Question

在C＃给出一个结构：

public class MainActivity : Activity
{
    ProgressBar pb;
    TextView tv;
    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Set our view from the "main" layout resource
        SetContentView(Resource.Layout.Main);
        pb = FindViewById<ProgressBar>(Resource.Id.pb);
        tv = FindViewById<TextView>(Resource.Id.tv);
        UpdatePB uptask = new UpdatePB(this,pb,tv);
        uptask.Execute(100);
    }

   public class UpdatePB : AsyncTask<int, int, string>
    {
        Activity mcontext;
        ProgressBar mpb;
        TextView mtv;
        public UpdatePB(Activity context,ProgressBar pb,TextView tv) {
            this.mcontext = context;
            this.mpb = pb;
            this.mtv = tv;
        }
        protected override string RunInBackground(params int[] @params)
        {
            // TODO Auto-generated method stub
            for (int i = 1; i <= 4; i++)
            {
                try
                {
                    Thread.Sleep(3000);
                }
                catch (InterruptedException e)
                {
                    // TODO Auto-generated catch block
                    Android.Util.Log.Error("lv",e.Message);
                }
                mpb.IncrementProgressBy(25);
                PublishProgress(i * 25);

            }
            return "finish";
        }

        protected override void OnProgressUpdate(params int[] values)
        {
            mtv.Text = String.ValueOf(values[0]);
            Android.Util.Log.Error("lv==", values[0] + "");
        }
        protected override void OnPostExecute(string result)
        {
            mcontext.Title = result;
        }


    }
}

我们可以这样写：

struct Point {int x, int y}

如何使用lambda函数在C ++中表达这个逻辑？

Answer 1

我觉得你想在(y, x) ¹上进行词典排序。您可以利用库函数std::tie。那个返回一个引用元组，而std::tuple有一个小于运算符，它执行字典比较。因此，您只需要指定比较项目的顺序。

以下是它如何查找std::vector（您在C ++中使用容器类型，始终以std::vector开头）：

std::vector<Point> my_list;
// Fill my_list
std::sort(begin(my_list), end(my_list), [](auto const& l, auto const& r){
  return std::tie(l.y, l.x) < std::tie(r.y, r.x);
});

<子> ¹ - 我只是基于方法名称，所以这可能不就是你真正追求的。

Answer 2

您可以使用STL函数std :: sort。例如：

    void CMFCApplication2Dlg::log2File(CString newData) {

        //Open or Create new log file
        if (!(file.Open(_T(FILE_NAME), CFile::modeNoTruncate | CFile::modeCreate | CFile::modeReadWrite)))
        {
            AfxMessageBox(_T("Couldn't open file."));
            return;
        }
        file.Close();

        CString oldData;

        //read binary data from file --> decrypt it and return decrypted oldData.
        readFile(oldData);      

        //Add at the end of file new data to be encrypted.
        oldData += newData;
        newData = oldData;  

        CByteArray arBytes;

        //Encypt new data
        //Derive a key from a password.
        crypto.DeriveKey(CRYPTO_PASS);

        //put encypted newData to arBytes.
        crypto.Encrypt(newData, arBytes);   

        //Delete file (we will write a new one ==> overwite)
        CFile::Remove(_T(FILE_NAME));       

        //Create new log file
        if (file.Open(_T(FILE_NAME), CFile::modeNoTruncate | CFile::modeCreate |CFile::modeReadWrite))
        {
            //Write the encrypted data (byte array) to a "new" file 
            //(we deleted the original one and creating a new one with the same name (overwrite)
            file.Write(arBytes.GetData(), static_cast<UINT>(arBytes.GetCount()));
            file.Close();
        }
        else {
            AfxMessageBox(_T("Couldn't write newData to file."));
        }


        //For Debug only ==> popup the file content
        CString str1;
        readFile(str1);
        AfxMessageBox((str1));

    }
    void CMFCApplication2Dlg::readFile(CString &str) {

        //Open the file in read mode
        if (  (file.Open(_T(FILE_NAME), CFile::modeRead) == TRUE) && 
            (file.GetLength() == 0) )
        {
            //There is no file ==> first time
            //Nothing to read, return empty string
            file.Close();
            str = "";
            return;
        }


        CByteArray arBytes;
        CString m_strData;

        //Size the array to accomodate the file bytes.
        arBytes.SetSize(static_cast<INT_PTR>(file.GetLength()));        

        //  Copy the data and close the file.
        file.Read(arBytes.GetData(), static_cast<UINT>(file.GetLength()));
        file.Close();

        //  Try and deserialize the data.
        //Derive a key from a password.
        crypto.DeriveKey(CRYPTO_PASS);
        if (crypto.Decrypt(arBytes, m_strData) == false)
            AfxMessageBox(_T("Coudln't decrypt data- check password."));
        else
        {
            //We have data !!
            str = m_strData;
        }
    }

Answer 3

两种方式 - 要么排序两次，先按y排序，然后在x上使用稳定排序（请注意，这与C＃完全相反！）。但std::sort运气不好，因为它不稳定，幸运的是，正如本杰明暗示的那样，std::stable_sort也是......

另一种方法是确保两点比较，以便x中的差异首先适用，并且仅在相等的情况下，y被视为：

std::sort
(
    // range to sort
    list.begin(), list.end(),
    // next the comparator - you wanted a lambda? OK, here it is:
    [](point const& a, point const& b)
    {
        return a.x < b.x || a.x == b.x && a.y < b.y;
    }
);

Answer 4

您只需要将lambda函数指定为std::list::sort()。您可以决定如何排序。

list.sort([](Point  i,Point  j)
            {
               if(i.x!=j.x)
                   return i.x<j.x;
               else
                   return i.y<j.y;
             }
          );

使用lambda函数排序，然后在c ++中排序

4 个答案: