.. _fileInputOutputXMLYAML:

File Input and Output using XML and YAML files
**********************************************

Goal
====

You'll find answers for the following questions:

.. container:: enumeratevisibleitemswithsquare

   + How to print and read text entries to a file and OpenCV using YAML or XML files?
   + How to do the same for OpenCV data structures?
   + How to do this for your data structures?
   + Usage of OpenCV data structures such as :xmlymlpers:`FileStorage <filestorage>`, :xmlymlpers:`FileNode <filenode>` or :xmlymlpers:`FileNodeIterator <filenodeiterator>`.

Source code
===========

You can :download:`download this from here <../../../../samples/cpp/tutorial_code/core/file_input_output/file_input_output.cpp>` or find it in the :file:`samples/cpp/tutorial_code/core/file_input_output/file_input_output.cpp` of the OpenCV source code library.

Here's a sample code of how to achieve all the stuff enumerated at the goal list.

.. literalinclude:: ../../../../samples/cpp/tutorial_code/core/file_input_output/file_input_output.cpp
   :language: cpp
   :linenos:
   :tab-width: 4
   :lines: 1-7, 21-154

Explanation
===========

Here we talk only about XML and YAML file inputs. Your output (and its respective input) file may have only one of these extensions and the structure coming from this. They are two kinds of data structures you may serialize: *mappings* (like the STL map) and *element sequence* (like the STL vector). The difference between these is that in a map every element has a unique name through what you may access it. For sequences you need to go through them to query a specific item.

1. **XML/YAML File Open and Close.** Before you write any content to such file you need to open it and at the end to close it. The XML/YAML data structure in OpenCV is :xmlymlpers:`FileStorage <filestorage>`. To specify that this structure to which file binds on your hard drive you can use either its constructor or the *open()* function of this:

   .. code-block:: cpp

      string filename = "I.xml";
      FileStorage fs(filename, FileStorage::WRITE);
      //...
      fs.open(filename, FileStorage::READ);

   Either one of this you use the second argument is a constant specifying the type of operations you'll be able to on them: WRITE, READ or APPEND. The extension specified in the file name also determinates the output format that will be used. The output may be even compressed if you specify an extension such as *.xml.gz*.

   The file automatically closes when the :xmlymlpers:`FileStorage <filestorage>` objects is destroyed. However, you may explicitly call for this by using the *release* function:

   .. code-block:: cpp

      fs.release();                                       // explicit close

#. **Input and Output of text and numbers.** The data structure uses the same << output operator that the STL library. For outputting any type of data structure we need first to specify its name. We do this by just simply printing out the name of this. For basic types you may follow this with the print of the value :

   .. code-block:: cpp

      fs << "iterationNr" << 100;

   Reading in is a simple addressing (via the [] operator) and casting operation or a read via the >> operator :

   .. code-block:: cpp

      int itNr;
      fs["iterationNr"] >> itNr;
      itNr = (int) fs["iterationNr"];

#. **Input/Output of OpenCV Data structures.** Well these behave exactly just as the basic C++ types:

   .. code-block:: cpp

      Mat R = Mat_<uchar >::eye  (3, 3),
          T = Mat_<double>::zeros(3, 1);

      fs << "R" << R;                                      // Write cv::Mat
      fs << "T" << T;

      fs["R"] >> R;                                      // Read cv::Mat
      fs["T"] >> T;

#. **Input/Output of vectors (arrays) and associative maps.** As I mentioned beforehand, we can output maps and sequences (array, vector) too. Again we first print the name of the variable and then we have to specify if our output is either a sequence or map.

   For sequence before the first element print the "[" character and after the last one the "]" character:

   .. code-block:: cpp

      fs << "strings" << "[";                              // text - string sequence
      fs << "image1.jpg" << "Awesomeness" << "baboon.jpg";
      fs << "]";                                           // close sequence

   For maps the drill is the same however now we use the "{" and "}" delimiter characters:

   .. code-block:: cpp

        fs << "Mapping";                              // text - mapping
        fs << "{" << "One" << 1;
        fs <<        "Two" << 2 << "}";

   To read from these we use the :xmlymlpers:`FileNode <filenode>` and the :xmlymlpers:`FileNodeIterator <filenodeiterator>` data structures. The [] operator of the :xmlymlpers:`FileStorage <filestorage>` class returns a :xmlymlpers:`FileNode <filenode>` data type. If the node is sequential we can use the :xmlymlpers:`FileNodeIterator <filenodeiterator>` to iterate through the items:

   .. code-block:: cpp

      FileNode n = fs["strings"];                         // Read string sequence - Get node
      if (n.type() != FileNode::SEQ)
      {
          cerr << "strings is not a sequence! FAIL" << endl;
          return 1;
      }

