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  1. C++ Examples: Matrix CS-410, Fall 2004 Michael Weiss CS410 – Software Engineering C++: Matrix example

  2. Today’s Lecture • Remember “the Big Four” member methods of a C++ class: • class Foo { • public: • Foo(); //default constructor • Foo(const Foo & f); //copy constructor • ~Foo(); //destructor • Foo& Foo::operator=(const Foo& rhs); • //copy assignment • } We will illustrate these with a class Matrix. A Matrix will be a dynamic two-dimensional array. CS410 – Software Engineering C++: Matrix example

  3. The class Matrix dx dy p 3 4 • We will use a pointer-to-pointer-to-base type structure. Our base type will be long (although the base type doesn’t make a big difference to the code). Matrix • Types: • long* p[i] • long** p p[0] p[1] p[2] CS410 – Software Engineering C++: Matrix example

  4. Matrix Class Definition • class Matrix { • public: • Matrix(int sizeX, int sizeY); • Matrix(); • ~Matrix(); • Matrix(const Matrix& m); • Matrix& operator=(const Matrix& rhs); • ... • private: • int dx, dy; // dimensions, dx ´ dy • long **p; // pointer to a pointer to a long integer • }; CS410 – Software Engineering C++: Matrix example

  5. Matrix Class Definition • It will be convenient to add a private method to allocate the array p and the p[i] arrays: • class Matrix { • private: • ... • void allocArrays() { • p = new long*[dx]; • for (int i = 0; i < dx; i++) { • p[i] = new long[dy]; • } • } • }; CS410 – Software Engineering C++: Matrix example

  6. Matrix Constructor • // Dynamically allocate a sizeX´ sizeY matrix, • // initialized to all 0 entries • Matrix::Matrix(int sizeX, int sizeY) • : dx(sizeX),dy(sizeY) { • allocArrays(); • for (int i = 0; i < dx; i++) { • for (int j = 0; j < dy; j++) { • p[i][j] = 0; • } • } • } CS410 – Software Engineering C++: Matrix example

  7. Matrix Default Constructor • Matrix::Matrix() : Matrix(1,1) {} • // We could also have given default arguments • // to our previous constructor: • Matrix::Matrix(int sizeX=1, int sizeY=1) • // the rest is the same as before CS410 – Software Engineering C++: Matrix example

  8. Matrix Copy Constructor • Matrix::Matrix(const Matrix& m) • : dx(m.dx), dy(m.dy) { • allocArrays(); • for (int i=0; i<dx; ++i) { • for (int j=0; j<dy; ++j) { • p[i][j] = m.p[i][j]; • } • } • } • “this” has been allocated, but not initialized at this point. CS410 – Software Engineering C++: Matrix example

  9. Matrix Destructor • Matrix::~Matrix() { • for (int i = 0; i < dx; i++) { • delete [] p[i]; • } • delete [] p; • } CS410 – Software Engineering C++: Matrix example

  10. Matrix Copy Assignment • Matrix &Matrix::operator = • (const Matrix &m) { • if (this == &m) { • // avoid self-assignment • return *this; • } else { • if (dx != m.dx || dy != m.dy) { • this->~Matrix(); • dx = m.dx; dy = m.dy; • allocArrays(); • } for (int i = 0; i < dx; i++) { for (int j = 0; j < dy; j++) { p[i][j] = m.p[i][j]; } } return *this; } CS410 – Software Engineering C++: Matrix example

  11. The Copy Assignment Return Type • Remember the signature of copy assignment: Foo& Foo::operator=(const Foo& rhs); • “this” points at the left-hand-side: • x = y; rhs this The code makes “this” have the same contents as the rhs. So why not return void? Answer: “x = y = z”, which is parsed as “x = (y = z)” CS410 – Software Engineering C++: Matrix example

  12. The Copy Assignment Return Type • The traditional return type “Foo&” even supports code like this: • ++(x=y) • if you’ve defined Foo::operator++(). This copies the contents of y into x, then increments x. CS410 – Software Engineering C++: Matrix example

  13. Some More Matrix Operations We next define matrix addition, output, and element access. class Matrix { public: … Matrix operator+(const Matrix & m); Matrix& operator+=(const Matrix & m); friend ostream &operator<< (ostream &out, const Matrix &m); long &operator()(int x, int y); … }; CS410 – Software Engineering C++: Matrix example

  14. Matrix Addition Matrix& Matrix::operator+=(const Matrix& m) { // x+=y adds the y-entries into the x-entries for (int i=0; i<dx; ++i) { for (int j=0; j<dy; ++j) { p[i][j] += m.p[i][j]; } } return *this; } CS410 – Software Engineering C++: Matrix example

  15. Matrix Addition Matrix Matrix::operator+(const Matrix& m) { Matrix temp(*this); //copy constructor return (temp += m); } The assignment form, +=, does the real work. The copy constructor does the allocation. This trick is less useful for matrix multiplication. CS410 – Software Engineering C++: Matrix example

  16. +, +=, and = • In C++, defining operator+ and operator= does not automatically give the right meaning to +=. • This language-design bug is fixed in C#. CS410 – Software Engineering C++: Matrix example

  17. Overloading the << Operator • cout << age << text2; • cout << text2; • … • cout << text1 << age << text2; • ostream &operator<<(ostream &ostr, string &s); • cout << age << text2; • ostream &operator<<(ostream &ostr, int i); CS410 – Software Engineering C++: Matrix example

  18. Matrix Output • ostream &operator<< • (ostream &out, const Matrix &m) • { • for (int i = 0; i < m.dx; ++i) { • for (int j = 0; j < m.dx; ++j) • out << m.p[i][j] << " "; • out << endl; • } • return out; • } CS410 – Software Engineering C++: Matrix example

