Laboratory 8: Design Patterns

I. File Accessing

1. Notes

2. Source Codes

3. example1

4. example2

5. Exercise

II. 1999 Winter Project

Description

Model Solution

• Expressions Bonus

  The trick is to observe that the evaluation tree can be wrong in two ways:

          
            *               *
           / \             / \
          2   +           +   4
             / \         / \
            3   4       2   3
  
          (WRONG)       (WRONG)
           
             |             |
             |             |
             V             V
      
             +            +
            / \          / \
           *   4        2   *
          / \              / \
         2   3            3   4
        
          (GOOD)         (GOOD)
  
         

  So we declare a new class called binary_operator_adaptor which holds one binary operator expression. In its constructor, if the binary expression's left-hand-side or right-hand-side has lower precedence than itself, then we swing the tree (recursivelly) so it looks good.
  
  Although I did not ask, an expression like (3+5)/(1+3) should not really be stored as a tree but as 2. In other words expressions can be simplified (check out the simplify method). The only place where we cannot simplify if there is a variable inside the expression (ie, if you really went all the way...). But even then, it can be partially simplified: (2+6)*x = 8*x, or (x-1)/(3+4) = (x-1)/7.
  
  The evaluate signature also has to change, because an expression may contain variables, hanve a double* argument represents an array of bindings in the current context.

• Drawing Bonus

  The strategy is to find one point on the perimeter of the figure which is inside. Once we have that point p then we can recursivelly halve the distance between p and o (towards p) until it is inside the figure. To find p, we can use one of the corners, or if there is no corner inside we can find two intersecting points and use their median. Check out shape::inside, shape::intersects and shape::find_in. It is the same for the ellipse, after all, it is drawn with lines between points. There is another case, maybe the figure totally surrounds the canvas and the middle is still not in. In that case, you should check that at least one of the points have to have a mapped x coordinate which is less than 0, one which is larger than width, one mapped y coordinate less tan 0 and one larger than height
  
  All of the above is only true for convex shapes, but that is all what we have.
  
  These files test if your bonus implementation is OK:

           e1.pic // ellipse test
           t1.pic // triangle test
           r1.pic // rectangle test
        

  If your (bonus) implementation does not work, you will only see lines.

• Parameter Passing, Recursion, ....

  You would have to build a new hierarchy (check out statement ), which would store the actual statements of the user defined figure, puts them into a figure_descriptor and registers it in a symbol_table....
  
  

• CODE

• Test Files: