help on prime number list c program

[cchris]

Hi, I'm a beginning C student and I'm working on a program that is supposed to display all the prime numbers between 50 and 100. I'm having trouble though. Can anyone help me out. This is what I have so far:

/*
* File: prime.cpp
* This program displays all the prime
* numbers between 50 and 100.
*/

#include<stdio.h>
#include<math.h>

int main()

{
int n,i,limit;
printf("The prime numbers between 50 and 100 are:");
for(n=50;n<100;n++)
{
limit=sqrt((double)n+1);

for (i=3; i<=limit; i+=2)
{
if (n%2==0) break;
if (n%i==0) break;
printf("\n%d",n); break;
}

}
printf("\n");
}


Any help would be greatly appreciated. Thanks.

[Ganon11]

Does this not work?

[ruskalym]

Hi,

There is another approach that is simpler in the case of restricted range.
Looks like that :

int tab[101];
int i;
int j
 
/* suppose all numbers are prime */
for (i = 0; i < 101; ++i) {
  tab[i] = 1;
}
 
/* process even numbers */
tab[0] = 0;
for (i = 4; i < 101; i += 2) {
  tab[i] = 0;
}
 
/* process odd numbers by marking multiples as not prime */
for (i = 3; i < 101; i += 2) {
  if (tab[i]) {
    for (j = 2 * i; j < 101; j += i) {
      tab[j] = 0;
    }
  }
}

Finally, you can extract prime numbers from array (tab[i] == 1).

[cchris]

Does this not work?

Yes, it does not work. Right now, if you run the program, it displays 53,55,59,61,65,67,71,73,77,79,83,89,91,95,97. (the bold numbers aren't pime.) Any ideas why those non-prime numbers are showing up?

Hi,
There is another approach that is simpler in the case of restricted range.
Looks like that :

Thank you for responding but the code you provided is too advanced for me. I'm only a beginner and I don't know what tab[] does. Also, I forgot to mention that my assignment requires me to use the sqrt() function. But thanks a lot for responding and maybe your code will be able to help someone else.

[Savage]

Here seems to be your problem:

for (i=3; i<=limit; i+=2)
{
   if (n%2==0) break;
   if (n%i==0) break;
   printf("\n%d",n);
   break;
 
}

Because of break not all conditions are checked,reform it to something like:

     k=0;
for (i=3; i<=limit; i+=2)
{
   if (n%2==0) k=1;
   if (n%i==0) k=1;
}
if(!k)printf("\n%d",n);

and it should work.

Savage

[cchris]

Thanks a lot Savage! The program works now.