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Berkeley Logo

Logo (programming language)

Logo is a computer programming language used for functional programming. It is an easier-to-read adaptation and dialect of the Lisp language; some have called it Lisp without the parentheses. Today, it is known mainly for its turtle graphics, but it also has significant facilities for handling lists, files, I/O, and recursion.

Logo was created for educational use, more so for constructivist teaching, by Daniel G. Bobrow, Wally Feurzeig and Seymour Papert. It can be used to teach most computer science concepts, as UC Berkeley Lecturer Brian Harvey does in his Computer Science Logo Style trilogy.

History

Logo was created in 1967 at Bolt, Beranek and Newman (BBN), a Cambridge, Massachusetts research firm, by Wally Feurzeig and Seymour Papert Its intellectual roots are in artificial intelligence, mathematical logic and developmental psychology. The first four years of Logo research, development and teaching work was done at BBN. The first implementation of Logo, called Ghost, was written in LISP on an SDS 950. The goal was to create a math land where kids could play with words and sentences. Modeled on LISP, the design goals of Logo included accessible power and informative error messages. The use of virtual Turtles allowed for immediate visual feedback and debugging.

The first working turtle robot was created at MIT in 1969. A display turtle preceded the physical floor turtle. Modern Logo has not changed too much from the basic concepts before the first turtle. The first turtle was a tethered floor roamer, not radio-controlled or wireless. Later, BBN developed a turtle named Irving that had touch sensors and could move forwards, backwards, rotate, and ding its bell. The earliest year-long school users of Logo were in 1968-69 at Muzzey Jr High, Lexington MA. The virtual and physical turtles were first used by fifth graders at the Bridge School in Lexington, MA in 1970-71.

Implementations

As of July 2008 there were 187 implementations and dialects of Logo, each with its own strengths. Most of those 187 are no longer in wide use, but many are still under active development.

As yet there is no single agreed-upon Logo language definition or standard, though there is a broad consensus on core aspects of the language. There are substantial differences between the many dialects of Logo. The situation is confused by the regular appearance of turtle graphics programs that mistakenly call themselves Logo.

The most broadly used and prevalent early implementation of Logo was Apple Logo, which was developed by LCSI for the Apple II computer and popular during the 1980s.

The closest thing to a de facto Logo standard today is UCBLogo, also known as Berkeley Logo. It is free and cross-platform. UCBLogo has only a rudimentary graphical user interface, so several projects exist that provide a better interface. MSWLogo, and its successor FMSLogo, for Windows, are commonly used in schools in the United Kingdom and Australia. Commercial Logos that are still widely used in schools include MicroWorlds Logo and Imagine Logo

Some modern derivatives of Logo allow thousands of independently moving turtles. There are two popular implementations: MIT's StarLogo and CCL's NetLogo. They allow for the exploration of emergent phenomena and come with many experiments in social studies, biology, physics, and other areas.

Most Logos are 2D, but the Elica interpreter is notable for supporting 3D graphics. Most Logo implementations are interpreted, but some compilers have been built, including the Lhogho compiler, by the same author as Elica. Although most often used for graphics, Logo can also control robots. It was interfaced with Lego bricks, although Lego decided later to use another language in the commercial Lego Mindstorms products. The Mindstorms GUI interface is based on LabVIEW by National Instruments. An interface also exists for Cricket robots.

ObjectLOGO is a variant with object-oriented extensions.

Influence

Logo was a primary influence on the Smalltalk programming language. It is also the main influence on the Etoys educational programming environment and language, which is essentially a Logo written in Squeak (a variant of Smalltalk).

Programming

Logo's best-known feature is the turtle, which is an on-screen cursor (derived originally from a robot of the same name), which can be given movement and drawing instructions, and is used to programmatically produce line graphics. It is traditionally and most often represented pictorially either as a triangle or a turtle icon (though it can be represented by any icon). Turtle graphics were added to the Logo language by Seymour Papert in the late 1960s to support Papert's version of the turtle robot, a simple robot controlled from the user's workstation that is designed to carry out the drawing functions assigned to it using a small retractable pen set into or attached to the robot's body.

As a practical matter, the use of turtle geometry instead of a more traditional model mimics the actual movement logic of the turtle robot. Turtle geometry works somewhat differently from (x,y) addressed Cartesian geometry, rather operating in a Euclidean space (i.e., relative measures and angles without an origin, unlike coordinate-addressed systems such as PostScript). The turtle moves with commands that are relative to its own position, LEFT 90 meant rotate left by 90 degrees. A student could understand (and predict and reason about) the turtle's motion by imagining what they would do if they were the turtle. Papert called this body-syntonic reasoning. Some Logo implementations, particularly those that allow the use of concurrency and multiple turtles, support collision detection and allow the user to redefine the appearance of the turtle cursor, essentially allowing the Logo turtles to function as sprites.

