Programming in C

[mical700]

I need to output in four columns.
1) The first will contain n.
2) The second will contain pi(n)
3) The third will contain n/ln(n)
4) The fourth will contain the ration of pi(n) to n/ln(n).

#include <stdio.h>
#include <string.h>
#include <stdint.h>



#define MAXN  100000000  /* maximum value of N */
#define P1    1562501    /* = ceil(MAXN/64) */
#define P2    50000000   /* = ceil(MAXN/2) */
#define P3    5000       /* = ceil(ceil(sqrt(MAXN))/2) */

uint32_t sieve[P1];

#define GET(b) ((sieve[(b)>>5]>>((b)&31))&1)

void make()
{
    uint32_t i, j, k;
    memset(sieve, 0, sizeof(sieve));
    for (k = 1; k <= P3; k++)
        if (GET(k)==0) for(j=2*k+1,i=2*k*(k+1);i<P2;i+=j) sieve[i>>5]|=1<<(i&31);
}

int isprime(int p) { return p==2 || (p>2 && (p&1)==1 && (GET((p-1)>>1)==0)); }

int main()
{
    int i, n;
    make();
    for (n = 0, i = 0; i <= MAXN; i++)
        if (isprime(i)) n++;
    printf("The number of primes below 10^8 is %d.\n", n);
    return 0;
}

[Ancient Dragon]

So, what is the problem? You stated the objective of the program but didn't tell us what's wrong with the code you posted.

[mical700]

Sorry. The code is ok.
Start with the implementation of the Sieve of Erastosthenes:
The program to compute pi(n), which is the number of prime numbers < n. It will print out pi(n) for values of n that are powers of 10 from 10 to 10^8.
For example, pi(2) = 1 and pi(10) = 4.1

Output will be in four columns.
1) The first will contain n.
2) The second will contain pi(n)
3) The third will contain n/ln(n)
4) The fourth will contain the ration of pi(n) to n/ln(n).
An extremely important theorem of number theory says that as n goes to pi, the ratio of pi(n) to n/ln(n) converges to 1.

So what should I do to output as above?
Thanks'

[Ancient Dragon]

You need to write a function named pi so that it can calculate pi(x) Or does that mean pi as in the circumference of a circle (3.14...) ? Same with the function ln()

If the array is correctly calculated outputting the result is trivial -- just call printf() in a loop

line 21: make it easy on yourself and put a couple carrage returns in that line, split up the statements instead of putting them all on the same line. For example the following is much easier to read.

if (GET(k)==0)
{
    for(j=2*k+1,i=2*k*(k+1);i<P2;i+=j)
        sieve[i>>5]|=1<<(i&31);
}

[mical700]

Thanks'.