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.
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.
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.
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.
Commands may be written on one line, or more. Many commands have mnemonic short forms; for example
RIGHT are coded
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
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.
print [Hello World!]
make "startup [main]
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
REPEAT 4 [FD 100 LEFT 90]
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]]
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.
REPEAT 4 [FD 100 RT 90] FD 200
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.
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
Now any future FD movements will erase anything beneath them. The pen can be restored with the command
EDALL;(to enter the editor mode, then the actual procedure)
BK 200 REPEAT 4 [FD 100 RT 90]
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
CS REPEAT 9 [CHAIR 50 RT 20 CHAIR 100 WAIT 50 RT 20]
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
false, are conventionally written with a final
p. Examples include
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
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
:- this means the contents of.
This is an extremely useful symbol that keeps reminding students that a variable is really some 'place' in memory.
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 "x sum :y 3or
make "x sum :y "3
maketakes 2 parameters, the second of which here is
sum :y "3.
sumtakes two 'parameters' and is an 'operation', thus the calculation is possible.
:ytakes 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.
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.
There are iteration commands
Recursion is Logo's preferred processing paradigm.
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.
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 listSee Associative array
print, showand to the graphics window using
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.
home ;returns the turtle to (0,0)
sety yy ; sends the turtle, still drawing to (xx,yy)
seth nn ; sets the turtle on a heading or compass bearing of (nn)
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.
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