Turtle geometry is also sometimes used in environments other than Logo as an alternative to a strictly coordinate-addressed graphics system. For instance, the idea of turtle graphics is also useful in Lindenmayer system for generating fractals.

Syntax

Commands may be written on one line, or more. Many commands have mnemonic short forms; for example FORWARD and RIGHT are coded FD and RT respectively. This makes the input less onerous. Anything written after the ; (semicolon) is ignored, allowing the coder to insert comments.

FORWARD 100 ; draws a square with sides 100 units long
LEFT 90
FORWARD 100
LEFT 90
FORWARD 100
LEFT 90
FORWARD 100
LEFT 90

FD 100 RT 120 FD 100 RT 120 ; draws a triangle
FD 100 RT 120

The Hello World program in Logo looks like this:

print [Hello World]

Loading a file will include all its defined procedures into the currently open file. If the file being opened has commands that are not in a defined procedure they will be run on startup and will be removed from the workspace (except for variable assignments), so if the file is then saved it becomes blank. For example, one could have a helloworld.lgo which prints Hello World and becomes blank when EDALL is called:

print [Hello World]

Alternatively, one could put the command in a procedure that is run when the program is loaded.

to main
 print [Hello World!]
end
make "startup [main]

The pen

The analogy of a turtle with a pen attached to its tail is often used. The turtle's pen can be lifted and lowered, thus drawing a rudimentary dotted line.

FD 20    ; drawing a line and moving
PENUP    ; lifting the pen so it won't draw anything
FD 20    ; not drawing but moving
PENDOWN  ; lowering the pen so it draws again
FD 20    ; drawing a line and moving
PENUP     ;lifting the pen so it won't draw anything
FD 40    ; not drawing but moving
PENDOWN  ;lowering the pen so it draws again
RT 20    ;drawing a line and moving

Loops

There are three loop (repeat) commands; REPEAT is one. This draws the a box.

REPEAT 4 [FD 100 LEFT 90]

The command FD 100 LEFT 90 is executed four times. An approximation of a circle can be constructed easily with 360 small rotations and a step forward: REPEAT 360 [FD 1 RIGHT 1]. Loops may be embedded, giving spectacular results with little effort.

REPEAT 36[RT 10 REPEAT 360 [FD 1 RT 1]]
FD 25
RT 90

Defining procedures, the need for an editor

Procedures can be defined on the command line, using the TO END pair:

TO CHAIR  REPEAT 4 [FD 100 RT 90]  FD 200  END

However, in some early Logos the procedure is limited to the physical line length of the input device.

All Logos can invoke an Editor, usually by EDALL. In the editor, procedures may be written over many lines, as nothing is interpreted until the edit is complete.

EDALL


TO CHAIR
REPEAT 4 [FD 100 RT 90]  FD 200
END

The new word is saved into the available vocabulary, but the definition will be lost once the Logo session is over. Internally procedures are words and in this case, any time CHAIR is entered, the sequence REPEAT 4 [FD 100 LEFT 90] FD 200 will be executed. The word CHAIR can be used as a command; for example, REPEAT 4 [CHAIR] would repeat the CHAIR operation four times.

Animation

Logo was designed in spirit of low threshold and no ceiling, which enables easy entry by novices and yet meet the needs of high-powered users. Animations require both the ability to draw shapes and to erase shapes. The process is the same, except that in the former a line is deposited on the display device and in the latter a line is removed. Using the turtle analogy, the turtle's pen must paint, and the turtle's pen must erase.

In UCBLogo, the turtle can be set to erase using the command PENERASE (PE). Now any future FD movements will erase anything beneath them. The pen can be restored with the command PENPAINT (PPT).

EDALL ;(to enter the editor mode, then the actual procedure)
TO ERASECHAIR
PE
BK 200 REPEAT 4 [FD 100 RT 90]
PPT
END

CS CHAIR WAIT 200 ERASECHAIR

A WAIT delay between the drawing and the erasing introduces the illusion of motion.

CS REPEAT 20 [CHAIR WAIT 200 ERASECHAIR FD 20]

Logo can pass extra information to its words, and return information. The procedure, (word) is instructed to expect something and give that something a name. The colon is used for this purpose. It passes the information by value and the colon is pronounced as the value of. When the procedure is run with a command such as CHAIR 200, the word :thesize takes the value 200 so when FD :thesize is executed, the interpreter understands FD, the value of 200.

