Thread: Project Euler problem 3

What is the largest prime factor of the number 600851475143 ?

My attempt:

Code:

#include <stdafx.h>

int main() 
{
	int num1, num2, num3, num4, lrgprime, sum;
	lrgprime=num2=num4=sum=0;
	for (num1=2; num1<300425737572; num1++) {
		num2=600851475143%num1; 

		if (num2==0) {
			for (num3=2; num3<=num1; num3++) {
				num4=num1%num3;
				if (num4==0 && num1==num3 && num1>=lrgprime)
					lrgprime=num1; 
			}		
		}	
	}
	printf("This is the largest prime factor: %d", lrgprime);
	return 0;
}

The machine is just not running this code

What did I do wrong?

I've been trying to solve this problem for the last 3 days without success can someone help me?

All I need is a function that checks whether a parameter is a prime or not.

Originally Posted by SELFMADE

What is the largest prime factor of the number 600851475143 ?

My attempt:

Code:

#include <stdafx.h>

int main() 
{
	int num1, num2, num3, num4, lrgprime, sum;
	lrgprime=num2=num4=sum=0;
	for (num1=2; num1<300425737572; num1++) {
		num2=600851475143%num1; 

		if (num2==0) {
			for (num3=2; num3<=num1; num3++) {
				num4=num1%num3;
				if (num4==0 && num1==num3 && num1>=lrgprime)
					lrgprime=num1; 
			}		
		}	
	}
	printf("This is the largest prime factor: %d", lrgprime);
	return 0;
}

The machine is just not running this code

What did I do wrong?

Hello,
first of all, integers are usually 32 bit long, i.e. you are limited to numbers between 0 an ~4.000.000.000 for unsigned integers.
For Euler problem #3 you need integers of 64 bit length.
Try to declare these numbers using one of these types:

long long n = 600851475143LL, i, prime;
or
int64_t n = 600851475143LL, i, prime;
or
__int64 n = 600851475143LL, i, prime;

Important is to add 'LL' at the end of the large numbers to tell the compiler to interprete this as an integer of 64 bit. (This is what I just learned and helped me to solve Euler #3).

Hope that helps.
A.