c++ - 了解 C 中的线程以及如何使用它们进行排序

Question

我有一个关于线程的问题，想知道是否有人可以帮助我。所以，我正在研究这段代码，对如何更改正在使用的线程数感到困惑：

// multi-threading
#include <iostream>
#include <pthread.h>
#include <time.h>

// number of elements in array
#define MAX 20

// number of threads
#define THREAD_MAX 4

using namespace std;

// array of size MAX
int a[MAX];
int part = 0;

// merge function for merging two parts
void merge(int low, int mid, int high)
{
    int* left = new int[mid - low + 1];
    int* right = new int[high - mid];

    // n1 is size of left part and n2 is size
    // of right part
    int n1 = mid - low + 1, n2 = high - mid, i, j;

    // storing values in left part
    for (i = 0; i < n1; i++)
        left[i] = a[i + low];

    // storing values in right part
    for (i = 0; i < n2; i++)
        right[i] = a[i + mid + 1];

    int k = low;
    i = j = 0;

    // merge left and right in ascending order
    while (i < n1 && j < n2) {
        if (left[i] <= right[j])
            a[k++] = left[i++];
        else
            a[k++] = right[j++];
    }

    // insert remaining values from left
    while (i < n1) {
        a[k++] = left[i++];
    }

    // insert remaining values from right
    while (j < n2) {
        a[k++] = right[j++];
    }
}

// merge sort function
void merge_sort(int low, int high)
{
    // calculating mid point of array
    int mid = low + (high - low) / 2;
    if (low < high) {

        // calling first half
        merge_sort(low, mid);

        // calling second half
        merge_sort(mid + 1, high);

        // merging the two halves
        merge(low, mid, high);
    }
}

// thread function for multi-threading
void* merge_sort(void* arg)
{
    // which part out of 4 parts
    int thread_part = part++;

    // calculating low and high
    int low = thread_part * (MAX / 4);
    int high = (thread_part + 1) * (MAX / 4) - 1;

    // evaluating mid point
    int mid = low + (high - low) / 2;
    if (low < high) {
        merge_sort(low, mid);
        merge_sort(mid + 1, high);
        merge(low, mid, high);
    }
}

// Driver Code
int main()
{
    // generating random values in array
    for (int i = 0; i < MAX; i++)
        a[i] = rand() % 100;

    // t1 and t2 for calculating time for
    // merge sort
    clock_t t1, t2;

    t1 = clock();
    pthread_t threads[THREAD_MAX];

    // creating 4 threads
    for (int i = 0; i < THREAD_MAX; i++)
        pthread_create(&threads[i], NULL, merge_sort,
                                        (void*)NULL);

    // joining all 4 threads
    for (int i = 0; i < 4; i++)
        pthread_join(threads[i], NULL);

下面这部分特别难，因为我不确定这些“(MAX / 2, MAX/2 + (MAX-1-MAX/2)/2, MAX - 1);”是什么 实际工作。例如，我如何制作 2 个或 8 个线程？

    // merging the final 4 parts
    merge(0, (MAX / 2 - 1) / 2, MAX / 2 - 1);
    merge(MAX / 2, MAX/2 + (MAX-1-MAX/2)/2, MAX - 1);
    merge(0, (MAX - 1)/2, MAX - 1);

    t2 = clock();

    // displaying sorted array
    cout << "Sorted array: ";
    for (int i = 0; i < MAX; i++)
        cout << a[i] << " ";

    // time taken by merge sort in seconds
    cout << "Time taken: " << (t2 - t1) /
            (double)CLOCKS_PER_SEC << endl;

    return 0;
}

在这种情况下我们该如何进行？

谢谢！

标签： c++multithreadingmergesort