C++ for Mathematicians An Introduction for Students and Professionals phần 5 pdf

The Projective Plane 185
30 int main() {
31 Base B(1,2);
32 Child C(3,4);
33 GrandChild D(5,6,7);
34
35 B.print(); cout << endl;
36 // cout << B.sum() << endl; // Illegal, sum is protected
37
38 C.print(); cout << " > ";
39 C.increase_b();
40 C.print(); cout << endl;
41
42 D.print(); cout << " > ";
43 D.increase_b();
44 D.print(); cout << endl;
45
46 return 0;
47 }
In this program we define three classes: Base, Child,andGrandChild; each is
used to derive the next.
The
Base class has two data members: a private integer a and a protected integer
b. The class also includes a protected method named sum, a public constructor, and
a public method named
print.
Class
Child has no additional data members. It has a public method increase_b
that increases the data member b by one. Note that it would not be possible for
Child to have a similar method for increasing a. The constructor for Child passes
its arguments on to its parent,

Base, but then takes no further action (hence the curly
braces on line 17 do not enclose any statements).
The
print method for Child uses Base’s print method and sum method.
Class
GrandChild adds an extra p rivate data element, c. The constructor for
GrandChild passes its first two arguments to its parent’s constructor and then uses
the third argument to set the value of
c.
The
print method for GrandChild uses its grandparent’s print method. Al-
though
Base::print() invokes the sum method, the GrandChild methods cannot
directly call
sum because it is protected in Base, hence implicitly private in Child,
and hence inaccessible in
GrandChild.
A
main to illustrate all these ideas begins on line 30 . The output of the program
follows.
✞
☎
(1,2)
(3,4)=7 > (3,5)=8
(5,6)/7 > (5,7)/7
✝ ✆
186 C++ for Mathematicians
10.5 Class and file organization for PPoint and PLine
Because of the duality between points and lines in the projective plane, most of
the data and code we use to represent these concepts in C++ are the same. If at all

possible, we should avoid writing the same code twice for two reasons. First, the
initial work in creating the programs is doubled. More important, maintaining the
code is also made more difficult; if a change is required to the code, we need to
remember to make that change twice. When we are fussing with our programs and
making a number of minor modifications, it is easy to forget to update both versions.
To illustrate this, we deliberately include a subtle flaw in the first version of the
program; we then repair the problem once (not twice).
To this end, we define three classes: a parent class named
PObject that contains
as much of the code as possible and two derived classes,
PPoint and PLine,that
include code particular to each. These classes are defined in two files each: a header
.h file and a code .cc file. Figure 10.3 illustrates the organ ization.
PObject
PObject.h
PObject.cc
PPoint
PPoint.h
PPoint.cc
PLine
PLine.h
PLine.cc
Figure 10.3: Hierarchy of the PObject classes.
The header file PObject.h is used to declare the PObject class. Both PPoint.h
and PLine.h require the directive #include "PObject.h". Programs that use
projective geometry need all three. Rather than expecting the user to type multiple
#include directives, we create a convenient header file that includes everything we
need. We call this header file
Projective.h;hereitis.
Program 10.3: Header file for all pro jective geometry classes, Projective.h.

1 #ifndef PROJECTIVE_H
2 #define PROJECTIVE_H
3 #include "PPoint.h"
4 #include "PLine.h"
5 #endif
The Projective Plane 187
Notice that we do not require
#include "PObject.h" because that file is al-
ready
#includedbyPPoint.h and PLine.h. All these files have the usual struc-
ture to prevent multiple inclusion.
All told, we have seven files that implement points and lines in the projective
plane.
PObject.h The header file for the
PObject class.
PObject.cc The C++ code for the
PObject class.
PPoint.h The header file for the
PPoint class.
PPoint.cc The C++ code for the
PPoint class.
PLine.h The header file for the
PLine class.
PLine.cc The C++ code for the
PLine class.
Projective.h The only header file subsequent programs need to include to use
projective points and lines.
The decision to write the program in seven different files is based on the principle
of breaking a problem down to manageable sizes and working on each piece sep-
arately. We could have packed all this work into two larger files (one .h and one

.cc).
10.6 The parent class PObject
Data and functionality common to PPoint and PLine are implemented in the
class
PObject. Using the ideas presented in Section 10.2, we map out the class
PObject.
Data A point or line in RP
2
is represented by homogeneous coordinates: (x,y,z) or
[x,y,z]. To hold these coordinates, we declare three private double variables,
x, y,andz. Let us write x,y,z for the homogeneous coordinates of a generic
projective object (either a point of a line). (See Program 10.4, line 9.)
Because 2,1, −5 and −4,−2, 10 name the same object, it is useful to
choose a canonical triple. One idea is to make sure that x,y,z is a unit vector
(but then we have a sign ambiguity). The representation we use is to make the
last nonzero coordinate of the triple equal to 1. The motivation for this choice
is that a point in the Euclidean plane at coordinates (x,y) corresponds to the
point (x, y,1) in the projective plane.
If later we are unhappy with this decision, we can choose another manner
to store the homogeneous coordinates. Because the coordinates are private
members of
PObject, we would only n eed to update the code for PObject.
188 C++ for Mathematicians
We provide public methods
getX(), getY(),andgetZ() to inspect (but
not modify) the values held in
x, y,andz, respectively. (See Program 10.4
lines 33–35.)
We reserve 0, 0,0to stand for an invalid projective object. We provide a pub-
lic method

