Review of ScreenTurtle1 (Archimedes version)

Micro User May, 1992

Rog Frost

YET another Logo variant? For the last 10 years schools have been bombarded with programs offering the turtle graphics subset of Seymour Papert's complete learning environment. For the last 10 years too, a few enthusiasts have embraced what these programs have to offer while the vast majority of teachers muttered quietly and the program has drifted to the back of a dusty cupboard.

Now, though, the National Curriculum almost makes turtle graphics compulsory. Schools are dusting off their old versions and find a colourless, boring, slow and difficult program which suffers from the need to remember strange commands and a very fussy syntax.

Worse still, every version has its own dialect which meant that skills learned on one version are difficult to transfer to another.

Topologika have now taken the plunge and produced a new program. It is purpose-written for the 32-bit range of computers and features the ease of use we associate with that machine. The screens are immediately attractive and encourage users to keep at work. It has the required speed expected from the Archimedes range and a comprehensive set of features. There is a well laid out manual with plenty of suggestions for use and a good collection of pupil worksheets.

So what does Screen Turtle offer? Just in case you have locked yourself away recently, perhaps it is as well to say that the program essentially links graphics and mathematics. Drawings are created by moving a pen - always called a turtle - around the screen. The pen is moved by commands such as FORWARD 100 or turned around by commands such as RIGHT 90. These simple commands can be put together to produce complex and even beautiful screens.

Screen Turtle has a comprehensive range of ready made words - in Logo parlance they are called primitives. These cover the expected range FORWARD, BACKWARD, LEFT, RIGHT, PENUP and PENCOLOUR, REPEAT and so on. The clever part of this program is that alternative words are permissible. For example, FORWARD can be abbreviated to FD or FOR, but an entirely different word, AHEAD, is also an option.

When these commands are mistyped the program will attempt to guess the intended word. Every other Logo that I've used has been particular about spaces between words and numbers. Not Screen Turtle. You can leave a gap or miss it. It's not fussy at all.

Command mode is just the beginning. The ability to create your own words is really what these programs are all about. The method here is to type BUILD - or DEFINE or TO or CREATE - followed by your word name. This takes you to an editor where you put together your new command. The first new word for nearly all users is SQUARE. The style of Screen Turtle makes it all obvious. To BUILD SQUARE you would type:





As you type these commands, the action is shown on screen. Problems can be ironed out immediately because editing is very straightforward. Once you are satisfied with your new word, SQUARE, it can be used like any other word. It can also be saved on the disc for later recall. Of course, it is possible to use SQUARE to BUILD another pattern. Saving the pattern to disc will automatically save the words that go to make it up.

When words are re-loaded, there can be a problem. It is all too easy to use the same word more than once. Screen Turtle recognises this problem and sticks an extra letter on to a word to avoid it. Don't worry, it tells you as well.

Like other versions of Logo, Screen Turtle can work with variables. Instead of FORWARD 100 you can give a value to a variable and then go FORWARD by the variable. If you define a word, it is possible to change the value of the variable within it you can produce very interesting shapes this way.

When using other Logos, I've always found that I lose turtles. When drawing triangles, the first two sides are easy but just what angle is needed, and how far must the turtle move to complete the third side? Screen Turtle overcomes this problem by using Basic type commands MOVE and DRAW so that I can shunt the turtle around to grid positions such as (200,200).

There is also a VECTOR option which allows the turtle to move to a new point relative to the current position in terms of co-ordinates rather than angles and distances. For me, this one feature turns an older, frustrating Logo experience into one I can cope with and enjoy.

One of the best features of Screen Turtle are its colour options. PENCOLOUR can be given one of 14 different values and this means that lines can be drawn in the various shades given. 1 have to say that on my pre-release version the choice of colours was odd with two nearly identical reds, two yellows and three whites. These are not adjustable at the palette.

But it’s the fill options which really make the program. You just move the turtle into a space, set a PENCOLOUR or INK and type FILL and you get instant colour. There is though, one small problem. The colour flows from the tail end of the beast so you must place the rear end of the turtle in the shape. Also, FILLing is a one attempt job. It is only, possible to fill on the background.

FILL is probably most useful to build into words. To colour up a screen there is a mouse point and shoot technique called PAINT. Then is no need to type anything for this. Select a colour from a menu, move the mouse arrow and click. Simple.

Whole screens can be saved to disc as compressed sprites. These can be converted to standard sprites for use in other programs such as a DTP package. They can also be loaded back in for a quick background for further work. Ready-made backgrounds, such as mazes and maps are provided on the disc and these can form the basis of first rate tasks for youngsters.

Screen Turtle is not, and does not pretend to be, a full version of the Logo programming language. It is just the turtle graphics part of it - the part most widely used in schools.

It's a shame that Screen Turtle is not a multi-tasking program and fails to use the standard Risc OS environment. But it is faithful to other, more traditional, versions of the program and will be easy for the intended young users. It is very friendly to use and is surely destined to become the standard turtle graphics program in both junior and secondary schools.