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~                             Amos Logo.                             ~
~                          By J.Rutherford                           ~
~                                                                    ~
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One of the things that I always wanted to make was LOGO. I didn't know
how to make it, then I thought;  "Why  not make it with Amos!!!" after
all, Amos is the ideal language  to  use for creating other languages.
O.K. it doesn't have a compiled speed anywhere as near as machine code
or intuition commands as good as C, but  it's easy to get work out and
quick to write programs on it.

Can anyone remeber DART on the  BBC?  You know, that program where you
typed in things like "Forward 10"  to  make it move forwards and stuff
like that. That was  probably  the  best  known  example of the 'logo'
programming language, it worked on  set  commands  to do with moving a
pointer around the screen, drawing  lines  and  things like that. This
month, we are going tostart creating a  LOGO language in Amos that can
load and save pictures, print pictures  and draw shapes on the screen.
All the above will take quite a long  time  to make so this can be the
start of a multi  part  series.  Who  knows,  when you've finished the
program, you could add a graphical  user interface and release it into
P.D. Fame !!!

The 1 command that will really  be  the  basis of our logo programming
language is:

        GR LOCATE.

The usage of this command is:-

        Gr Locate x,y

Where X is the x  co-ordinate  of  the  point  you  want to locate the
cursor at and y is the y co-ordinate.

Eg. If we wanted to locate  the  graphics  cursor at x(100), y(150) we
would type:-

        Gr Locate 100,150.

We will use  this  command  to  enable  us  to  draw  from the current
position on the screen to anywhere else  without having to use lots of
different drawing routines and complex  stuff. Remember the best thing
you can do when programming is keep it simple. Why make something more
complicated than it really needs to be?

We will use it to set the  position  of  the cursor, or where where we
are drawing the line from, when  we  type  in forward 10. This way, we
can even say the co-ordinates of the last drawn point when we save our
drawing !!!

If you're still confused by what  the  graphics cursor is, think of it
as being the same as the text cursor. You can locate it where you want
and then instead of typing some text we  type a command to draw a line
using the command "draw to". For usage of this command see the article
in this beginners section.
Right, down to the nitty gritty:
Here's the source code.

        '***OPEN SCREEN***
        Screen Open 0,320,200,2,Lowres
        '
        '***Set Graphics Cursor Co-ordinates***
        X=150 : Y=100
        '
        '***Start Main Loop***
        Do
        Gr Locate X,Y
        '***Ask for an input
        Locate 0,0 : Input A$
        '
        '*** Convert input to uppercase.
        A$=Upper$(A$)
        '
        '
        '***Move forward routine
        If Left$(A$,1)="F"
        Input A
        Add Y,-A
        Draw Xgr,Ygr To X,Y
        End If
        '
        '
        '***Move down routine***
        If Left$(A$,1)="D"
        Input B
        Add Y,B
        Draw Xgr,Ygr To X,Y
        End If
        '
        '
        '***Move left routine***
        If Left$(A$,1)="L"
        Input C
        Add X,-C
        Draw Xgr,Ygr To X,Y
        End If
        '
        '
        '***Move right routine***
        If Left$(A$,1)="R"
        Input D
        Add X,D
        Draw Xgr,Ygr To X,Y
        End If
        '
        '***Go back to start***
        Loop



After we've loaded in this code from the disk, and ran  the program we
should type "Forward" or  something  resembeling  "Forward" to make it
draw a line forward, it's so  intelligent we could even type "FurWald"
or "Floppy rabbits" and it would still work. We are then confronted by
another prompt. This time we have to  type  in a value that we want to
move the cursor forward, in  a  straight  line.  Once we've typed in a
value, BINGO!, we  should  see  a  line  appear  on  the  screen.  The
majority of  this  program,  is  just  taking  an  input,  setting the
Graphics Cursor  and  moving  the  cursor  in  a  different direction,
according to the input.

Next issue, a continuation of the series.

Jonathan.