is_invalid() to check if an object is invalid. ( See Program 10.4
lines 37–39.)
Constructors It is natural to define a constructor for a
PObject with three parame-
ters that set the homogeneous coordinates of the object. The user might invoke
the constructor like this:
PObject P(2., 3., 5.);
Rather than holding this point as 2,3,5, we use the canonical representation
0.4,0.6, 1. (See Program 10.4 lines 25–30.)
To facilitate the conversion of user-supplied coordinates to canonical coordi-
nates, we create a private helper procedure
scale. (See Program 10.4 line 10
and Program 10.5 lines 4–19.)
All C++ classes ought to define a zero-argument constructor that creates a
default object of that class. In this case, a sensible choice is the object 0,0,1.
This corresponds to the origin (as a point) or the line at infinity (as a line) . (See
Program 10.4 lines 21–24.)
Relations We want to be able to co mpare projective points (and lines) f or equality,
inequality, and < (for sorting). To this end, we might be tempted to define
operator== in the public portion of PObject like this:
public:
bool operator==(const PObject& that) const {
return ( (x==that.x) && (y==that.y) && (z==that.z) );
}
Although this would give the desired result when comparing points to points
or lines to lines, it would also provide the unfortunate ability to compare points
to lines. Were we to use the above method for testing equality, then we might
run into the following situation.
PPoint P(2., 3., 5.);
PLine L(2., 3., 5.);

if (P == L) {
cout << "They are equal." << endl;
}
When this code runs, the message They are equal wou ld be written on the
computer’s screen.
There are at least two problems with this approach. First, the point (2,3,5) and
the line [2 , 3,5] are not equal even though they have the same homogeneous
The Projective Plane 189
coordinates. Second, lines and points should not even be comparable by
==.
Code that compares a point to a line is alm ost cer tainly a bug; the best thing in
this situation is for the compiler to flag such an expression as an error.
We therefore take a different approach to equality testin g. We want the fun-
damental code that checks for equality to reside inside the
PObject class be-
cause that code is common to both
PPoint and PLine. We d o this by declar-
ing a protected method called
equals inside PObject (see Program 10.4
line 14):
protected:
bool equals(const PObject& that) const;
In the file PObject.cc we give the code for this method (see Program 10.5
lines 34–36):
bool PObject::equals (const PObject& that) const {
return ( (x==that.x) && (y==that.y) && (z==that.z) );
}
Then, in the class definitions for PPoint and PLine we give the necessary
operator definitions. For example, in
PLine we have this:

public:
bool operator==(const PLine& that) const {
return equals(that);
}
bool operator!=(const PLine& that) const {
return !equals(that);
}
In this way, if (indeed, when) we decide to change the manner in which we test
for equality, we only need to update
PObject’s protected equals method.
Similarly, rather than defining a
< operator for PObject,wedefine a protected
less method. The children can access less to define their individual, public
operator< methods. (See Program 10.4 line 15 and Program 10.5 lines 38–
45.)
Meet/Join Operation Given two
PPoints P and Q,wewantP+Q to return the line
through those points. Dually, given
PLines L and M,wewantL
*
M to return
the point of intersection of these lines.
In both cases, the calculations are the same: given the triples x
1
,y
1
,z
1
 and
x

2
,y
2
,z
2
 we need to find a new triple x
3
,y
3
,z
3
 that is orthogonal to the first
two. To do this, we calculate the cross product:
x
3
,y
3
,z
3
 = x
1
,y
1
,z
1
×x
2
,y
2
,z

2
.
Therefore, in
PObject we declare a protected method called op that is invoked
by
operator+ in PPoint and operator
*
in PLine. (See Program 10.4
line 18 and Program 10.5 lines 138–148.)
190 C++ for Mathematicians
Incidence Given a point and a line, we want to be able to determine if the point lies
on the line. If the coordinates for these are (x,y,z) and [a,b,c], r espectively,
then we simply need to test if ax + by + cz = 0.
It does not make sense to ask if one line is incident with another, so we do not
make an “is incident with” method publicly available in
PObject.Rather,we
make a protected
incident method (that calls, in turn, a private dot method
for calculating dot product). (See Program 10.4 lines 11,16 and Program 10.5
lines 21–23,47–49.)
Then,
PPoint and PLine can declare their own public methods that access
incident. Details on this later.
Collinearity/Co ncurrence Are three given points collinear? Are three given lines
concurrent? If the three objects have coordinates x
1
,y
1
,z
1

, x
2
,y
2
,z
2
,and
x
3
,y
3
,z
3
, then the answer is yes if and only if the vectors are linearly depen-
dent. We check this by calculating
det
⎡
⎣
x
1
y
1
z
1
x
2
y
2
z
2

x
3
y
3
z
3
⎤
⎦
and seeing if we get zero. In
PObject.h we declare a procedure dependent
that takes three PObject arguments and returns true or false. We then use
dependent to implement procedures collinear and concurrent for the
classes
PPoint and PLine (respectively). (See Program 10.4 line 44 and
Program 10.5 lines 60–77.)
Random points/lines There is no way to generate a point uniformly at random in
the Euclid ean plane, but there is a sensible way in which we can do this for
the projective plane. Recall that points in RP
2
correspond to lines through the
origin in R
3
. Thus, to select a point at random in RP
2
we generate a point
uniformly at random on the unit ball centered at the origin. An efficient way to
perform this latter task is to select a vector v uniformly in [−1,1]
3
.Ifv> 1,
then we reject v and try again.

