1 #include <iostream>
2 #include <time.h>
3 #include <random>
4 #include <iomanip>
5 using namespace std;
6
7 //求指数函数
8 inline double myPow(double x, int y)
9 {
10 unsigned int n;
11
12 //指数可能为负
13 if (y >= 0)
14 {
15 n = y;
16 }
17 else
18 {
19 n = -y;
20 }
21
22 //每次右移，相应x*=x，若当前最低位为1，则result *= x
23 for (double result = 1; ; x *= x)
24 {
25 if ((n & 1) != 0)
26 {
27 result *= x;
28 }
29 if ((n >>= 1) == 0)
30 {
31 return (y >= 0)?result:1/result;//指数为负，取倒数
32 }
33 }
34 }
35 double polynomial(double a[], double x, int n)
36 {
37 double re = 0;
38 for (int i = 0; i < n; i++)
39 {
40 re += a[i] * myPow(x, i);
41 }
42 return re;
43 }
44 double polynomial_horner(double a[], double x, int n)
45 {
46 double re = 0;
47 for (int i = n - 1; i > 0; i--)
48 {
49 re = (a[i] + re) * x;
50 }
51 re += a[0];
52 return re;
53 }
54 int main()
55 {
56 mt19937 eng;
57 eng.seed((unsigned)time(NULL));
58 uniform_real<double> unif;
59 const size_t size = 10000;
60 double *a = new double[size];
61 for (int i = 0; i < size; i++)
62 {
63 a[i] = unif(eng);
64 }
65
66 clock_t start = clock();
67 cout << polynomial(a, 0.9, size) << endl;
68 clock_t end = clock();
69 double d = double(end - start)/CLOCKS_PER_SEC;
70 cout << setprecision(4) << fixed << d << endl;
71
72 start = clock();
73 cout.unsetf(ostream::floatfield);
74 cout << polynomial_horner(a, 0.9, size) << endl;
75 end = clock();
76 d = double(end - start)/CLOCKS_PER_SEC;
77 cout << setprecision(4) << fixed << d;
78 }