project euler problem 1

[kapo]

// problem1.cpp
/* If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.  
Find the sum of all the multiples of 3 or 5 below 1000. */

#include <iostream>
using namespace std;

int main(){
	int num = 0;
	int x;
	for (x = 3; x < 1000; x++){
		if ( x % 3 == 0 ){
			num += x;
			cout << x << " ";
		}
	 	if ( x % 5 == 0 ){
			num += x;
			cout << x << " ";
		}
	}
	cout << endl << "total sum: " << num << endl;
	return 0;
}

when ii put in 10 it works but when ii put in a 1000 its wrong ?_?

[StanEEPhys]

Please note that I am a beginner as well but I'm tearing through project euler pretty well.

This was my algorithm (correct term?) or solution for this problem.

Assume min and max are defined and everything else is kosher.

 for (int counter = min; counter < max; ++counter) {
          if ((counter % 3 == 0) || (counter % 5 == 0)) 
               sum += counter;
    }

I believe, correct me if I am wrong, that you are double counting...

By having 2 if statements then anything that satisfies bot gets counted twice.

Say 30.

30%3 = 0 so -> num+= x

but

30%5=0 is also true so num+=x happens again.

Try putting both tests in the same if statement like mine.


if ((counter % 3 == 0) || (counter % 5 == 0))

This way whether the first test is true or the second is true or both are true, you will only count once.

[kapo]

yeaa that solved it. ii was thinking ii was suppose to solve all of multiples of 3 && 5.
thx for your help ^_^

[StanEEPhys]

No problem. I actually had that problem for awhile. Was pulling my hair out. I basically had to print each number to the console which was when I noticed some numbers printing twice. LOL

[kapo]

what project are you on now ?

[StanEEPhys]

I think 32? My problem now is the math lol... About 75% of my time is figuring out the problem. Once that happens the code is not so bad.

[kapo]

are you teaching yourself c++ or do you learn from an university.
sorry if im being too nosey ^_^

[StanEEPhys]

I'm in an intro class this semester but I way beyond it now.

I just find it easier to have a book, the web and concrete problems to solve when learning.