Understand the Problem

• When do two cubes intersect?

  • When the squares projected on the planes intersect.

• When do two squares intersect?

  • When the line segments projected on the axes intersect.

• When do two line segments intersect?

  • These are line segments on the same line.

Initialize Variables

• What's wrong with this code?

  class cube {
  
    public:
            
      cube(int s) { side=s; }
  
      void move(int x, int y, int z) {
        xcord = x;
        ycord = y;
        zcord = z;
        }
  
      bool interferes_with(cube c) {
        // uses xcord, ycord, and zcord.
        }
  
    private:
  
      int side;
      int xcord, ycord, zcord;
    }; 
  

• move() isn't implemented right.

• xcord, ycord, and zcord aren't defined until moved.

Make Sure Variables are Initialized

• What's wrong with this code?

  class cube{
  
    public:
  
      cube(int s){
        int myX = s;
        int myY = s;
        int myZ = s;
        }
  
      void move(int x, int y, int z){
        // whatever
        }
  
      bool interferes_with(cube c){
        // whatever
        }
  
    private:
  
      int myX, myY, myZ;
    };  
  

• The compiler can help you here:

  $ g++ -g -ansi -pedantic -Wall -c game.cc
  In file included from game.cc:3:
  cube.h: In method cube::cube(int):
  cube.h:8: warning: unused variable int myZ
  cube.h:7: warning: unused variable int myY
  cube.h:6: warning: unused variable int myX
  

Don't Duplicate Code

• Duplicated code is bad code.

  if (((xbeg >= c.xbeg) && (xbeg < c.xend) ||
       (xend > c.xbeg) && (xend <= c.xend)) &&
      ((ybeg >= c.ybeg) && (ybeg < c.yend) ||
       (yend > c.ybeg) && (yend <= c.yend)) &&
      ((zbeg >= c.zbeg) && (zbeg < c.zend) ||
       (zend > c.zbeg) && (zend <= c.zend)))
  	return true;
  

• Duplicated code can bite you.

  • if ((c.a  <=  a  && c.a >= d) || (c.d >= d  && c.d <= a))
      if ((c.b <= b && c.b >= e)  || (c.e >= e &&  c.e <= b))
        if ((c.r <= r && c.r >  f) || (c.f >= f  && c.f < r))
          return 1;
      

  • if (((c.lOne >= lTwo) && (c.lOne <= lOne)) || 
        ((c.lTwo >= lTwo) && (c.lTwo <= lOne))) flag1 = true;
    if (((c.bOne >= bTwo) && (c.bOne <= bOne)) || 
        ((c.bTwo >= bTwo) && (c.bTwo <= bOne))) flag2 = true;
    if (((c.hOne > hTwo) && (c.hOne <= hOne)) || 
        ((c.hTwo >= hTwo) && (c.hTwo < hOne))) flag3 = true;
      

Document Your Code

• What's wrong with this code?

  return ((c.x_coorda > x_coordb && c.x_coordb < x_coorda) &&
  	(c.y_coorda > y_coordb && c.y_coordb < y_coorda) &&
  	(c.z_coorda > z_coordb && c.z_coordb < z_coorda));
  

• Here's the explanation.

  // The problem statement says that "One cube interferes
  // with another if there exists some point p in the 
  // environment such that p is part of both cubes."
  // Therefore, if the cubes touch they should interfere
  // unless we assume that a point on the edge of a cube
  // is not part of the cube.  I will assume that this is
  // true so that there's no interference if the cube touch.
  

• That's good for another 5 points.

Use Meaningful Names

• Compare

  if (((c.lOne >= lTwo) && (c.lOne <= lOne)) || 
      ((c.lTwo >= lTwo) && (c.lTwo <= lOne))) flag1 = true;
  if (((c.bOne >= bTwo) && (c.bOne <= bOne)) || 
      ((c.bTwo >= bTwo) && (c.bTwo <= bOne))) flag2 = true;
  if (((c.hOne > hTwo) && (c.hOne <= hOne)) || 
      ((c.hTwo >= hTwo) && (c.hTwo < hOne))) flag3 = true;
  

  to

  return ((c.x_coorda > x_coordb && c.x_coordb < x_coorda) &&
  	(c.y_coorda > y_coordb && c.y_coordb < y_coorda) &&
  	(c.z_coorda > z_coordb && c.z_coordb < z_coorda));
  

This page last modified on 3 July 2001.