Solution to exercise 6.1.1

<sxh c> h = figure(); axis([-5,5,-5,5]); xy = []; # array of vertex coordinates stp = 0; quitchar = [-1, toascii(“Q”)]; closechar = [toascii(“cC”)]; while stp == 0

[x,y,b] = ginput(1);
if sum(b == quitchar) # request for exit
  break;
endif
xy= [x;y];
while 1
  [x,y,b] = ginput(1);
  cp = [x;y];
  if b == 1 # draw new segment apped coords to xy
    xs = [xy(1,end), x];
    ys = [xy(2,end), y];
    line(xs,ys);
    xy = [xy, cp];
  elseif sum(b == closechar) # close the polyline finish drawing
    xs = [xy(1,1), xy(1,end)];
    ys = [xy(2,1), xy(2,end)];
    line(xs,ys);
    break;
  elseif sum(b == quitchar) # request for exit
    stp = 1;
    break;
  endif 
endwhile

endwhile </sxh>

<texit> \begin{lstlisting} h = figure(); axis([-5,5,-5,5]); xy = []; # array of vertex coordinates stp = 0; quitchar = [-1, toascii(“Q”)]; closechar = [toascii(“cC”)]; while stp == 0

[x,y,b] = ginput(1);
if sum(b == quitchar) # request for exit
  break;
endif
xy= [x;y];
while 1
  [x,y,b] = ginput(1);
  cp = [x;y];
  if b == 1 # draw new segment apped coords to xy
    xs = [xy(1,end), x];
    ys = [xy(2,end), y];
    line(xs,ys);
    xy = [xy, cp];
  elseif sum(b == closechar) # close the polyline finish drawing
    xs = [xy(1,1), xy(1,end)];
    ys = [xy(2,1), xy(2,end)];
    line(xs,ys);
    break;
  elseif sum(b == quitchar) # request for exit
    stp = 1;
    break;
endif 

endwhile endwhile \end{lstlisting} </texit>

Description:

• variable stp defined in line 4 is used to indicate that the user requested termination of the program,
• in line 5 and 6 we calculate vector of codes for the keys (we also handle the case of the pressed caps lock). The letter “q” is special because its code is -1,
• in line 9, 21, 26 we check if the even key 'b' is equal to any of the defined key values,
• the variable cp defined in line 15 is used to hold coordinates of the mouse cursor as a column vector,
• in line 20 we append the current point coordinates the the matrix that stores coordinates of already clicked points.