Question

从我读过的内容以及我理解内存结构如何工作的方式来看，向量创建了一个数组，其中所有值都相互对齐，而我在下面放在一起的链表将指向一个随机点分配下一个对象的内存。因此，该代码的矢量应该更快。

当我编译下面的代码时，链接列表的运行时间为4秒，而向量在21秒内运行，这是向量的5倍速度增加。

我在这里遗漏了什么吗？或者有人对此有解释吗？

#include <vector>
#include <iostream>
#include <ctime>

using namespace std;

struct point{
    float t;
    float v;
};

struct pointlist{
    float t;
    float v;
    struct pointlist *next;
};

void fillpoints(std::vector<struct point> &points, struct pointlist *pointlist) {
    int i;

    for (i = 0; i<8;i++) {
        struct point newpoint; // Fill vector
        newpoint.t = (float)i;
        newpoint.v = (float)i/2;
        points.push_back(newpoint);

        pointlist->t = (float)i; // Fill linked list
        pointlist->v = (float)i/2;
        struct pointlist *temppoint = (struct pointlist *)malloc(sizeof(struct pointlist));
        pointlist->next = temppoint;
        pointlist = temppoint;
    }

    struct point newpoint;
    newpoint.t = 0;
    newpoint.v = (float)i/2;
    points.push_back(newpoint);
    pointlist->t = 0;
    pointlist->v = (float)i/2;
    pointlist->next = NULL;
}

float areavec(std::vector<struct point> pointlist) {
    float y1, z1, y2, z2, area;

    area = 0;
    y2 = pointlist[0].t - pointlist[0].v;
    z2 = pointlist[0].t + pointlist[0].v;

    for (unsigned int i = 1; i<pointlist.size();i++) {
        y1 = y2;
        z1 = z2;
        y2 = pointlist[i].t - pointlist[i].v;
        z2 = pointlist[i].t + pointlist[i].v;
        area += (y1*z2) - (y2*z1);
    }
    return area;
}

float arealist(struct pointlist *pointlist) {
    float y1, z1, y2, z2, area;

    area = 0;
    y2 = pointlist->t - pointlist->v;
    z2 = pointlist->t + pointlist->v;

    while (pointlist->next != NULL) {
        y1 = y2;
        z1 = z2;
        y2 = pointlist->next->t - pointlist->next->v;
        z2 = pointlist->next->t + pointlist->next->v;
        area += (y1*z2) - (y2*z1);

        pointlist = pointlist->next;
    }
    return area;
}

void main() {
    int runs = 200000000;
    float a;
    std::vector<struct point> points;
    struct pointlist *pointlist = (struct pointlist *)malloc(sizeof(struct pointlist));

    struct pointlist *test = pointlist;
    fillpoints(points, pointlist);

    clock_t start = clock();
    for (int i=0; i<runs; i++) {
        a = arealist(pointlist);
    }
    cout << "Time: " << (clock()-start)/CLOCKS_PER_SEC << " Area: " << a << endl;

    start = clock();
    for (int i=0; i<runs; i++) {
        a = areavec(points);
    }
    cout << "Time: " << (clock()-start)/CLOCKS_PER_SEC << " Area: " << a << endl;

    cin.get();
}

Answer 1

float areavec(std::vector<struct point> pointlist)

在这里，我会解决这个问题：

float areavec(const std::vector<struct point>& pointlist)

您正在复制矢量200000000次。而对于链表，你只是复制一个指针。

c风格链表和c ++ std :: vector的速度差异很大

1 个答案: