c++ - 时间未被计算

Question

我想看看哪个访问速度更快，结构体还是元组，所以我写了一个小程序。但是，当它完成运行时，两者记录的时间均为 0.000000。我很确定程序没有那么快完成(因为我不在家所以在在线编译器上运行)

#include <iostream>
#include <time.h>
#include <tuple>
#include <cstdlib>
using namespace std;

struct Item
{
    int x;
    int y;
};

typedef tuple<int, int> dTuple;

int main() {

    printf("Let's see which is faster...\n");
    //Timers
    time_t startTime;
    time_t currentTimeTuple;
    time_t currentTimeItem;

    //Delta times
    double deltaTimeTuple;
    double deltaTimeItem;

    //Collections
    dTuple tupleArray[100000];
    struct Item itemArray[100000];

    //Seed random number
    srand(time(NULL));
    printf("Generating tuple array...\n");
    //Initialize an array of tuples with random ints
    for(int i = 0; i < 100000; ++i)
    {
        tupleArray[i] = dTuple(rand() % 1000,rand() % 1000);
    }

    printf("Generating Item array...\n");
    //Initialize an array of Items
    for(int i = 0; i < 100000; ++i)
    {
        itemArray[i].x = rand() % 1000;
        itemArray[i].y = rand() % 1000;
    }

    //Begin timer for tuple array
    time(&startTime);
    //Iterate through the array of tuples and print out each value, timing how long it takes
    for(int i = 0; i < 100000; ++i)
    {
        printf("%d: %d", get<0>(tupleArray[i]), get<1>(tupleArray[i]));
    }
    //Get the time it took to go through the tuple array
    time(&currentTimeTuple);
    deltaTimeTuple = difftime(startTime, currentTimeTuple);

    //Start the timer for the array of Items
    time(&startTime);
    //Iterate through the array of Items and print out each value, timing how long it takes
    for(int i = 0; i < 100000; ++i)
    {
        printf("%d: %d", itemArray[i].x, itemArray[i].y);
    }
    //Get the time it took to go through the item array
    time(&currentTimeItem);
    deltaTimeItem = difftime(startTime, currentTimeItem);
    printf("\n\n");
    printf("It took %f seconds to go through the tuple array\nIt took %f seconds to go through the struct Item array\n", deltaTimeTuple, deltaTimeItem);
    return 0;
}

根据 www.cplusplus.com/reference/ctime/time/，difftime 应该返回两个 time_t 之间的差值。

Answer 1

time()通常返回自 1970 年 1 月 1 日 UTC(即当前 unix 时间戳)00:00 开始的秒数。因此，根据您的库实现，任何低于 1 秒的内容都可能显示为 0。

您应该更喜欢使用 <chrono> 用于基准测试目的:

chrono::high_resolution_clock::time_point t;
t = high_resolution_clock::now();
// do something to benchmark 
chrono::high_resolution_clock::time_point t2 = chrono::high_resolution_clock::now();
cout <<"Exec in ms: "<< chrono::duration_cast<milliseconds>(t2 - t).count() <<endl;

您仍然需要考虑时钟分辨率。例如，对于 windows，它是 15 毫秒，所以如果你接近 15 毫秒左右，甚至更低，你应该增加迭代次数。