EDALL ;(to enter the editor mode, then the actual procedure)


TO CHAIR  :thesize
REPEAT 4 [FD :thesize  RT 90] FD :thesize FD :thesize
END

CS REPEAT 9 [CHAIR 50 RT 20 CHAIR 100 WAIT 50 RT 20]

The language

Logo is generally known as an interpreted language, although recently there have been developed compiled Logo dialects—such as Lhogho or Liogo. Logo is not case-dependent but retains the case used for formatting. It is written in lines. It is a compromise between a sequential programming language with block structures, and a functional programming language. There is no standard LOGO, but UCBLogo is highly regarded. It is a teaching language but its list handling facilities make it remarkably useful for producing useful scripts.

Functions and procedures

Each line is made up of function calls, or subroutines in programming terminology, of which there are two types:

  • commands (which do something—effects—but don't return a value) like print.
  • operations (which just return a value, its output) like sum, first or readlist.

A command is similar to a Pascal procedure, and an operation is similar to a Pascal function. (See also: command-query separation, where a query is an operation in Logo). A special subset of operations, called predicates, which just output the word true or false, are conventionally written with a final p. Examples include emptyp, wordp, and listp.

  • Expressions can be primitives, or can be defined by the user.
  • Expressions can take zero, one or more parameters.

Mathematics in Logo uses prefix notation, like: sum :x :y, product :x :y, difference :x :y, quotient :x :y. Infix is also available.

help "keyword ;(will bring up a full description of the expression).

Logo allows for recursion, the process where a procedure calls itself.

to spiral :size
   if  :size > 30 [stop] ; an exit condition
   fd :size rt 15        ; many lines of action
   spiral :size *1.02    ; the tailend recursive call
end

spiral 10

Data

There are three datatypes in UCBLogo,

  • the word,
  • the list,
  • the array.

A number is a special case of word.

There is no static typing. The interpreter detects the datatype by context.

There are two important symbols

  • The colon :- this means the contents of.

This is an extremely useful symbol that keeps reminding students that a variable is really some 'place' in memory.

  • The quote- this means '"the word is evaluated as itself"', or '"its value after evaluation is the same as it was before"'. This is important.

A number is a special case of self evaluation—it really could be written with a quote 2 is really "2

Variable assignment (eg. x := y + 3) is handled in Logo with the make command:

   make "x sum :y 3
or
   make "x sum :y "3
make takes 2 parameters, the second of which here is sum :y "3. sum takes two 'parameters' and is an 'operation', thus the calculation is possible. "3 evaluates to 3, and :y takes the contents of the thing called y, these are summed giving a number.

The effect of make is to place the result into the first parameter. From a programmatical perspective, the first argument to make is passed by reference, while the second is passed by value.

Scoping

Variables don't have to be declared before use; their scope is then global. A variable may be declared local, then its scope is limited to that procedure and any procedures that it calls (a.k.a. dynamic scope). Calling a procedure with inputs (the name usually used for arguments in the Logo literature) also creates local variables that hold the argument values.

Lists

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.

Control structure commands

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

Logo also provides list-based control structures. The basic idea is of 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.

Property lists

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
See Associative array

I/O Commands

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.

Graphics

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)

MSWLogo extensions

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.

Simple GIF animations may also be produced on MSWlogo version 6.5 with the gifsave command.

See also

  • Lego Logo, which uses Logo to control Lego bricks

Further reading

  • Computer Science Logo Style, Brian Harvey, MIT Press (3 volumes) ISBN 0-262-58148-5, ISBN 0-262-58149-3, ISBN 0-262-58150-7. Available online
  • Practical logo for the Atari ST (Paperback), Martin Sims, Glentop ISBN 1851810307 ,ISBN 978-1851810307. Available online
  • How to Think Like a Computer Scientist: Logo Version (Paperback), Allen Downey & Guido Gay, Lulu .
  • The Great Logo Adventure, Jim Muller, Doone Publications ISBN 0-9651934-6-2 (Now out of print but downloadable free of charge in pdf form from The MSWLogo website - from where you can also download the freeware MSWLogo program)
  • To Artifical Intelligence (1976) Early AI textbook where Logo is used extensively. (Using the Edinburgh University dialect, AI2LOGO)
  • Turtle Geometry Abelson and diSessa
  • Children Designers, Idit Harel Caperton, Ablex Publishing Corporation ISBN 0-89391-8788-5. Available online

External links

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