Untitled

#include <iostream>
#include <random>
#include <vector>
using namespace std;

double vector_mul_funtion_1(const vector<double>& data, double x, size_t n) {
	int64_t start_time = clock();

	long i;
	double result = data[0];
	double xpower = x;
	for (i = 1; i < n; ++i) {
		result += data[i] * xpower;
		xpower = xpower * x;
	}

	double used_time = 1.0 * (clock() - start_time) / CLOCKS_PER_SEC;

	printf("vector_mul_funtion_1 deal n = [%lld] need time [%f] second\n", n, used_time);
	return result;
}

double vector_mul_funtion_2(const vector<double>& data, double x, size_t n) {
	int64_t start_time = clock();

	long i;
	double result = data[n - 1];
	for (i = n - 2; i >= 0; --i) {
		result = data[i] + x * result;
	}

	double used_time = 1.0 * (clock() - start_time) / CLOCKS_PER_SEC;

	printf("vector_mul_funtion_2 deal n = [%lld] need time [%f] second\n", n, used_time);
	return result;
}

void vector_mul_test() {
	std::random_device rd;
	std::mt19937 gen(rd());
	std::uniform_real_distribution<> dist(0, 1);

	size_t n = 5e7 + 7;
	vector<double> data(n, 0);
	for (size_t i = 0; i < n; ++i) {
		data[i] = dist(gen);
	}
	double x = dist(gen);

	printf("start\n");

	double y1 = vector_mul_funtion_1(data, x, n);
	double y2 = vector_mul_funtion_2(data, x, n);

	printf("y1 = [%f]; y2 = [%f]; y1 == y2 = [%d]\n", y1, y2, y1 == y2);

	double y3 = vector_mul_funtion_2(data, x, n);
	double y4 = vector_mul_funtion_1(data, x, n);

	printf("y3 = [%f]; y4 = [%f]; y3 == y4 = [%d]\n", y3, y4, y3 == y4);
}

int main() {
	vector_mul_test();
	return 0;
}