The method for generating a random line is precisely the same.
We therefore include a public method
randomize that resets the coordinates
of the
PObject by the algorithm we just described. (See Program 10.4 line 31
and Program 10.5 lines 25–32.)
To choose a random line through a point is similar. Suppose the point is
(x,y,z). The line [a,b,c] should be chosen so that [a, b,c] is orthogonal to
(x,y,z).Todothis,wefind an orthonormal basis for (x,y,z)
⊥
that we denote
{(a
1
,b
1
,c
1
),(a
2
,b
2
,c
2
)}. We then choose t uniformly at random in [0,2
π
].
The random line [a,b,c] is given by
a = a
1
cost + a

2
sintb= b
1
cost + b
2
sintc= c
1
cost + c
2
sint.
The Projective Plane 191
Rather than generate t and then compute two trigonometric functions, we can
obtain the pair (cost,sint) by choosing a point uniformly at random in the unit
disk (in R
2
) and then scaling.
The technique for selecting a random point on a g iven line is exactly the same.
Thus, we define a protected
rand_perp method in PObject (Program 10.4
line 17 and Program 10.5 lines 79–136).
The
rand_perp method is used by rand_point in PLine and rand_line
in PPoint.
Output Finally,
PObject.h declares a procedure for writing to an output stream.
The format is
<x,y,z>.Theostream operator<< defined in PObject
is overridden by like-named p rocedures in PPoint and PLine.(SeePro-
gram 10.4 line 42 and Program 10.5 lines 51–58.)
With these explanations in place, we now g ive the header and code files for the

class
PObject.
Program 10.4: Header file for the PObject class (version 1).
1 #ifndef POBJECT_H
2 #define POBJECT_H
3
4 #include <iostream>
5 using namespace std;
6
7 class PObject {
8 private:
9 double x,y,z;
10 void scale();
11 double dot(const PObject& that) const;
12
13 protected:
14 bool equals(const PObject& that) const;
15 bool less(const PObject& that) const;
16 bool incident(const PObject& that) const;
17 PObject rand_perp() const;
18 PObject op(const PObject& that) const;
19
20 public:
21 PObject() {
22 x=y=0.;
23 z=1.;
24 }
25 PObject(double a, double b, double c) {
26 x=a;
27 y=b;

28 z=c;
29 scale();
30 }
31 void randomize();
32
33 double getX() const { return x; }
192 C++ for Mathematicians
34 double getY() const { return y; }
35 double getZ() const { return z; }
36
37 bool is_invalid() const {
38 return (x==0.) && (y==0.) && (z==0.);
39 }
40 };
41
42 ostream& operator<<(ostream& os, const PObject& A);
43
44 bool dependent(const PObject& A, const PObject& B, const PObject& C);
45
46 #endif
Program 10.5: Program file for the PObject class (version 1).
1 #include "PObject.h"
2 #include "uniform.h"
3
4 void PObject::scale() {
5 if (z != 0.) {
6 x/=z;
7 y/=z;
8 z=1.;
9 return;

10 }
11 if (y != 0.) {
12 x/=y;
13 y=1.;
14 return;
15 }
16 if (x != 0) {
17 x=1.;
18 }
19 }
20
21 double PObject::dot(const PObject& that) const {
22 return x
*
that.x + y
*
that.y + z
*
that.z;
23 }
24
25 void PObject::randomize() {
26 do {
27 x = unif(-1.,1.);
28 y = unif(-1.,1.);
29 z = unif(-1.,1.);
30 } while (x
*
x+y
*

y+z
*
z > 1.);
31 scale();
32 }
33
34 bool PObject::equals(const PObject& that) const {
35 return ( (x==that.x) && (y==that.y) && (z==that.z) );
36 }
37
38 bool PObject::less(const PObject& that) const {
39 if (x < that.x) return true;
The Projective Plane 193
40 if (x > that.x) return false;
41 if (y < that.y) return true;
42 if (y > that.y) return false;
43 if (z < that.z) return true;
44 return false;
45 }
46
47 bool PObject::incident(const PObject& that) const {
48 return dot(that)==0.;
49 }
50
51 ostream& operator<<(ostream& os, const PObject& A) {
52 os << "<"
53 << A.getX() << ","
54 << A.getY() << ","
55 << A.getZ()
56 << ">";

57 return os;
58 }
59
60 bool dependent(const PObject& A, const PObject& B, const PObject& C){
61 double a1 = A.getX();
62 double a2 = A.getY();
63 double a3 = A.getZ();
64
65 double b1 = B.getX();
66 double b2 = B.getY();
67 double b3 = B.getZ();
68
69 double c1 = C.getX();
70 double c2 = C.getY();
71 double c3 = C.getZ();
72
73 double det = a1
*
b2
*
c3 + a2
*
b3
*
c1+a3
*
b1
*
c2
74 -a3

*
b2
*
c1 - a1
*
b3
*
c2-a2
*
b1
*
c3;
75
76 return det == 0.;
77 }
78
79 PObject PObject::rand_perp() const {
80 if (is_invalid()) return PObject(0,0,0);
81
82 double x1,y1,z1; // One vector orthogonal to (x,y,z)
83 double x2,y2,z2; // Another orthogonal to (x,y,z) and (x1,y1,z1)
84
85 if (z == 0.) { // If z==0, take (0,0,1) for (x1,y1,y2)
86 x1 = 0;
87 y1 = 0;
88 z1 = 1;
89 }
90 else {
91 if (y == 0.) { // z != 0 and y == 0, use (0,1,0)
92 x1=0;

