算法的快慢不是一眼看上去就能决定的（一）

  1 #include <iostream>
  2 #include <ctime>
  3 #include <vector>
  4 #include <memory.h>
  5 #include <cmath>
  6 
  7 #define TOP_NUM 1500000
  8 #define ARRAY_SIZE 150000
  9 
 10 using namespace std;
 11 
 12 int find_by_normal_loop()
 13 {
 14     int prime_count = 0;
 15     bool isPrime = true;
 16 
 17     for(int i=2; i<TOP_NUM; ++i)
 18     {
 19         isPrime = true;
 20 
 21         for(int j=2; j<=sqrt(i); ++j)
 22         {
 23             if(0 == i%j)
 24             {
 25                 isPrime = false;
 26                 break;
 27             }
 28         }
 29 
 30         if(isPrime)
 31         {
 32             ++prime_count;
 33         }
 34     }
 35 
 36     return prime_count;
 37 }
 38 
 39 int find_by_division_vector()
 40 {
 41     vector<int> res_vec;
 42     bool isPrime = true;
 43 
 44     for(int i=2; i<TOP_NUM; ++i)
 45     {
 46         isPrime = true;
 47 
 48         for(unsigned int j=0; j<res_vec.size()&&res_vec[j]<=sqrt(i); ++j)
 49         {
 50             if(0 == i%res_vec[j])
 51             {
 52                 isPrime = false;
 53                 break;
 54             }
 55         }
 56 
 57         if(isPrime)
 58         {
 59             res_vec.push_back(i);
 60         }
 61     }
 62 
 63     return res_vec.size();
 64 }
 65 
 66 int find_by_division_vector_iterator()
 67 {
 68     vector<int> res_vec;
 69     vector<int>::iterator itor;
 70     bool isPrime = true;
 71 
 72     for(int i=2; i<TOP_NUM; ++i)
 73     {
 74         isPrime = true;
 75 
 76         for(itor=res_vec.begin(); res_vec.end()!=itor&&(*itor)<=sqrt(i); ++itor)
 77         {
 78             if(0 == i%(*itor))
 79             {
 80                 isPrime = false;
 81                 break;
 82             }
 83         }
 84 
 85         if(isPrime)
 86         {
 87             res_vec.push_back(i);
 88         }
 89     }
 90 
 91     return res_vec.size();
 92 }
 93 
 94 int find_by_division_array()
 95 {
 96     int res_vec[ARRAY_SIZE] = {0};
 97     bool isPrime = true;
 98     int current_prime_count = 0;
 99 
100     for(int i=2; i<TOP_NUM; ++i)
101     {
102         isPrime = true;
103 
104         for(int j=0; j<current_prime_count&&res_vec[j]<=sqrt(i); ++j)
105         {
106             if(0 == i%res_vec[j])
107             {
108                 isPrime = false;
109                 break;
110             }
111         }
112 
113         if(isPrime)
114         {
115             res_vec[current_prime_count] = i;
116             ++current_prime_count;
117         }
118     }
119 
120     return current_prime_count;
121 }
122 
123 int find_by_signal_array()
124 {
125     bool numArray[TOP_NUM];
126     int prime_count = 0;
127     memset(numArray, true, sizeof(numArray));
128 
129     numArray[0] = false;
130     numArray[1] = false;
131 
132     for(int i=2; i<TOP_NUM; ++i)
133     {
134         for(int j=2; i*j<TOP_NUM; ++j)
135         {
136             numArray[i*j] = false;
137         }
138     }
139 
140     for(int i=0; i<TOP_NUM; ++i)
141     {
142         if(numArray[i])
143         {
144             ++prime_count;
145         }
146     }
147 
148     return prime_count;
149 }
150 
151 int main()
152 {
153     cout << "Hello world!" << endl;
154 
155     int result_count = 0;
156     clock_t t_begin,t_end;
157 
158     t_begin = clock();
159     result_count = find_by_normal_loop();
160     t_end = clock();
161 
162     cout << "粗暴法：" << endl << "All count : " << result_count << endl << "Time : " << ( double)( ( t_end-t_begin)/1000.0) << endl;
163 
164     t_begin = clock();
165     result_count = find_by_division_vector();
166     t_end = clock();
167 
168     cout << "除素数法：" << endl << "All count : " << result_count << endl << "Time : " << ( double)( ( t_end-t_begin)/1000.0) << endl;
169 
170     t_begin = clock();
171     result_count = find_by_division_vector_iterator();
172     t_end = clock();
173 
174     cout << "除素数法(with iterator)：" << endl << "All count : " << result_count << endl << "Time : " << ( double)( ( t_end-t_begin)/1000.0) << endl;
175 
176 
177     t_begin = clock();
178     result_count = find_by_division_array();
179     t_end = clock();
180 
181     cout << "除素数法(with array)：" << endl << "All count : " << result_count << endl << "Time : " << ( double)( ( t_end-t_begin)/1000.0) << endl;
182 
183     t_begin = clock();
184     result_count = find_by_signal_array();
185     t_end = clock();
186 
187     cout << "标志位法：" << endl << "All count : " << result_count << endl << "Time : " << ( double)( ( t_end-t_begin)/1000.0) << endl;
188 
189     return 0;
190 }