sieve of Eratosthenes

[plike922]

i am having trouble fixing this program... its not printing the number any tips???
i am using sieve of Eratosthenes

#include <cstdio>
#include "simpio.h"
#include "strlib.h"
#include <iostream>
using namespace std; 


int main()
{
	bool is_prime[300];
	int num[300];
	int i, n;
	
	printf("This program prints all prime numbers from 1 to 300.\n");

	// Identifing the numbers
	for (i = 0; i<=300; i++)
	{
	is_prime[i] = true;
	num[i] = i;
	}
	is_prime[0] = false;
	is_prime[1] = false;
	
	//Identifing the non prime
	for(i = 1; i<=300; i++)
	{
		for(n = i; n<=300;)
		{
			is_prime[n] = false;
			n = n*2;
		}
	}
	//Displaying numbers
	for(n = 0; n<=300; n++)
	{
		if (is_prime[n] == true)
		{
			printf("%d\n",num[n]);
		}
	}
	//Pausing program
	system("pause");
}

[Salem]

That you're stepping off the ends of your arrays would be one problem.

[plike922]

???

[Salem]

> for (i = 0; i<=300; i++)
Arrays run from 0 to N-1
So it's < 300, not <= 300

[plike922]

that doesn't solve the problem its still list every number from 2 to 299

[Salem]

Well the next step would be to go read up on how the sieve actually works.
Because your code looks nothing like it.

For a start, the outer loop should stop at the square root of 300.

[plike922]

shit i missed something big!

[Aia]

#include <cstdio>
#include "simpio.h"
#include "strlib.h"
#include <iostream>
using namespace std; 


int main()
{
	bool is_prime[300];
	int num[300];
	int i, n;
	
	printf("This program prints all prime numbers from 1 to 300.\n");

	// Identifing the numbers
	for (i = 0; i<=300; i++)
	{
	is_prime[i] = true;
	num[i] = i;
	}
	is_prime[0] = false;
	is_prime[1] = false;
	
	//Identifing the non prime
	for(i = 1; i<=300; i++)
	{
		for(n = i; n<=300;)
		{
			is_prime[n] = false;
			n = n*2;
		}
	}
	//Displaying numbers
	for(n = 0; n<=300; n++)
	{
		if (is_prime[n] == true)
		{
			printf("%d\n",num[n]);
		}
	}
	//Pausing program
	system("pause");
}

Why do you keep posting these aberrations of C++ in the C forum?

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

Those are header files and syntax that belongs to C++
And so does this:

bool is_prime[300];

C language do not have a key word bool.
Very likely you are compiling a .cpp file.

[technology]

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>

//*************************************************
//skips even numbers and only checks squared values
//reduces the loop size runtime
//*************************************************

int IsPrime(long number);

int main(int argc, char *argv[]){
	long n;
   printf("Enter number to check for primality:");
   scanf("%ld",&n);
	if(IsPrime(n)) {
   	printf("Number is a prime\n");
   } else {
      printf("Number is not a prime\n");
   }

   int c;
   printf("Press any key to continue..");
   c = getch();
}

int IsPrime(long number) {
    long i;



    i =2;
    if (number % i == 0) return 0;
    
    for (i=3; (i*i)<=number; i+=2) {
        if (number % i == 0) return 0;
    }
    return 1;
}