首页 \ 问答 \ For循环，map和forEach Javascript(For-loop, map and forEach Javascript)

For循环，map和forEach Javascript(For-loop, map and forEach Javascript)

 我正在尝试创建一副52张牌。 我可以使用double for-loop轻松创建它，但它具有O（n2）复杂度。 所以我试图使用map（）和forEach（）数组方法，但事情很复杂，需要返回东西。 这是我的代码如下。 
 

 

 
 
 
  
(function deckCreate() {
  var values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
  var suits = ["clubs", "diamonds", "hearts", "spades"];
  var newDeck = values.map(function(xValue) {
    suits.forEach(function(xSuit) {
      return [xSuit,xValue];
    });
  });
  return newDeck;
}());
 
 
 

 

 

 它给出了一个长度为13的数组，内部都是未定义的。 我尝试在map（）之前交换forEach（），但结果是一样的。 
 

 我在这些函数中的console.log（）时发现的问题是元素没有相互映射，而是单独打印。 可能是什么问题？ 

I am trying to create a deck of 52 cards. I can create it easily with double for-loop but it has O(n2) complexity. So I was trying to play with map() and forEach() array methods but things are complex with them needing to return stuffs. Here is my code below.
 

 

 
 
 
  
(function deckCreate() {
  var values = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
  var suits = ["clubs", "diamonds", "hearts", "spades"];
  var newDeck = values.map(function(xValue) {
    suits.forEach(function(xSuit) {
      return [xSuit,xValue];
    });
  });
  return newDeck;
}());
 
 
 

 

 

It gives an array of length 13 all undefined inside. I tried swapping forEach() before map() just incase but the result was the same. 
 

The issue I found while console.log() inside those functions was that the elements were not being mapped to each other but were printed all separately. What could be the issue be? 

原文：
更新时间：2022-10-15 06:10

最满意答案

 当master_socket被标记为可读时，您将在new_socket上调用recv() 。 假设master_socket是您的侦听服务器套接字，则需要调用accept()来接收新客户端。 除非select()告诉您特定客户端是可读的，然后从该客户端读取，否则不要调用recv() 。 
 

 尝试更像这样的东西： 
 

for (i = 0; i < max_clients; ++i)
    client_socket[i] = -1;

...

while (TRUE) 
{
    FD_ZERO(&readfds);

    //add master socket to set
    FD_SET(master_socket, &readfds);
    max_sd = master_socket;

    //add child sockets to set
    for (i = 0; i < max_clients; ++i)
    {
        //socket descriptor
        sd = client_socket[i];
        if (sd > -1)
        {
            FD_SET(sd, &readfds);
            if (sd > max_sd)
                max_sd = sd;
        }
    }

    //waiting for activity
    activity = select(max_sd + 1, &readfds, NULL, NULL, NULL);
    if (activity < 0)
    {
        if (errno != EINTR)
            printf("select error");
    }
    else
    {
        //check master socket to accept from
        if (FD_ISSET(master_socket, &readfds)) 
        {
            new_socket = accept(master_socket, NULL, NULL);
            if (new_socket < 0)
            {
                printf("accept error");
            }
            else
            {
                //add new socket to array of sockets
                for (i = 0; i < max_clients; ++i)
                {
                    //if position is empty
                    if (client_socket[i] == -1)
                    {
                        client_socket[i] = new_socket;
                        printf("Adding to list of sockets as %d\n" , i);
                        send(new_socket, "here", strlen("here"), 0);
                        new_socket = -1;
                        break;
                    }
                }

                // close new socket if no room for it
                if (new_socket > -1)
                    close(new_socket);
            }
        }

        //check child sockets to read from
        for (i = 0 ; i < max_clients; ++i) 
        {
            sd = client_socket[i];
            if (sd > -1)
            {
                if (FD_ISSET(sd, &readfds)) 
                {
                    len = recv(sd, buffer, 1025, 0);
                    if (len > 0)
                    {
                        // data received
                        buffer[len] = 0; // make sure it is null terminated
                        printf("data: %s", buffer);
                        // or: printf("data: %.*s", len, buffer);
                    }
                    else
                    {
                        if (len == 0)
                            printf("disconnected");
                        else
                            printf("recv error");

                        printf("Removing from list of sockets at %d\n", i);
                        close(sd);
                        client_socket[i] = -1;
                    }
                }
            }
        }
    }


When master_socket is marked as readable, you are calling recv() on new_socket instead. Assuming master_socket is your listening server socket, you need to call accept() instead to receive a new client. Do not call recv() unless select() tells you that a specific client is readable, and then read from that client.
 

Try something more like this instead:
 

for (i = 0; i < max_clients; ++i)
    client_socket[i] = -1;

...

while (TRUE) 
{
    FD_ZERO(&readfds);

    //add master socket to set
    FD_SET(master_socket, &readfds);
    max_sd = master_socket;

    //add child sockets to set
    for (i = 0; i < max_clients; ++i)
    {
        //socket descriptor
        sd = client_socket[i];
        if (sd > -1)
        {
            FD_SET(sd, &readfds);
            if (sd > max_sd)
                max_sd = sd;
        }
    }

    //waiting for activity
    activity = select(max_sd + 1, &readfds, NULL, NULL, NULL);
    if (activity < 0)
    {
        if (errno != EINTR)
            printf("select error");
    }
    else
    {
        //check master socket to accept from
        if (FD_ISSET(master_socket, &readfds)) 
        {
            new_socket = accept(master_socket, NULL, NULL);
            if (new_socket < 0)
            {
                printf("accept error");
            }
            else
            {
                //add new socket to array of sockets
                for (i = 0; i < max_clients; ++i)
                {
                    //if position is empty
                    if (client_socket[i] == -1)
                    {
                        client_socket[i] = new_socket;
                        printf("Adding to list of sockets as %d\n" , i);
                        send(new_socket, "here", strlen("here"), 0);
                        new_socket = -1;
                        break;
                    }
                }

                // close new socket if no room for it
                if (new_socket > -1)
                    close(new_socket);
            }
        }

        //check child sockets to read from
        for (i = 0 ; i < max_clients; ++i) 
        {
            sd = client_socket[i];
            if (sd > -1)
            {
                if (FD_ISSET(sd, &readfds)) 
                {
                    len = recv(sd, buffer, 1025, 0);
                    if (len > 0)
                    {
                        // data received
                        buffer[len] = 0; // make sure it is null terminated
                        printf("data: %s", buffer);
                        // or: printf("data: %.*s", len, buffer);
                    }
                    else
                    {
                        if (len == 0)
                            printf("disconnected");
                        else
                            printf("recv error");

                        printf("Removing from list of sockets at %d\n", i);
                        close(sd);
                        client_socket[i] = -1;
                    }
                }
            }
        }
    }

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