93 y1=1;
94 z1=1;
95 }
194 C++ for Mathematicians
96 else { // y and z both nonzero, use (0,-z,y)
97 x1=0;
98 y1=-z;
99 z1=y;
100 }
101 }
102
103 // normalize (x1,y1,z1)
104 double r1 = sqrt(x1
*
x1 + y1
*
y1 + z1
*
z1);
105 x1 /= r1;
106 y1 /= r1;
107 z1 /= r1;
108
109 // (get x2,y2,z2) by cross product with (x,y,z) and (x1,y1,z1)
110 x2 = -(y1
*
z)+y
*
z1;
111 y2 = x1

*
z-x
*
z1;
112 z2 = -(x1
*
y)+x
*
y1;
113
114 // normalize (x2,y2,z2)
115 double r2 = sqrt(x2
*
x2 + y2
*
y2 + z2
*
z2);
116 x2 /= r2;
117 y2 /= r2;
118 z2 /= r2;
119
120 // get a point uniformly on the unit circle
121 double a,b,r;
122 do {
123 a = unif(-1.,1.);
124 b = unif(-1.,1.);
125 r=a
*
a+b

*
b;
126 } while (r > 1.);
127 r = sqrt(r);
128 a/=r;
129 b/=r;
130
131 double xx = x1
*
a+x2
*
b;
132 double yy = y1
*
a+y2
*
b;
133 double zz = z1
*
a+z2
*
b;
134
135 return PObject(xx,yy,zz);
136 }
137
138
139 PObject PObject::op(const PObject& that) const {
140
141 if (equals(that)) return PObject(0,0,0);

142
143 double c1 = y
*
that.z - z
*
that.y;
144 double c2 = z
*
that.x - x
*
that.z;
145 double c3 = x
*
that.y - y
*
that.x;
146
147 return PObject(c1,c2,c3);
148 }
The Projective Plane 195
10.7 The classes PPoint and PLine
With the code for PObject in place, we are ready to finish our work by writing the
files for the classes
PPoint and PLine. We do this work in four files: PPoint.h,
PPoint.cc, PLine.h,andPLine.cc (Programs 10.6 through 10.9).
As we start to write these files, we meet a chicken-and-egg problem. Which do
we define first: the class
PPoint or the class PLine? For us, this is more than
a philosophical conundrum. Some
PLine methods need to refer to PPoints. For

example,
operator
*
acts on lines to produce points and PLine’s rand_point
method returns a PPoint. Dually, some of the PPoint methods require the PLine
class.
Here is how we solve this dilemma. In the
PPoint.h file,beforewegivethe
definition of
PPoint we have the following statement (see line 6 of Program 10.6),
class PLine;
This lets the C++ compiler know that a class named PLine is defined somewhere
else. Therefore, when the compiler sees
PLine, it knows that th is identifier refers
to some class. However, it does not know anything else about
PLine other than the
fact it is a class. This means that we may not give an inline definition for any method
that refers to objects of type
PLine. Later, in the file PPoint.cc, we include all
the head ers (conveniently with the sing le d irective
#include "Projective.h").
Therefore, the code in
PPoint.cc can make full use of the PLine class.
Dually, we include the statement
class PPoint; in the file PLine.h.
We focus our attention o n the class
PPoint. The analysis of PLine is similar.
For the class
PPoint we have five constructors. At first, this may seem to be too
many, but we show how to do this easily.

Of course, we want a three-argument constructor
PPoint(a,b,c) that creates
the point (a, b,c). We also want a zero-argument constructor
PPoint() that creates
the point (0,0,1) corresponding to the origin.
It makes sense to define a two-argument constructor
PPoint(a,b) to create the
point (a,b,1). In a sense, this maps the Euclidean point (a, b ) to its natural cor re-
spondent in the RP
2
.
What should be the action of a single-argument constructor? The real number a
can be identified with the poin t (a,0) on the x-axis, and this in turn corresponds to
(a,0,1) in RP
2
. In summary, we have the following constructors and their effects.
Constructor form Point created
PPoint P(); (0,0,1)
PPoint P(a); (a,0,1)
PPoint P(a,b); (a,b,1)
PPoint P(a,b,c); (a,b,c)
The great news is that we can implement these four constructors with a single
definition using default parameters.
196 C++ for Mathematicians
For any C++ procedure (class method or free-standing procedure), default values
can be specified. In the
.h file, where procedures are declared, we use a syntax such
as this:
return_type procedure_name(type arg1 = val1, type arg2 = val2, );
Then, when the procedure is used, any missing parameters are replaced by their de-

fault values. Let’s look at a concrete example. We declare a procedure named
next
that produces the next integer after a given integer. (This is a contrived example, but
we want to keep things simple.) In the head er file we put the following,
int next(int num, int step=1);
Andinthe.cc file, we have the code,
int next(int num, int step) {
return num+step;
}
Notice that the argument step is given a default value, 1. However, the argument
num is not given a default. It is permissible to specify only a subset of the arguments
that receive default values, but if an argument has a d efault value, all arguments to
its right must also have default values.
Notice that the optional arguments are not reiterated in the
.cc file.
Consider the following code.
int a,b,c;
a = next(5);
b = next(5,1);
c = next(5,2);
This will set a and b equal to 6 and c equal to 7.
Alternatively, we could have given an inline definition of
next in the header file
like this:
inline int next(int num, int step=1) { return num+step; }
Returning to PPoint, the four constructors (with zero to three double arguments)
can all be declared at once like this:
PPoint(double a=0., double b=0., double c=1.)
See line 10 of Program 10.6. The action of this constructor is simply to pass the
three arguments up to its parent (line 11) and then there is nothing else to do. To

