Question

typedef struct {
   double a; // coefficient for x
   double b; // coefficient for y
   double c; // constant term
} line;

points_to_line(point p1, point p2, line *l)
{
   if (p1[X] == p2[X]) {
      l->a = 1;
      l->b = 0;
      l->c = -p1[X];
} else {
      l->b = 1;
      l->a = -(p1[Y]-p2[Y])/(p1[X]-p2[X]);
      l->c = -(l->a * p1[X]) - (l->b * p1[Y]);
}

I’m reading Programming Challenges book by (Steven S. Skiena and Miguel Revilla), maybe someone should have been read this book.

p1[X] is point p1’s X value, and [Y] is Y value.

This function makes a linear equation line. (line *l)
First If sentense makes x + C (p1[X]) = 0 to l.

What I don’t know is:

• In C language, how can I represent p1[X]??

• Is p1 a one dimentional array? then, X and Y is constant macro??

• Or, using C++, Is this a overloaded operator[] ?? then, point type has overloaded operator[]???

• Or is there point type in C?? I searched google “point type C” and, I found nothing.

Answer 1

Assuming it is C code, as p1 is accessed with [] operator, my guess is point type is likely to be a typedef to an array.

#define X  0
#define Y  1
typedef double point[2];

I think it is double because (p1[Y]-p2[Y])/(p1[X]-p2[X]) expression is written without double casts.