Loops - Problem

[Majeth]

void createPrimes (int max){
	int i, j;
	bool k = 0;

	for ( i = 2; i <= max; i++){

		do {

			for( j = i-1; j > 1; j--){
		
				if (isWhole((i/j), 0) == true){
				k = false;
				}else{
				k = true; 
				}

			}

			}while (k == true);
		if (k == true){
			writeFile (i);
			cout << "Prime.\n";
		}
	}
}

Hi. I can't seem to get this working. Staring at it too long I imagine. All other functions work perfectly, exept this.

I'm trying to write a list of all prime numbers from 2 through max. Sofar I've just gotten every number, none, or just 2.

isWhole() checks if a number is whole
writeFile() writes to file, just incase some didn't get that.

Anyone seing any errors?

[ne555]

isWhole() checks if a number is whole

wonder how are you doing that.
cout << (5/3);

		do {

			for( j = i-1; j > 1; j--){
		
				if (isWhole((i/j), 0) == true){
				k = false;
				}else{
				k = true; 
				}

			}

			}while (k == true);

The while loop will end when k will be false. In the for loop you are setting
k = not isWhole(i/j, 0);, but the only value that will mater is k = not isWhole( i/2, 0 );
Maybe you want to break;

Still you may want to modularise a little more

for (int K = 2; K <= max; K++)
  if( is_prime(K) )
    writeFile(K);

Also k==true. Don't do that, k is a boolean, just ask for its value, if( k )

[Majeth]

That certainly cleaned it up, but the problem still exist.

void createPrimes (int max){
	int i, j;
	bool k;
	bool K;

	for ( i = 2; i <= max; i++){          //Creating numbers from 2 through max
		K = true;			//Assuming i is a prime before disproven by j-loop
		for ( j = i -1; j > 1; j-- ){	//For each number created, creates all numbers from max - 1 through 2
			k = (isWhole(i/j, 0));	//Gets truth about i/j is a whole number. true = whole
			if(k){			//If k is a whole
				K = !k;		//Masterswitch K is false, and the whole thing will 
			break;			//break to the next i-loop step
			}
		}
		if(K)
		writeFile (i);	//If K still is true after j-loop then write i to file
		}
}

I now only get the number '2' And I have the nasty feeling I'm missing an important point somehow.


The isWhole function calls upon a number and accuracy (here = 0), both floats, then forces the number into an int, before subtracting them. If the remainder = 0, it returns True.

[ne555]

Put the code of isWhole, ¿what floats?
I don't explain it properly. The integer division returns an integer.

int a, b;
a/b; //returns an int 

[Majeth]

bool isWhole(float x, float e){
	return ( (x - int (x)) <= e);
}

Hang on, so it processes i/j before it forces it into a float, thereby making sure that every number comes out as a whole, none as prime. But then the 2 shouldn't be written at all... hmm...

[Majeth]

Sorry, my mind is going. I tested isWhole earlier with division inside the func. call and it wored perfectly... Any idea on what's holding the script back?

[ne555]

Hi, the 2 is being printed because the inner loop is not being executed

for ( j = i -1; j > 1; j-- ){ 
//when i=2 that translates to
for( j=1; j>1; j--) //the condition is false, so is never executed 

To test if the number is divisible, use the modulo o remainder
i%j ==0

(is like doing n - d * floor(n/d))

[Majeth]

Yes! it's working! My evil minions can now utilize the power of primes, soon the world shall thank the saint ne555!

But really thanks for pointing towards %. Before I implemented it I reversed the order, now counting downwards, but I do not know if this has any significance. (after i=2, i=3 comes along through the loop one time and so forth) I'll probably figure exactly what went wrong and why, but now I'm just too happy. Thanks again ne.