Project Euler problem 3

What is the largest prime factor of the number 600851475143 ?

My attempt:

Code:

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#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;
}

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The machine is just not running this code

What did I do wrong?

Quote:

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The machine is just not running this code

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Not very descriptive.

Quote:

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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?

---

What's that? int, float char, double, long double.........................

Quote:

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Originally Posted by SELFMADE

What is the largest prime factor of the number 600851475143 ?

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You need a long int ("%li").

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.

Quote:

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Originally Posted by SELFMADE

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.

---

A prime number is one that is divisible by 1 and itself only. So, you just run a loop starting from 2 till the number and check if it leaves any remainder while dividing with numbers in between. If it leaves a remainder, you need to continue, if the remainder is zero it's not a prime number.

Quote:

---

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;
}

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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.

I suggest you start with smaller values. Your program says the largest prime factor of 200 is 50.

It also looks like the complexity of the algorithm is too bad - you find all factors, and then you attempt to run a primality test on each of those? Just so you know, there must be better ways.

However I don't think this is where better solutions should be discussed. The point of a challenge is that you do it yourself.