小算法笔记

#include <stdio.h>
#define N 1000
int num = 0;
int prime(int n)
{
	int i;
	if(n % 2 == 0)
		return (n == 2);
	if(n % 3 == 0)
		return (n == 3);
	if(n % 5 == 0)
		return (n == 5);
	for(i = 7; i*i <= n; ++i)
		if(n % i == 0)
			return 0;
	return 1;
}

void main()
{
	int i;
	for(i = 2; i < N; ++i)
		if(prime(i))
			printf("%-4d %d\n", ++num, i);
}

#include <stdio.h>
#include <math.h>
#include <time.h>
#define N 1000
bool tag[N+1] = {0};
int num = 0;
int prime(int n)
{
	int i;
	if(n % 2 == 0)
		return (n == 2);
	if(n % 3 == 0)
		return (n == 3);
	if(n % 5 == 0)
		return (n == 5);
	for(i = 7; i*i <= n; i += 2)
		if(n % i == 0)
			return 0;
	return 1;
}
void main()
{
	clock_t tic;
	tic = clock();
	int i, v, bound;
	bound = (int)sqrt((float)N);
	for(i = 2; i <= bound; ++i)
	{
		if(!prime(i))  tag[i] = 1;
		if(!tag[i]){
			v = i + i;
			while(v <= N){
				tag[v] = 1;
				v += i;
			}
		}
	}
	for(i = 2; i <= N; ++i)
		if(!tag[i])
			printf("%-4d %d\n", ++num, i);
	printf("%d ms\n", clock()-tic);
}