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Re: Cabri n'estil qu'un jouet?
Posted:
Jul 1, 1993 6:30 PM


Peter Appelbaum said:
>If it is like the sketchpad, and I know about Sketchpad, should I still want >to find out more about it? I mean, are there any differences that you could >tell me about?
I use both Sketchpad and Cabri. There are reasons for using both. I have intended to write out a full comparison of both but have not been able to get around to it. I have time for a few rough points.
First, before anyone flames me, understand that I like these programs a lot. Enough that I have bought copies of both of these programs out of my own pocket and for my own use. I do geometry for fun (read passion), not for work. Any examples I give below are indended to bring out an issue and are not intended to imply there is only one way to do the particular construction. Now that we are passed the disclaimers...
Sketchpad has about twice as much functionality than Cabri. This, of course, is a subjective statement at best, but is my earnest estimate. For instance, Sketchpad has facility for doing demo's on student machines, supports color and shading, etc.
However, there are three features for which I prefer Cabri. First, Cabri can prove things for you. You can do a construction and ask Cabri if three points are collinear and it will tell you if in general if they are or are not. If not, most of the time offer you a counter example. This blows me away. This makes Cabri worth its price alone.
Second, Cabri recognizes when two points are the same, although they may have been defined through different constructions. I think this feature is required to allow Cabri to check properties (like collinearity & membership) for you. In any case, this issue can cause you subtle difficulties in constructions. Try this: Construct a circle with center O. Place a point A on the circle and draw a line through A and O. Intersect the line with the circle. In Cabri there are now two points on the circle. In Sketchpad there are three  two points are at the location of A, A and a new one constructed from the intersection. There are situations where having these duplicate points has caused me trouble.
The third feature is a interface issue. It has to do with whether you select a tool first or the objects of the tool first. Macintosh interface guidelines aside, when I draw a line through two points with a ruler and paper, I pick up the ruler and then line it up on the points and draw the line. This is the way Carbi works. In Sketchpad you pick the points and then select the line operation from the menu. Although I have learned to anticipate this requirement, this is counter intuitive and requires me to use more key strokes.
That was it for the short time I have. I would like to hear what others have to say? Anyone else use both? Are there plans to make Sketchpad or Cabri more like the other?
Michael Thwaites, UCDLA



