Tomer Daloomi - 4 months ago 15

C++ Question

so, I've been trying to solve this problem for a few hours now.

shortly I arrived to a solution that logically - should work and is working, but only for numbers no bigger than 10^7. I guess I could just const the specific number they asked for (600851475143) but I would really love to know - why my code isn't working with big numbers?

this is my code for the solution :

`#include <iostream>`

#include <stdlib.h>

#include <cmath>

using namespace std;

//enter any number and its largest prime factor will be detected.

int main()

{

long largest(1),num(0);

bool primecheck;

cout<<"enter the desired number :"<<endl;

cin>>num;

if (num%2==0)

largest=2;

cout<<"the relevant factors are: ";

for (int i=3;i<=int((sqrt(num))/2);i+=2)

{

primecheck=true;

for(int j=2;j<i;j++)

{

if(i%j==0)

primecheck=false;

}

if(primecheck)

if(num%i==0)

{

largest=i;

cout<< largest<<"\t";

}

}

cout<<endl<< "the largest prime factor of the number you have entered is: " <<largest;

return 0;

}

thanks in advance! :)

Answer

**Problem with your code-**

Use **long long int** as data type instead of **long int** since C++ recognizes **long int** as having same range as **int**.

**If you want to refer my concise solution in C/C++ -**

*For C++ just change the header file to iostream.

```
#include <cstdio>
#define MAX 775147
#define NUM 600851475143
int main()
{
long long n = NUM;
int max = 3;
for(int i = 3; i <= MAX; i+=2)
{
if(n%i == 0)
max = i;
while(n%i == 0)
{
n /= i;
}
}
printf("%d\n", max);
return 0;
}
```