show there is nothing else, we give a pair of open/close braces that enclose empty
space (line 12).
The class
PPoint needs one more constructor. Recall that PObject provides
methods such as
rand_perp and op that return PObjects. However, when these are
used by
PPoint or PLine,thePObject values need to be converted to type PPoint
or PLine as appropriate. To do this PPoint provides a constructor that accepts a
single argument of type
PObject. Here is the simple code (see also lines 14–15 of
Program 10.6).
The Projective Plane 197
PPoint(const PObject& that) :
PObject(that.getX(), that.getY(), that.getZ()) { }
This code sends the x, y,andz values held in that up to the parent constructor and
then does nothing else. Now, if we want to assign a
PObject value to a PPoint
object, we can do it in the following ways.
PObject X(2.,3.,5.);
PPoint P(X); // invoke the constructor at declaration of P
PPoint Q;
Q = PPoint(X); // invoke the constructor as a converter procedure
PPoint R;
R = X; // implicit conversion (compiler figures out what to do)
Here is the PPoint.h header file.
Program 10.6: Header file for the PPoint class.
1 #ifndef PPOINT_H
2 #define PPOINT_H
3

4 #include "PObject.h"
5
6 class PLine;
7
8 class PPoint : public PObject {
9 public:
10 PPoint(double a = 0., double b = 0., double c = 1.) :
11 PObject(a,b,c)
12 {}
13
14 PPoint(const PObject& that) :
15 PObject(that.getX(), that.getY(), that.getZ()) { }
16
17 bool operator==(const PPoint& that) const {
18 return equals(that);
19 }
20
21 bool operator!=(const PPoint& that) const {
22 return !equals(that);
23 }
24
25 bool operator<(const PPoint& that) const {
26 return less(that);
27 }
28
29 PLine operator+(const PPoint& that) const;
30
31 bool is_on(const PLine& that) const;
32
33 PLine rand_line() const;

34
35 };
198 C++ for Mathematicians
36
37 ostream& operator<<(ostream& os, const PPoint& P);
38
39 inline bool
40 collinear(const PPoint& A, const PPoint& B, const PPoint& C) {
41 return dependent(A,B,C);
42 }
43
44 #endif
On lines 17–26 we give inline definitions of operator==, operator!=,and
operator<.However,operator+, is_on,andrand_line may not be given in-
line because these refer to the (as yet) unknown class
PLine. (See lines 29–33.)
The
collinear procedure simply invokes the dependent procedure defined in
PObject.h,sowegiveitinlinehere. Theinline keyword in mandatory here
because
collinear is not a member of any class.
The p arts of
PPoint not given in PPoint.h are defined in PPoint.cc which we
present next.
Program 10.7: Program file for the PPoint class.
1 #include "Projective.h"
2
3 ostream& operator<<(ostream& os, const PPoint& P) {
4 os << "(" << P.getX() << "," << P.getY() << "," << P.getZ() << ")";
5 return os;

6 }
7
8 PLine PPoint::operator+(const PPoint& that) const {
9 return PLine(op(that));
10 }
11
12 PLine PPoint::rand_line() const {
13 return PLine(rand_perp());
14 }
15
16 bool PPoint::is_on(const PLine& that) const {
17 return incident(that);
18 }
The code for the class PLine is extrem ely similar to that of PPoint.Herearethe
files
PLine.h and PLine.cc for your perusal.
Program 10.8: Header file for the PLine class.
1 #ifndef PLINE_H
2 #define PLINE_H
3
4 #include "PObject.h"
5
6 class PPoint;
7
8 class PLine : public PObject {
The Projective Plane 199
9 public:
10 PLine(double a = 0., double b = 0., double c = 1.) :
11 PObject(a,b,c)
12 {}

13
14 PLine(const PObject& that) :
15 PObject(that.getX(), that.getY(), that.getZ()) { }
16
17 bool operator==(const PLine& that) const {
18 return equals(that);
19 }
20
21 bool operator!=(const PLine& that) const {
22 return !equals(that);
23 }
24
25 bool operator<(const PLine& that) const {
26 return less(that);
27 }
28
29 PPoint operator
*
(const PLine& that) const;
30
31 bool has(const PPoint& X) const;
32
33 PPoint rand_point() const;
34
35 };
36
37 ostream& operator<<(ostream& os, const PLine& P);
38
39 inline bool
40 concurrent(const PLine& A, const PLine& B, const PLine& C) {

41 return dependent(A,B,C);
42 }
43
44 #endif
Program 10.9: Program file for the PLine class.
1 #include "Projective.h"
2
3 ostream& operator<<(ostream& os, const PLine& P) {
4 os << "[" << P.getX() << "," << P.getY() << "," << P.getZ() << "]";
5 return os;
6 }
7
8 PPoint PLine::rand_point() const {
9 return PPoint(rand_perp());
10 }
11
12 PPoint PLine::operator
*
(const PLine& that) const {
13 return PLine(op(that));
14 }
15
16 bool PLine::has(const PPoint& that) const {
200 C++ for Mathematicians
17 return incident(that);
18 }
10.8 Discovering and repairing a bug
With the projective point and line classes built, it is time to test our code. Here is
a simple main to perform some checks.
Program 10.10: A main to test the RP

