Thread: Prime number generator

ok i've taken the suggestions in and changed the code.
i got the scrap book out and wrote down a flow diagram and then worked out the the code, adding the square root idea and incrementing by 2 idea in.

i try to compile but i get lots of errors still. but it's close.

Code:

#include <stdio.h>
#include <math.h> /* included for sqrt() function */
main()
{
int p,prevprime,root,count,max;

count=0;
p=3;
prevprime=0;
FILE *fwrite;
fwrite=fopen("/home/primes.txt","a+");  /*adds to the end of file */

FILE *fread;
fread=fopen("/home/primes.txt","r");  /*reads values of file */

root=sqrt(p);

printf("Enter how many primes to find:");
scanf("%d",&max);

while(max<=count)
{count++

while(prevprime<=root)  /*First logic argument checks previous prime value isn't greater then root value of p, since p divided by previous prime values (no greater than root) are the only ones needed to be checked*/ 
{if((prevprime%p)==0) /*2nd logic argument check if p is actually a prime value */ 
 {printf("not a prime:%d\n",p); /* not a prime... dang */
  fclose(fread);       /*closes file so it will read from start next time */
  prevprime=0;      
  p=p+2;            /* incrementing up by 2, primes are only odds */
  root=sqrt(p);     /* new root value when new p value*/
 }
 else{fscanf(fread, "%d" &prevprime); /* changes preprime value so it can be checked again by 1st and 2nd logic argument */ 
      printf("prevprime value:%d",prevprime); /* i just want to see the preprime value to check whats happening */ 
     }
}
fclose(fread);
printf("Prime:%d\n",p);   /* YAY prime number found*/
fprintf(fwrite,"%d\n",p);     /* calls primes.txt and adds value at end of file */
p=p+2;         /* incrementing up by 2, primes are only odds */
root=sqrt(p); /* new root value when new p value*/
prevprime=0;  /* resets preprime value for logic arguments */
}
printf("finnished finding %d primes", max);

}

Originally Posted by st00ch

i try to compile but i get lots of errors still. but it's close.

Of course, it would be nice to see what those errors were! <hint, hint>

A few suggestions:

1) ALWAYS int main(void) or int main(int argc, char *argv[]), and return 0 at the end of main.

2) The whole writing to the file, while reading from the file, and re-closing and opening the file - that part I'd dump. Put the primes you find, into a file, AND IF YOU WANT TO, keeping the last (most recent) big bunch of them (maybe 1,000 or 100,000 or whatever you think is a reasonable number. It depends on how many primes you're going to be generating), into the array.

For example, using the basic strategy outlined in your code, you'll generate the first 1,000 prime numbers, in a second or so (maybe less, depending on your system). So being practical, although using the previous array is a big speed up, how much time do you want to spend optimizing the program, to save a half a second?

3) The number two is a prime number (one is not but two is). So don't leave out 2.

4) So I would suggest opening the file in write mode, only. Not append, and not for reading, either.


5) Take an example of a prime: 101 - the previous prime will be WAY more than 10, which is the square root of 101 (about). previous primes should work down starting with the square root of the number being checked, and going down, or working from a very low prime like 5, and working upward.

I don't get the whole previous prime logic, and I can't tell what "count" is counting.

You have the two nested loops, and the basic idea, but I can't quite tell whats up with the logic in them.

Just when I thought it was "safe" to go back into the prime number water, too!