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 else18 {
19 n = -y;
20 }
21 22 //每次右移，相应x*=x，若当前最低位为1，则result *= x23 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 }
转载于:https://www.cnblogs.com/yysblog/archive/2011/11/25/2263491.html