2
classes.
1 #include <iostream>
2 #include "Projective.h"
3 #include "uniform.h"
4
5 int main() {
6 seed();
7 PPoint P;
8
9 P.randomize();
10 cout << "The random point P is " << P << endl;
11
12 PLine L,M;
13
14 L = P.rand_line();
15 M = P.rand_line();
16
17 cout << "Two lines through P are L = " << L << endl
18 << "and M = " << M << endl;
19
20 cout << "Is P on L? " << P.is_on(L) << endl;
21 cout << "Does M have P? " << M.has(P) << endl;
22
23 PPoint Q;
24 Q=L
*
M;
25
26 cout << "The point of intersection of L and M is Q = " << Q << endl;

27
28 cout << "Is Q on L? " << Q.is_on(L) << endl;
29 cout << "Does M have Q? " << M.has(Q) << endl;
30
31 if (P==Q) {
32 cout << "P and Q are equal" << endl;
33 }
34 else {
35 cout << "P and Q are NOT equal" << endl;
36 }
37
38 return 0;
39 }
The Projective Plane 201
When this program is run, we have the following output:
✞
☎
The random point P is (-1.32445,0.591751,1)
Two lines through P are L = [6.51303,12.8875,1]
and M = [0.871229,0.260071,1]
IsPonL?0
Does M have P? 0
The point of intersection of L and M is Q = (-1.32445,0.591751,1)
IsQonL?0
Does M have Q? 0
P and Q are NOT equal
✝ ✆
Much of what we see here doesn’t make sense. First, the lines L and M are random
lines through P. Yet the output indicates that P is on neither of these lines. Then,
we generate the point Q at the intersection of L and M. The good news is that the

coordinates of P and Q are the same: (−5.64488,2.562,1). However, the computer
still tells us that Q is on neither L nor M. Worse, it thinks that P = Q. What is going
on here!?
All these problems stem from the same underlying cause: roundoff. Remember
that a
double variable is a rational approximation to a real number. Two real quanti-
ties that are computed differently may result in
double values that are different. For
example, consider this code:
#include <iostream>
using namespace std;
int main() {
double x = 193./191.;
double y = 1./191.;
y
*
= 193;
if (x == y) {
cout << "They are equal" << endl;
}
else {
cout << "They are different" << endl;
cout << "Difference = " << x-y << endl;
}
return 0;
}
Here is the output from this program.
✞
☎
They are different

Difference = -2.22045e-16
✝ ✆
The computer reports that
193
191
= 193 ×
1
191
.
The eq uality test we created for
PObjects checks if the three coordinates are
exactly the same. We need to relax this.
The bad news is we need to rewrite some of our code to correct this problem. The
great news is that we only need to repair
PObject. The children PPoint and PLine
inherit the improvements.
202 C++ for Mathematicians
To begin, let us identify the places in the code for
PObject wher e exact equality
is sought.
• The
equals method requires exact equality of the three coordinates.
• The
incident method requires zero to be the exact result of the dot product
method.
• The
dependent procedure requires zero to be the exact value of the determi-
nant.
In lieu of exact equality, we can require that the values be within a small tolerance.
What tolerance should we use? We make that quantity user selectable initialized with

some default value (say 10
−12
).
To implement this idea we add a private static
double variable named tolerance
and define a constant named default_tolerance set to 10
−12
.
Inside
PObject we define two inline public static methods: set_tolerance and
get_tolerance. Here is the revised header file.
Program 10.11: Header file for the PObject class (version 2).
1 #ifndef POBJECT_H
2 #define POBJECT_H
3 #include <cmath>
4 #include <iostream>
5 using namespace std;
6
7 const double default_tolerance = 1e-12;
8
9 class PObject {
10 private:
11 double x,y,z;
12 void scale();
13 double dot(const PObject& that) const;
14 static double tolerance;
15
16 protected:
17 bool equals(const PObject& that) const;
18 bool less(const PObject& that) const;

19 bool incident(const PObject& that) const;
20 PObject rand_perp() const;
21 PObject op(const PObject& that) const;
22
23 public:
24 PObject() {
25 x=y=0.;
26 z=1.;
27 }
28 PObject(double a, double b, double c) {
29 x=a;
30 y=b;
31 z=c;
32 scale();
The Projective Plane 203
33 }
34 void randomize();
35
36 static void set_tolerance(double t) {
37 tolerance = abs(t);
38 }
39
40 static double get_tolerance() {
41 return tolerance;
42 }
43
44 double getX() const { return x; }
45 double getY() const { return y; }
46 double getZ() const { return z; }
47

48 bool is_invalid() const {
49 return (x==0.) && (y==0.) && (z==0.);
50 }
51
52 };
53
54 ostream& operator<<(ostream& os, const PObject& A);
55
56 bool dependent(const PObject& A, const PObject& B, const PObject& C);
57
58 #endif
Inside PObject.cc we need to declare PObject::tolerance and we g ive it an
initial value. (See Program 10.1 2, line 4.)
We also need to modify the
equals, incident,anddependent procedures to
test for near eq uality instead of exact equality. You can find these modifications on
lines 37–39, 52, and 80 o f the new
PObject.cc file which we present here.
Program 10.12: Program file for the PObject class (version 2).
1 #include "PObject.h"
2 #include "uniform.h"
3
4 double PObject::tolerance = default_tolerance;
5
6 void PObject::scale() {
7 if (z != 0.) {
8 x/=z;
9 y/=z;
10 z=1.;
11 return;

