Project Euler #3 Get largest prime factor of a number

Question

So the Question was as follows:

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ?

I made a code for the question whose logic and algorithm seems correct but I can't seem to find the mistake, there's the code.

#include <iostream>

using namespace std;

int main()
{
    long int x = 600851475143;
    for(long int i=x-1; x%i; --i);
    cout << i;
    return 0;
}

RE: Didn't know about the 'i' scope, Teachers in my school didn't tell me =) [not blaming them]

---

RE: Thanx for all your responses, I got the answer. =)

Answer 1

There are two small issues and one big issue:

• Your i variable is out of scope for printing - the scope of variables declared in the header of the statement ends with the statement itself. In this case, i is not visible after the semicolon, so you need to declare it outside the for
• Your variables may not necessarily hold the values that you want to put into them - long is allowed to be a 32-bit type, and on many platforms, it is. To make it at least a 64-bit type, change the declaration to long long .

Once you fix these problems, your code will compile. However, it will take ages to run because of the third, big, problem with your code: your algorithm is too slow.

Answer 2

What about this instead? Just declare the i outsude of the for.

int main()
{
  long int x = 600851475143, i;
  for (i = x-1; x%i; --i);
  cout << i;
  return 0;
}

In your version i doesn't exist when you call cout .

int main()
{
    long int x = 600851475143;    /* here i doesn't exist */
    for(long int i=x-1; x%i; --i) /* here i is declared */ ;
    /* the for is closed by ; so the scope of i is already closed */
    cout<<i;
    return 0;
}

However, there are also other problems in your solution as pointed out by other answers and comments.

Answer 3

i is not accessible outside for loop.

long int i=0;
for(i= x-1;x%i;--i);
cout<<i<<endl;

Answer 4

At least the following code compiles correctly and has no overflow problems:

#include <iostream>

using namespace std;

int main()
{
    unsigned long long x = 600851475143, i;
 // ^^^^^^^^ Note the unsigned!
    for(i= x-1;x%i;--i);
    cout<<i;
    return 0;
}

Answer 5

There's also a bug:

if x is prime, you won't find the answer. i should start from the same value.