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#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <assert.h>
#include <math.h>
// int *primes = primes_upto(n,&numprimes);
// Generate a dynamic array of integers containing
// all primes up to $n. The array is zero-terminated.
// We also set numprimes to the number of primes found.
// The caller should free(primes) when they've finished
// using it.
//
int * primes_upto( int n, int *numprimep )
{
bool *isprime = (bool *) malloc( (n+1) * sizeof(bool) );
assert( isprime != NULL );
int i;
for( i=1; i<=n; i++ )
{
isprime[i] = 1; // initially
}
int upper = (int)(sqrt((double)(n)));
//printf( "debug: n=%d, upper=%d\n", n, upper );
for( i=2; i<=upper; i++ )
{
if( isprime[i] )
{
//printf( "debug: crossing out multiples of %d\n", i );
int j;
for( j=i*i; j<=n; j+=i )
{
isprime[j] = 0;
}
}
}
// count how many primes there are
int np = 0;
for( i=2; i<=n; i++ )
{
if( isprime[i] )
{
np++;
}
}
*numprimep = np;
// dynamically allocate an array of np+1 ints, and fill it in
int *primes = malloc( (np+1) * sizeof(int) );
int p = 0;
for( i=2; i<=n; i++ )
{
if( isprime[i] )
{
primes[p++] = i;
}
}
primes[p] = 0;
free( isprime );
return primes;
}
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