12 }
13 if (y != 0.) {
14 x/=y;
15 y=1.;
16 return;
17 }
18 if (x != 0) {
19 x=1.;
20 }
204 C++ for Mathematicians
21 }
22
23 double PObject::dot(const PObject& that) const {
24 return x
*
that.x + y
*
that.y + z
*
that.z;
25 }
26
27 void PObject::randomize() {
28 do {
29 x = unif(-1.,1.);
30 y = unif(-1.,1.);
31 z = unif(-1.,1.);
32 } while (x
*
x+y

*
y+z
*
z > 1.);
33 scale();
34 }
35
36 bool PObject::equals(const PObject& that) const {
37 double d = abs(x-that.x) + abs(y-that.y) + abs(z-that.z);
38
39 return d <= tolerance;
40 }
41
42 bool PObject::less(const PObject& that) const {
43 if (x < that.x) return true;
44 if (x > that.x) return false;
45 if (y < that.y) return true;
46 if (y > that.y) return false;
47 if (z < that.z) return true;
48 return false;
49 }
50
51 bool PObject::incident(const PObject& that) const {
52 return abs(dot(that)) <= tolerance;
53 }
54
55 ostream& operator<<(ostream& os, const PObject& A) {
56 os << "<"
57 << A.getX() << ","
58 << A.getY() << ","

59 << A.getZ()
60 << ">";
61 return os;
62 }
63
64 bool dependent(const PObject& A, const PObject& B, const PObject& C){
65 double a1 = A.getX();
66 double a2 = A.getY();
67 double a3 = A.getZ();
68
69 double b1 = B.getX();
70 double b2 = B.getY();
71 double b3 = B.getZ();
72
73 double c1 = C.getX();
74 double c2 = C.getY();
75 double c3 = C.getZ();
76
The Projective Plane 205
77 double det = a1
*
b2
*
c3 + a2
*
b3
*
c1+a3
*
b1

*
c2
78 -a3
*
b2
*
c1 - a1
*
b3
*
c2-a2
*
b1
*
c3;
79
80 return abs(det) <= PObject::get_tolerance();
81 }
82
83 PObject PObject::rand_perp() const {
84 if (is_invalid()) return PObject(0,0,0);
85
86 double x1,y1,z1; // One vector orthogonal to (x,y,z)
87 double x2,y2,z2; // Another orthogonal to (x,y,z) and (x1,y1,z1)
88
89 if (z == 0.) { // If z==0, take (0,0,1) for (x1,y1,y2)
90 x1 = 0;
91 y1 = 0;
92 z1 = 1;
93 }

94 else {
95 if (y == 0.) { // z != 0 and y == 0, use (0,1,0)
96 x1=0;
97 y1=1;
98 z1=1;
99 }
100 else { // y and z both nonzero, use (0,-z,y)
101 x1=0;
102 y1=-z;
103 z1=y;
104 }
105 }
106
107 // normalize (x1,y1,z1)
108 double r1 = sqrt(x1
*
x1 + y1
*
y1 + z1
*
z1);
109 x1 /= r1;
110 y1 /= r1;
111 z1 /= r1;
112
113 // (get x2,y2,z2) by cross-product with (x,y,z) and (x1,y1,z1)
114 x2 = -(y1
*
z)+y
*

z1;
115 y2 = x1
*
z-x
*
z1;
116 z2 = -(x1
*
y)+x
*
y1;
117
118 // normalize (x2,y2,z2)
119 double r2 = sqrt(x2
*
x2 + y2
*
y2 + z2
*
z2);
120 x2 /= r2;
121 y2 /= r2;
122 z2 /= r2;
123
124 // get a point uniformly on the unit circle
125 double a,b,r;
126 do {
127 a = unif(-1.,1.);
128 b = unif(-1.,1.);
129 r=a

*
a+b
*
b;
130 } while (r > 1.);
131 r = sqrt(r);
132 a/=r;
206 C++ for Mathematicians
133 b/=r;
134
135 double xx = x1
*
a+x2
*
b;
136 double yy = y1
*
a+y2
*
b;
137 double zz = z1
*
a+z2
*
b;
138
139 return PObject(xx,yy,zz);
140 }
141
142

143 PObject PObject::op(const PObject& that) const {
144
145 if (equals(that)) return PObject(0,0,0);
146
147 double c1 = y
*
that.z - z
*
that.y;
148 double c2 = z
*
that.x - x
*
that.z;
149 double c3 = x
*
that.y - y
*
that.x;
150
151 return PObject(c1,c2,c3);
152 }
When the test program (Program 10.10) is run with the new PObject class, we
achieve the desired results.
✞
☎
The random point P is (-0.479902,-0.616199,1)
Two lines through P are L = [0.384191,1.32364,1]
and M = [1.66531,0.32589,1]
IsPonL?1

Does M have P? 1
The point of intersection of L and M is Q = (-0.479902,-0.616199,1)
IsQonL?1
Does M have Q? 1
P and Q are equal
✝ ✆
The user may invoke
PObject::set_tolerance(0.0); to revert to the previ-
ous behavior (exact checking).
Finally, the method we use for testing near equality can be improved. For example,
we check if two projective objects are equal by computing their L
1
distance and
comparing against
tolerance. Alternatively, to check x
1
,y
1
,z
1
 and x
2
,y
2
,z
2

for equality, we might consider a test such as this:
|x
1

−x
2
|+ |y
1
−y
2
|+ |z
1
−z
2
|
|x
1
|+ |x
2
|+ |y
1
|+ |y
2
|+ |z
1
|+ |z
2
|
≤
ε
.
Whatever equality test you feel is most appropriate, it is only necessary to edit one
method (
equals in PObject) to implement your choice.

