goldbach.c

自己做的常用库和实现的数据结构。public domain.

/* Demo of Goldbach Conjecture implementation, 
 * using dynamic link data structure.
 *
 * Written by Cyril Hu ([e-mail]), public domain.
 */

#include<stdio.h>
#include<stdlib.h>
#define M 1024

typedef struct pri {
	int e;
	struct pri *next;
}P;

P *t, *head;

void goldbach(int x, int flag);
P *get_pri(int x, int flag);
int is_p(int x);
void find(int x, P *head);
void freelist(P *head);

int main(void)
{
	int i;
	printf("4 =  2 + 2\n");
	for(i=4; i<=M; i+=2) {
		goldbach(i, i-4);
	}
	free(head);
	return 0; 
}

void goldbach(int x, int flag)
{
	P *p;
	p = get_pri(x, flag);
	find(x,p);
}

P *get_pri(int x, int flag)
{
	P *p;
	if(is_p(x-1) != 0) {
		if((p = malloc(sizeof(P))) == NULL) {
			fprintf(stderr,"Fatal: mem alloc failed !!!\n");
			exit(1);
		}
		p->e = is_p(x-1);	
		if(flag==0) {
			head = t = p;
		} else {
			t->next = p;
			t = p;
		}
		p->next = NULL;
	}
	return head;
}

int is_p(int x)
{
	int i;
	for(i=2; i<x; i++)
		if(x%i==0) 
			return 0;
	return x;
}

void find(int x, P *head)
{
	P *p, *q;
	for(p=head; p->next != NULL; p = p->next)
		for(q = p->next; q != NULL; q = q->next)
			if(p->e + q->e == x)
				printf("%d =  %d + %d\n", x, p->e, q->e);
}

void freelist(P *head)
{
	P *p, *q;
	for(p=head; p != NULL; p = q) {
		q = p->next;
		free(p);
	}
	head = t = NULL;
}