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POCannot calculate prime factors with a number with 12 digits in C
 

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  1. POCannot calculate prime factors with a number with 12 digits in C
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    <p>Im trying to learn alot more about C code by trying to do the examples on the Project Eular Page : <a href="http://projecteuler.net/problems" rel="nofollow">http://projecteuler.net/problems</a> At the moment im trying to work out the highest prime factor from the number 600851475143. Here is my code:</p> <pre><code>/* The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ?*/ #include &lt;stdio.h&gt; unsigned long long GetPrimeFactor(unsigned long long number) { /* printf("The number you typed was %d\n", number);*/ unsigned long long prime = 2; unsigned long long LargestPrime = 0; while (prime != 0) { if(number%prime == 0) { if(number == prime) { if(LargestPrime &lt; prime) LargestPrime = prime; printf("End of prime factors \nLargest Prime: %d\nEnding Number: %d\n", LargestPrime, number); break; } else if(LargestPrime &lt; prime) LargestPrime = prime; //divide number by prime number = number/prime; printf("Prime: %d\n", prime); } else { //get new prime prime = nextPrimeNumber(prime); } } } unsigned long long nextPrimeNumber(unsigned long long input) { unsigned long long nextPrimeNumber; while(input !=0) { if( input &lt; 0) { printf("The number you have entered is negative\n"); } else if (input &gt; 0) { nextPrimeNumber = input + 1; if(nextPrimeNumber%2 == 0 &amp;&amp; nextPrimeNumber != 2) { nextPrimeNumber += 1; } while(!isPrime(nextPrimeNumber)) { nextPrimeNumber += 2; } return nextPrimeNumber; } } } int isPrime(int number) { int i; int prime = 1; //true if(number == 2) prime = 0; //false if(number%2 == 0 || number &lt;= 1) prime = 0; else { for(i=3; i&lt;sqrt(number) &amp;&amp; prime == 1; i+=2) if(number%i == 0) prime = 0; } return prime; } int main(void) { unsigned long long number; printf("Enter a Number: \n"); scanf("%d", &amp;number); GetPrimeFactor(number); return 0; } </code></pre> <p>This code works for the number 13195 which is in the example, but goes bad when i try the larger numbers and im not sure whats wrong.</p> <p>output with large number 600851475143: prime: 3 prime: 29 prime: 5108231 etc. this cant be correct because the number cannot be divisible by 3 anyway.</p> <p>Cant work out whats going wrong so any help would be great</p>
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      1. POCannot calculate prime factors with a number with 12 digits in C
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    1. COthe answer should be 6857
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      1. POCannot calculate prime factors with a number with 12 digits in C
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    2. CO`600851475143` needs 40 bits to be represented in binary. Apparently your `int`s are 32 bits wide. Try with a larger type, eg: `long long unsigned`.
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      1. POCannot calculate prime factors with a number with 12 digits in C
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    3. COYour functions are defined in the wrong order. The compiler assumes they implicitly return `int`. This causes problems when trying to switch to a larger integer type. Define your functions before they are used.
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      1. USPascal Cuoq
 

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