Euler Project Problem #3

[Manga]

#include <iostream>
#include <cmath>
using namespace std;

int main() {

	long long int x;
	x = 0;
	while(x<=0) {
		cout << "Enter an integer greater than 0." << endl;
	  cin >> x;
	}

	for(int k=2; k<(sqrt((double)x)); k++){
		if(x % k == 0) {
			x = x/k;
		}
	}

	int count = 0;
	int temp = 0;
	int largestprime = 0;
	for(long long int i=(x/2); i>0; i--){
		if(x % i== 0) {
			count = 0;
			for(long long int j=(i/2); j>0; j--){
				if(i % j== 0) {
		          count++;
				}
			}
			if(count == 1 && largestprime == 0) largestprime= i;
		}
	}
	if (largestprime != 0) {
	cout << largestprime << " is the largest prime multiple of " << x << endl;;
	}
	else {
		cout << x << " is a prime number." << endl;
	}


	system("PAUSE"); 
	return 0;
}

here is my finished program. it works right and fast.
I changed it by looking for the lowest factor at first and then dividing the big number by it. This cut it down to smaller size.

[Manga]

but my program still has major flaws.

if for example the user inputs a letter at first instead of a number, my program farts. I don't know how to control user input yet.

[18hRX9L8]

Chervil wrote:

The Python code looks useful. However, I'd suggest that false=True shows an unfortunate choice of variable name.

Fixed that.

[MatthewRock]

but my program still has major flaws.

if for example the user inputs a letter at first instead of a number, my program farts. I don't know how to control user input yet.

You are solving a specific problem. You should only train your brain/learn some ways of solving specific problem. Your program shouldn't be The Best Program In The World; it just should do its job.

However, if you really do want to reuse this code later - your problem is that you take int as an input:

	long long int x;
         //code here
	 cin >> x;

This way, if someone write text string - program will break. If you want to fix it, I suggest using sstream, although I think this is really unnecessary here, as we want program to work fast and we know what input should be.


PS. Oh, and one thing: Try to avoid
system("PAUSE");

This is both not portable and not safe code.