The Projective Plane 207
10.9 Pappus revisited
We close this section with a program to illustrate Pappus’s Theorems and the use
of the near-equality testing.
Program 10.13: A program to illustrate Pappus’s theorem and its dual.
1 #include "Projective.h"
2 #include "uniform.h"
3
4 /
**
5
*
An illustration of Pappus’s theorem and its dual
6
*
/
7
8 void pappus() {
9 seed();
10
11 // two random lines
12 PLine L1,L2;
13 L1.randomize();
14 L2.randomize();
15 cout << "The two lines are " << endl << L1 << " and " << L2 << endl;
16
17 // get three points on the first
18 PPoint P1 = L1.rand_point();
19 PPoint P2 = L1.rand_point();
20 PPoint P3 = L1.rand_point();

21
22 cout << "Three points on the first line are " << endl
23 << P1 << endl << P2 << endl << P3 << endl;
24
25 // get three points on the second
26 PPoint Q1 = L2.rand_point();
27 PPoint Q2 = L2.rand_point();
28 PPoint Q3 = L2.rand_point();
29
30 cout << "Three points on the second line are " << endl
31 << Q1 << endl << Q2 << endl << Q3 << endl;
32
33 // find the three pairwise intersections
34 PPoint X1 = (P2+Q3)
*
(P3+Q2);
35 PPoint X2 = (P1+Q3)
*
(P3+Q1);
36 PPoint X3 = (P1+Q2)
*
(P2+Q1);
37
38 cout << "The three points constructed are " << endl;
39 cout << X1 << endl << X2 << endl << X3 << endl;
40
41 if (collinear(X1,X2,X3)) {
42 cout << "They are collinear, as guaranteed by Pappus’s theorem"
43 << endl;
44 }

45 else {
46 cout << "TROUBLE! The three points are not collinear!!"
208 C++ for Mathematicians
47 << endl;
48 }
49 }
50
51 void dual_pappus() {
52 // Two random points
53 PPoint A,B;
54 A.randomize();
55 B.randomize();
56 cout << "The two points are " << endl << A << " and " << B << endl;
57
58 // Three lines through the first
59 PLine L1 = A.rand_line();
60 PLine L2 = A.rand_line();
61 PLine L3 = A.rand_line();
62
63 cout << "The three lines through the first point are " << endl
64 << L1 << endl << L2 << endl << L3 << endl;
65
66 // Three lines through the second
67 PLine M1 = B.rand_line();
68 PLine M2 = B.rand_line();
69 PLine M3 = B.rand_line();
70
71 cout << "The three lines through the second point are " << endl
72 << M1 << endl << M2 << endl << M3 << endl;
73

74 // Get the three dual Pappus lines
75 PLine X1 = L2
*
M3 + L3
*
M2;
76 PLine X2 = L1
*
M3 + L3
*
M1;
77 PLine X3 = L1
*
M2 + L2
*
M1;
78
79 cout << "The three lines constructed are " << endl;
80 cout << X1 << endl << X2 << endl << X3 << endl;
81
82 if (concurrent(X1,X2,X3)) {
83 cout << "They are concurrent, as guaranteed by Pappus’s theorem"
84 << endl;
85 }
86 else {
87 cout << "TROUBLE! The three lines are not concurrent!!"
88 << endl;
89 }
90 }
91

92
93 int main() {
94 double t;
95 cout << "Enter desired tolerance > ";
96 cin >> t;
97 PObject::set_tolerance(t);
98 cout << "You set the tolerance to " << PObject::get_tolerance()
99 << endl << endl;
100
101 pappus();
102 cout << endl;
The Projective Plane 209
103 dual_pappus();
104
105 return 0;
106 }
Here are three runs of the program with the tolerance set to different values.
✞
☎
Enter desired tolerance > 0
You set the tolerance to 0
The two lines are
[2.23943,2.19462,1] and [-0.685646,2.15228,1]
Three points on the first line are
(14.3273,-15.0755,1)
(-0.434872,-0.0119093,1)
(-0.56911,0.12507,1)
Three points on the second line are
(-2.29319,-1.19516,1)
(0.43878,-0.324843,1)

(7.8271,2.02884,1)
The three points constructed are
(-0.323743,0.01554,1)
(6.49488,5.53439,1)
(-0.0728833,0.218581,1)
TROUBLE! The three points are not collinear!!
The two points are
(0.576837,0.361625,1) and (-0.407185,0.0903103,1)
The three lines through the first point are
[-1.45089,-0.450946,1]
[0.398283,-3.4006,1]
[18.9351,-32.9691,1]
The three lines through the second point are
[2.2305,-1.01622,1]
[2.46456,0.0391258,1]
[2.51677,0.274485,1]
The three lines constructed are
[2.42584,0.116742,1]
[1.63958,0.114103,1]
[3.10284,0.119014,1]
TROUBLE! The three lines are not concurrent!!
✝ ✆
✞
☎
Enter desired tolerance > 1e-16
You set the tolerance to 1e-16
The two lines are
[0.55364,0.547428,1] and [1.05044,-0.347064,1]
Three points on the first line are
(-0.0325509,-1.79381,1)

(-0.440495,-1.38123,1)
(1.76843,-3.61523,1)
Three points on the second line are
(-2.34784,-4.22478,1)
(-0.911666,0.122021,1)
(-0.367147,1.77009,1)
The three points constructed are