Re: A very weird bug......
Richard Herring wrote:
In message
<3b768e92-a91d-4e5b-81af-4fc268db63b9@n77g2000hse.googlegroups.com>,
xz <zhang.xi.cn@gmail.com> writes
Floor.h:
class Floor{
public :
private:
int m; //# of rows of vertices
int n; //# of columns of vertices
double r;
That's a member variable called "r". I'd recommend choosing a naming
convention that makes it clear when something is a member.
int meshOrder;
std::vector< std::vector<Vertex*> > floorVertices;
public:
Floor(double x0, double y0, double xM, double yM, double r = 1.0, int
meshOrder = 1);
//...
void describe();
Probably ought to be const.
};
in Floor.cpp:
Constructor:
Floor::Floor(double x0, double y0, double xM, double yM, double r,
That's a parameter called "r", local to the implementation of the
constructor. Although it happens to have the same name, it is
completely unrelated to the member variable called "r". The compiler
cannot read your mind.
int
meshOrder) {
m = int ((yM - y0)/r + 1);
n = int ((xM - x0)/r + 1);
You probably need to learn about initialization lists.
floorVertices = *(new vector< vector<Vertex*> >(m, vector<Vertex*>
(n) ) );
floorVertices[0][0] = new Vertex(x0, y0);
for(int j = 1; j < n; j++) {
floorVertices[0][j] = new Vertex(j+x0, y0);
floorVertices[0][j]->addAdjacency(floorVertices[0][j-1]);
}
for(int i = 1; i < m; i++) {
floorVertices[i][0] = new Vertex(x0, i+y0);
floorVertices[i][0]->addAdjacency(floorVertices[i-1][0]);
}
for(int i = 1; i < m; i++) {
for(int j = 1; j < n; j++) {
floorVertices[i][j] = new Vertex(j+x0, i+y0);
floorVertices[i][j]->addAdjacency(floorVertices[i-1][j]);
floorVertices[i][j]->addAdjacency(floorVertices[i][j-1]);
}
}
}
None of the above initializes the member variable "r", or assigns
anything to it. Its value is therefore undefined.
Floor has a self-describe function as follows:
void Floor::describe() {
cout << "The floor is represented by the mesh as follows: \n"
"(r = " << r << ", meshOrder = " << meshOrder << "\n";
That reports the member variable r, which has never been initialized.
for(int i = 0; i < floorVertices.size(); i++) {
for(int j = 0; j < floorVertices[i].size(); j++) {
floorVertices[i][j]->describe();
}
}
}
In a test file I have lines like:
Floor keck(-5.0, -5.0, 10.0, 10.0, 1.0);
//Floor keck(-5.0, -5.0, 10.0, 10.0);
keck.describe();
In the information printed by keck.describe(), I got:
(r = 2.33637e-310, meshOrder = 0)
With DDD I checked the value of "r" inside the constructor, it is *1*
all the
way to the end of the constructor.
That's the constructor parameter "r". It's not the member variable
"r".
I have also tried changing r to public (for debugging), and printed
it out right after the constructor, I found it being 2.36305e-310
there. Seems like the value is changed at the end of the constructor.
No, the value of the member variable is never set.
And the same applies equally to the two variables called "meshOrder".
Also, one way to fix it is with what is called an "initialization list" in
the constructor.
Floor(double x0, double y0, double xM, double yM, double r = 1.0,
int meshOrder = 1): r(r), meshOrder(meshOrder)
The initializaiton list is one place where you can use variables of the same
name and the compiler will know which one you are talking about. In this
case, r(r) means to initialize the class variable r with the parameter r,
meshOrder(meshOrder) means to initialize the class variable meshOrder with
the parameter meshOrder.
This is one reason I personally distinuish my local class variable names
from parameters. I personally use an underscore at the end of the variable,
some people use capital letters or all small, etc.. So my varialbes would
be:
private:
int m_; //# of rows of vertices
int n_; //# of columns of vertices
double r_;
int meshOrder_;
std::vector< std::vector<Vertex*> > floorVertices_;
and my constructor would look like:
Floor(double x0, double y0, double xM, double yM, double r = 1.0,
int meshOrder = 1): r_(r), meshOrder_(meshOrder)
--
Jim Langston
tazmaster@rocketmail.com