Logo Programming

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Logo inherits lists from Lisp, and they are its primary method of storing vectors. Arrays are also provided.

  • Operators exist to convert words into lists, and lists into arrays and back again.
  • This data type has the advantage over arrays that it is infinitely expandable. Data are extracted using the operations first, butfirst, last, butlast, butmember, member and item. Data elements are added using sentence fput and lput.
  • A list can be considered to be a queue with the operators queue and dequeue, or a stack with the operations push and pop.
  • Recursion rather than iteration is the natural method to process lists.

Example: using list primitives to extract the first five members of a list[edit]

One way would be to use iteration.

 to firstfive :alist
  ifelse lessp count :alist 6 [ op :alist ][make "olist []]
  repeat 2 [ make "olist lput first :alist  :olist   make "alist bf :alist ] output :olist
 end

 show firstfive [1 2 3 4 5 6 7 8 9 ]
 [1 2 3 4 5]
 foreach firstfive [1 2 3 4 5 6 7 8 9 ] [show 10 - ?]
 9 
 8 
 7 
 6 
 5

Another, more elegant way would be

 to firstn :num :list
  if :num = 0 [output []]
  output fput (first :list) (firstn :num-1 butfirst :list)
 end
 
 to firstfive :list
  output firstn 5 :list
 end

This method uses recursion, and is an example of a functional rather than an imperative programming approach.

And a more general way would be

 to tail :num :alist
  if or not numberp :num not listp :alist [print [Error: Bad argument(s)] stop]
  if not equalp 0 :num - int :num [make "num int :num]
  if :num=0 [output []]
  if equalp count :alist abs :num [op :alist]
  if lessp count :alist abs :num [output :alist]
  if :num>0 [output tail.left :num :alist] 
  if :num<0 [output tail.right abs :num :alist]
 end

  to tail.left :num :list
   output fput (first :list) (tail.left :num-1 butfirst :list)
  end

  to tail.right :num :list
   output lput (last :list) (tail.right :num-1 butlast :list)
  end

 show tail 5 [1 2 3 4 5 6 7 8 9]
 [1 2 3 4 5]
 show tail -5 [1 2 3 4 5 6 7 8 9]
 [5 6 7 8 9]

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Control structure commands[edit]

Logo provides several common control structures.

  • ifelse test [ do_if_true list ] [do_if_false list]

There are iteration commands

  • while condition [instruction list]
  • until condition [instruction list ]
  • repeat number [instruction list]

Recursion is Logo's preferred processing paradigm.

Template iteration[edit]

Logo also provides list-based control structures. The basic idea is that you have two lists

OPERATION [ a list of commands ] [ many data items ]

each of the commands is applied in turn to each of the data items. There are several of these template commands with names like MAP, APPLY, FILTER, FOREACH, REDUCE and CASCADE. They represent four flavours of template iteration, known as explicit-slot, named-procedure, named-slot (or Lambda), and procedure-text.

show map [? * ? ] [ 5 6 7 ]
[25 36 49 ]
show filter [ (count ?) > 4 ] [ the quick brown fox jumps over the lazy dog ]
[quick brown jumps]
show foreach [1 2 3 4 5] [ ? * 10 ]
[10 20 30 40 50]
RUN [ list of commands ] ;run a list of commands  (or programs) from in a program.

Property lists[edit]

A property list is a special list where the odd number items are property names, and the even are property values. There are three commands to process property list.

pprop :listname :name :value ;to add a new pair to the list
remprop :listname :name :value ;to remove a pair to the list
show gprop :listname :name  ;to get the matching value from the list

I/O Commands[edit]

Text may be written to the command window (output stream) using print, show and to the graphics window using label

The standard commands are readlist readword readchar with the normal input stream being the keyboard. In Unix tradition the input stream can be changed, so input can come from a disk file. Similarly, output can be redirected.

openread [filename]
setread[filename]
setreadpos nn
readchar
setread[]
close [filename].

There are equivalent commands to change the output stream, openwrite, openappend, setwrite, setwritepos nn.

dribble [filename]

Creates a transcript of everything that is typed in or outputted to the command window.

nodribble

This turns it off.

