[metapost] Drawing a urn

[Hartmut Henkel]

On Thu, 29 Nov 2007, Hans Hagen wrote:

> you can avoid using u (units) here; also, dots are a bit more efficient (but
> the loop isn't -)
>
> vardef balls(expr nx, ny, n, l) =
>     image (
>         save done, m, i, j ;
>         boolean done[][] ; numeric m, i, j ;
>         m := round(nx * ny * n) ;
>         forever :
>             i := ceiling(uniformdeviate(nx)) ;
>             j := ceiling(uniformdeviate(ny)) ;
>             if not known done[i][j] :
>                 done[i][j] := true ; m := m - 1 ; exitif m = 0 ;
>             fi ;
>         endfor ;
>         for i=1 upto nx : for j=1 upto ny :
>             drawdot(i,j)
>                 withcolor if known done[i][j] : .5white else : .7 white fi
>                 withpen pencircle scaled l ;
>         endfor ; endfor ;
>         draw (0,ny+l) -- (0,0) -- (nx+1,0) -- (nx+1,ny+l)
> 	    withcolor .3 white ;
>     )
> enddef ;
>
> draw balls(20,10,0.57,.4) scaled 10 ;

that works nice!

only, to get rid of the "(end occurred when if on line xx was
incomplete)" warning just move the exitif behind the enclosing if. And a
"draw origin shifted (i,j) ... withpen pencircle scaled 2l;" appears to
be more efficient than the "drawdot(i,j)" as it uses PostScript's own
circles (no curveto at all in the eps file).

Regards, Hartmut