(myints); cout &lt;&lt; endl &lt;&lt; "Largest prime factor is: " &lt;&lt; lcf; cout &lt;&lt; endl &lt;&lt; "Again?(1 for yes 2 for no): "; cin &gt;&gt; response; }while(response == 1); } uint64_t split(uint64_t split) { if(split%2 != 0) { if((split/2))%2 == 0) for(uint64_t i = (split/2)-1; i&gt;1; i=i-2) if(split%i == 0) return i; else for(uint64_t i = (split/2); i&gt;1; i=i-2) if(split%i == 0) return i; return 1; } else return 2; } int largestprimefactor(priority_queue&lt;int&gt; myints) { // largestfactor holds the next number to be tested for primeness in the queue do{ int largestfactor = myints.top(); myints.pop(); //splat will hold the first factor split finds of the top item in the queue int splat = split(largestfactor); //if it holds a 1 then that means that there are no factors if(splat != 1 &amp;&amp; largestfactor) { myints.push(splat); myints.push(largestfactor / splat); } else return largestfactor; }while(myints.top() &gt; 1); } </code></pre>
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