函數fun的功能是：統計所有小於等於n（n>2）的素數的個數，素數的個數作為函數返回。
#include <stdio.h>
int fun(int n)
{ int i,j, count=0;
printf("\nThe prime number between 3 to %d\n", n);
for (i=3; i<=n; i++) {
for (j=2; j<i; j++)
if (i%j == 0)
break;
if (j>=i)
{ count++; printf( count%15? "%5d":"\n%5d",i); }
}
return count;
}
main()
{ int n=20, r;
r = fun(n);
printf("\nThe number of prime is : %d\n", r);
}