      FileNodeIterator it = n.begin(), it_end = n.end(); // Go through the node
      for (; it != it_end; ++it)
          cout << (string)*it << endl;

   For maps you can use the [] operator again to acces the given item (or the >> operator too):

   .. code-block:: cpp

      n = fs["Mapping"];                                // Read mappings from a sequence
      cout << "Two  " << (int)(n["Two"]) << "; ";
      cout << "One  " << (int)(n["One"]) << endl << endl;

#. **Read and write your own data structures.** Suppose you have a data structure such as:

   .. code-block:: cpp

      class MyData
      {
      public:
            MyData() : A(0), X(0), id() {}
      public:   // Data Members
         int A;
         double X;
         string id;
      };

   It's possible to serialize this through the OpenCV I/O XML/YAML interface (just as in case of the OpenCV data structures) by adding a read and a write function inside and outside of your class. For the inside part:

   .. code-block:: cpp

      void write(FileStorage& fs) const                        //Write serialization for this class
      {
        fs << "{" << "A" << A << "X" << X << "id" << id << "}";
      }

      void read(const FileNode& node)                          //Read serialization for this class
      {
        A = (int)node["A"];
        X = (double)node["X"];
        id = (string)node["id"];
      }

   Then you need to add the following functions definitions outside the class:

   .. code-block:: cpp

      void write(FileStorage& fs, const std::string&, const MyData& x)
      {
      x.write(fs);
      }

      void read(const FileNode& node, MyData& x, const MyData& default_value = MyData())
      {
      if(node.empty())
          x = default_value;
      else
          x.read(node);
      }

   Here you can observe that in the read section we defined what happens if the user tries to read a non-existing node. In this case we just return the default initialization value, however a more verbose solution would be to return for instance a minus one value for an object ID.

   Once you added these four functions use the >> operator for write and the << operator for read:

   .. code-block:: cpp

      MyData m(1);
      fs << "MyData" << m;                                // your own data structures
      fs["MyData"] >> m;                                 // Read your own structure_

   Or to try out reading a non-existing read:

   .. code-block:: cpp

      fs["NonExisting"] >> m;   // Do not add a fs << "NonExisting" << m command for this to work
      cout << endl << "NonExisting = " << endl << m << endl;

Result
======

Well mostly we just print out the defined numbers. On the screen of your console you could see:

.. code-block:: bash

   Write Done.

   Reading:
   100image1.jpg
   Awesomeness
   baboon.jpg
   Two  2; One  1


   R = [1, 0, 0;
     0, 1, 0;
     0, 0, 1]
   T = [0; 0; 0]

   MyData =
   { id = mydata1234, X = 3.14159, A = 97}

   Attempt to read NonExisting (should initialize the data structure with its default).
   NonExisting =
   { id = , X = 0, A = 0}

   Tip: Open up output.xml with a text editor to see the serialized data.

Nevertheless, it's much more interesting what you may see in the output xml file:

.. code-block:: xml

   <?xml version="1.0"?>
   <opencv_storage>
   <iterationNr>100</iterationNr>
   <strings>
     image1.jpg Awesomeness baboon.jpg</strings>
   <Mapping>
     <One>1</One>
     <Two>2</Two></Mapping>
   <R type_id="opencv-matrix">
     <rows>3</rows>
     <cols>3</cols>
     <dt>u</dt>
     <data>
       1 0 0 0 1 0 0 0 1</data></R>
   <T type_id="opencv-matrix">
     <rows>3</rows>
     <cols>1</cols>
     <dt>d</dt>
     <data>
       0. 0. 0.</data></T>
   <MyData>
     <A>97</A>
     <X>3.1415926535897931e+000</X>
     <id>mydata1234</id></MyData>
   </opencv_storage>

Or the YAML file:

.. code-block:: yaml

   %YAML:1.0
   iterationNr: 100
   strings:
      - "image1.jpg"
      - Awesomeness
      - "baboon.jpg"
   Mapping:
      One: 1
      Two: 2
   R: !!opencv-matrix
      rows: 3
      cols: 3
      dt: u
      data: [ 1, 0, 0, 0, 1, 0, 0, 0, 1 ]
   T: !!opencv-matrix
      rows: 3
      cols: 1
      dt: d
      data: [ 0., 0., 0. ]
   MyData:
      A: 97
      X: 3.1415926535897931e+000
      id: mydata1234

You may observe a runtime instance of this on the `YouTube here <https://www.youtube.com/watch?v=A4yqVnByMMM>`_ .

.. raw:: html

  <div align="center">
  <iframe title="File Input and Output using XML and YAML files in OpenCV" width="560" height="349" src="https://www.youtube.com/embed/A4yqVnByMMM?rel=0&loop=1" frameborder="0" allowfullscreen align="middle"></iframe>
  </div>
