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--- docs:c_basics [2009/02/16 07:25]
gibson
+++ docs:c_basics [2010/02/02 07:55] (current)
@@ Line 6: / Line 6: @@
 linear algebra programs, like Matlab scripts.
-For example,
+====== Using classes ======
+For example, you could program with typical %%Matrix%% and %%Vector%% types like this
 <code c++>
   int M = 13;
   int N = 11;
-  Matrix A(M,N);      // Construct a matrix A
+  Matrix A(M,N);      // construct a matrix A
-  Vector x(N);        // Construct a vector x
+  Vector x(N);        // construct a vector x
-  // ...fill in values of A and x...
+  for (int i=0; i<M; ++i)
+    for (int j=0; j<N; ++ j)
-  Vector y = A*x;     // Multiply A*x and then set y = A*x
+      A(i,j) = drand48();    // assign random numbers to A
-  // Print the results
+  for (int j=0; j<N; ++j)
+     x(j) = drand48();       // ditto for x
+  Vector y = A*x;            // multiply A*x and then set y = A*x
+  // print the results
   cout << "y's dimension is " << y.dim() << endl;
   cout << "y's value is " << y << endl;
 </code>
-In C++
+Some C++ vocabulary
-   * a user-defined type is called a **class**. E.g. %%Matrix%% is a class. Classes are roughly like fundamental types.
+   * a user-defined type is called a **class**; e.g. %%Matrix%% is a class. Classes are roughly like fundamental types.
-   * variables of user-defined types are often called **objects**. E.g. %%A%% is an object of type %%Matrix%%.
+   * variables of user-defined types are often called **objects**, e.g.  %%A%% is an object of type %%Matrix%%.
-   * objects are initialized or **constructed** by **constructors**  E.g. %%Matrix A(M,N)%% constructs an object named %%x%%. The %%(M,N)%% is an argument list for the %%Matrix%% constructor, in this case the row and columns dimensions. Constructors typically allocate memory and assign initial values to the object's internal data structures.
+   * objects are initialized or **constructed** by **constructors**, e.g. the statement %%Matrix A(M,N);%% constructs an %%M x N%% Matrix object named %%x%%. The %%(M,N)%% is an argument list for the %%Matrix%% constructor, in this case the row and columns dimensions. Constructors typically allocate memory and assign initial values to the object's internal data structures.
-   * Classes have **member functions** and **operators**. E.g. %%A*x%% calls the %%Matrix, Vector%% multiplication operation, and %%y.dim()%% calls the %%dim()%% member function of object %%y%%.
+   * Classes have **member functions** and **operators**.  %%A*x%% calls the %%Matrix, Vector%% multiplication operator, and %%y.dim()%% calls the %%dim()%% member function of object %%y%%.
+   * %%cout << y << endl%% prints object %%y%% to standard output followed by a new line.
+====== Reading header files ======
+Of course, the code above requires the %%Matrix, Vector%% classes and their member functions to
+be defined elsewhere. Usually classes are declared in **header files** (%%matrix.h%% and %%vector.h%%)
+and implemented in **source files** (%%matrix.cpp%% and %%vector.cpp%%). The header file declarations
+define the high-level user interface to the class, and the source file defines the low-level
+programming that actually makes the classes work. To use a well-written library you will usually
+not have look at the source files. But you will want to able to read and understand the header files.
+The declarations of the %%Matrix%% and %%Vector%% classes used above might look like this
+((normally the class declarations would go in two files %%matrix.h%% and %%vector.h%% but
+I'll cram the text together here))
+<code c++>
+class Matrix {
+  public:
+    Matrix();                           // default ctor
+    Matrix(int M, int N);               // ctor for M x N matrix
+    operator=(const Matrix& A);         // assignment
+    double& operator()(int i, int j);   // get/set (i,j) element
+    int rows() const;                   // return # rows
+    int cols() const;                   // return # cols
+  private:
+    // ... ignore everything labelled private ...
+};
+class Vector {
+  public:
+    Vector();
+    Vector(int dim);
+    operator=(const Vector& x);         // assignment
+    double& operator()(int i);          // get/set (i) element
+    int dim() const;                    // return # rows
+  private:
+    // ...ignore...
+};
+Vector operator*(const Matrix& A, const Vector& x);         // Matrix * Vector operator
+operator ostream& operator<<(ostream& os, const Matrix& A); // Matrix print operator
+operator ostream& operator<<(ostream& os, const Vector& x); // Vector print operator
+</code>
+Each line in the class declaration declares an object or a function that can be used
+in programming with the class. how a declaration translate to usage is tricky. The
+following table will help you get started. Remember, "declaration" means how the
+function declaration appears in a header file, and "usage" means how it is used in
+high-level programming.
+^Declaration^Usage^Meaning^
+|%%Matrix()%%             | %%Matrix A;%% | construct a 0x0 Matrix named A  |
+|%%Matrix(int M, int N)%% | %%Matrix A(3,4);%% | construct a 3x4 Matrix named A  |
+|%%operator=(const Matrix& A)%% | %%A = B;%%   | assign Matrix B to Matrix A |
+|%%double& operator()(int i, int j)%% | %%A(i,j) = 0.34;%% | assign the i,j element of A... |
+| | %%double Aij = A(i,j)%% | ...or get the value of the i,j element |
+|%% rows()%% | %%int M = A.rows();%% | get the number of rows in A |
+|%%Vector operator*(const Matrix& A, const Vector& x);%% | %%Vector y = A*x;%% | multiply %%A%% times %%x%% |
+This barely scratches the surface of C++ programming but it's enough to get you started programming with
+channelflow. Please refer to C++ books and online documentation for more information.

channelflow.org

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