  19. Overloading the << Operator • operator<< must be a non-member function (an ordinary function), since the first operand is an ostream, and not “this”. • We make operator<< a friend of Matrix, so it has access to m.dx, m.dy, and m.p. • Friend functions are usually overloaded operators, for just this reason. CS410 – Software Engineering C++: Matrix example

  20. Matrix Element Access • class Matrix { • public: • … • long &operator()(int x, int y); • … • }; We will overload () so we can write things like: long x = myMatrix(1,2); myMatrix(0,1) = 25; CS410 – Software Engineering C++: Matrix example

  21. Overloading the () Operator • long &Matrix::operator()(int i, int j) { • return p[i][j]; • } • Note that operator() returns the matrix element by reference. Why? Answer: so we can put “myMatrix(i,j)” on the left-hand side of an assignment statement: myMatrix(0,1) = 25; CS410 – Software Engineering C++: Matrix example

  22. Matrix Multiplication • We conclude with matrix multiplication. We want to be able to write two kinds of statements: • matProduct = mat1 * mat2; • matDouble = 2 * mat1; • We write two ordinary, non-member functions, and let ordinary overloading pick the right one. CS410 – Software Engineering C++: Matrix example

  23. Matrix Multiplication class Matrix { public: … friendMatrix operator* (const Matrix & m1, const Matrix & m2); friendMatrix operator* (long c, const Matrix & m2); friendMatrix operator* (const Matrix & m1, long c); … }; CS410 – Software Engineering C++: Matrix example

  24. Matrix Multiplication x = Matrix operator*(const Matrix& m1, const Matrix& m2) { Matrix prod(m1.dx, m2.dy); for (int i=0; i<prod.dx; ++i) { for (int j=0; j<prod.dy; ++j) { for (int k=0; k<m1.dy; ++k) { prod.p[i][j] += m1.p[i][k] * m2.p[k][j]; } } } return prod; } CS410 – Software Engineering C++: Matrix example

  25. Matrix Multiplication Matrix operator*(long c, const Matrix& m2) { Matrix prod(m2); for (int i=0; i<prod.dx; ++i) { for (int j=0; j<prod.dy; ++j) { prod.p[i][j] = c * m2.p[i][j]; } } return prod; } Matrix operator*(const Matrix& m2, long c) { return c*m2; } CS410 – Software Engineering C++: Matrix example

  26. Testing the Code • int main() { • Matrix x(2,1), y(1,2), z(1,1); • x(0,0) = 1; • x(1,0) = 2; • y(0,0) = 3; • y(0,1) = 4; • cout << "Matrix x\n" << x • << "\nMatrix y\n" << y • << "\nMatrix z\n" << z << endl; • cout << "x*y = \n" << x*y << endl; • z = x*y; • cout << "Matrix z = x*y (note new dimensions)\n" << z << endl; • Matrix x2(2,1); • x2 = 2*x; • cout << "Matrix x2 = 2*x\n" << x2 << endl; • cout << "x + x2 = \n" << x+x2 << endl; • return 0; • } CS410 – Software Engineering C++: Matrix example

  27. Output • Matrix x • 1 • 2 • Matrix y • 3 4 • Matrix z • 0 • x*y = • 3 4 • 6 8 Matrix z = x*y (note new dimensions) 3 4 6 8 Matrix x2 = 2*x 2 4 x + x2 = 3 6 CS410 – Software Engineering C++: Matrix example

  28. Final Remarks • The full code Matrix.cpp is in the Examples section (see the course homepage) • ALL the code should have had error checking, for example: assert(sizeX > 0); if (m1.dy != m2.dx) throw MatMulException();The on-line copy of Matrix.cpp has asserts. • We didn’t code *= for matrices; this is left as an exercise. • Another exercise: define a class BinaryTree, and figure out what overloaded operators should be defined for it. (“BinTree x = y + z” ?) CS410 – Software Engineering C++: Matrix example

  29. Appendix • The class version of this Powerpoint file had a couple of bugs. These bugs are instructive, especially for those with a Java background. Hence this appendix. • We have three places in the code where we need to allocate the arrays p and the p[i] arrays: • Matrix(int sizeX, int sizeY); //constructor • Matrix(const Matrix& m); //copy constructor • Matrix& operator=(const Matrix& rhs); • //copy assignment CS410 – Software Engineering C++: Matrix example

  30. Appendix • We'd like to reuse the code that does these allocations. Above, I've introduced the private method allocArrays() to do this. • In Java and in C#, one constructor can call another constructor in the same class: // Java Matrix(Matrix m) { this(m.dx, m.dy); } // C# Matrix(Matrix m) : this(m.dx, m.dy) { } CS410 – Software Engineering C++: Matrix example

  31. Appendix • I tried to do something similar: • Matrix::Matrix(const Matrix& m) • : Matrix(m.dx, m.dy) {...} Illegal! Matrix::Matrix(const Matrix& m) { Matrix(m.dx, m.dy); ... } Legal, but just creates an anonymous Matrix object – doesn't affect "this" CS410 – Software Engineering C++: Matrix example

  32. Appendix • In operator=, I tried: • if (dx != m.dx || dy != m.dy) { • this->~Matrix(); • Matrix(m.dx, m.dy); • } legal! And does the right thing. Legal, but just creates an anonymous Matrix object – doesn't affect "this" Note: “this->Matrix(...)” is illegal! For more on this topic, see: http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.3 CS410 – Software Engineering C++: Matrix example