I/O commands can be applied to generate text to resemble a known bitmap, also seen in ASCII art conversion programs. An example of this, a set of crosses resembling the Mandelbrot set.

Graphics[edit]

Turtle graphics is a powerful method of introducing thinking but LOGO also has a few useful Cartesian commands

home         ;returns the turtle to (0,0)
setx xx      
sety yy      ; sends the turtle, still drawing to (xx,yy)
seth nn      ; sets the turtle on a heading or compass bearing of (nn)

Example: calculating and drawing a sundial for a given latitude[edit]

This is a typical garden dial. The graphic can be printed and transferred to wood or brass to make an accurate garden timepiece.

to dial
   
   cs show [Type in your latitude as a whole number]
   make "latitude readword  ;uses keyboard input
   for [i 0 6 1][
     make "ang arctan product sin :latitude tan product  :i 15  ;the calculation
     rt :ang fd 200 bk 200 lt :ang                      ;draw the morning line
     lt :ang fd 200 bk 200 rt :ang                      ;use symmetry to draw the afternoon line
   ]
   pu setx -300 sety -300 seth 90 pd                  ;send the turtle to the bottom
   fd 300 seth 270 rt 90 - :latitude fd 300           ;draw the [[style]] or [[gnomon]]
   pu home pd                                         ;tidy it up
 end

The same code in a different Logo dialect (in this case, NetLogo, would look like this.

to dial
   
   clear-all
   create-turtles 1
   let latitude 38  ;Just set this manually
   let ang 0
   ask turtles [pu set heading ang]
   foreach [0 1 2 3 4 5 6][
     set ang atan ((sin latitude) * (tan (? * 15))) 1  ;the calculation
     ask turtles [pd rt ang fd 10 bk 10 lt ang                      ;draw the morning line
     lt ang fd 10 bk 10 rt ang pu                     ;use symmetry to draw the afternoon line
     ]
   ]
   ask turtles [pu setxy -15 -15 set heading 90 pd                  ;send the turtle to the bottom
   fd 15 set heading 270 rt (90 - latitude) fd 15           ;draw the [[style]] or [[gnomon]]
   pu home pd                                         ;tidy it up
   ]
 end

A sundial plate must be adjusted by latitude using the formula

x= arctan( sin(latitude)*tan(HourDiff * 15) )

The Gnomon Angle = latitude.

Sundial in Berkeley, California


This dial above is set for 38N, the latitude of Berkeley, California. A small correction should also be made for longitude.

MSWLogo extensions[edit]

MSWLogo supports multiple turtles, and 3D Graphics. MSWLogo allows input from COM ports and LPT ports and also hardware ports. MSWLogo also supports a windows interface thus I/O is available through this GUI- and keyboard and mouse events can trigger interrupts.

GIF animations[edit]

Simple GIF animations may also be produced on MSWlogo version 6.5 with the gifsave command. Below is an example of code to generate a simple animation.

to squareani :number
 setactivearea [-5 -5 105 105]         ;we do not want to save the entire screen
 local "col
 setpensize [3 3]                      ;make it a noticeable square
 make "append "false                   ;signify that we want to start a new animation
  repeat :number [
   cs
   make "col 255*(repcount-1)/(:number-1)
   setpc (list :col :col :col)         ;change the square's color
    repeat 4 [                         ;
    fd 100 rt 90                       ;draw the square
    ]                                  ;
  (gifsave "squareani.gif 5 :append 0) ;save to squareani.gif
  make "append "true                   ;from now on, we want to keep adding frames to squareani.gif
  ]
end

Thus, with a :number variable of 15 frames, the result is given below, along with a slightly more complex example (note that this gifsave feature does not yield the same quality color gradients as Logo would save in bitmap form):

Squareani.gif
A square fading
from black to white
in 8 bits per pixel
(default for gifsave).

Save/Load Functions[edit]

MSWLogo and FMSLogo support saving and loading of LOGO procedures and files.

to clone
 make "generation :generation + 1
 make "newfilename (word "reproducer :generation ".lgo)
 
 save :newfilename
 load :newfilename
end

Make "generation 0
Make "newname "reproducer1.lgo

Make "startup [clone]