| Logo Programming Language |
Article Index for Logo |
Website Links For Logo |
Information AboutLogo Programming Language |
| CATEGORIES ABOUT LOGO PROGRAMMING LANGUAGE | |
| dynamically-typed programming languages | |
| educational programming languages | |
| functional languages | |
| lisp programming language family | |
| programming languages | |
| 1960s software | |
| robotics | |
The Logo programming language is a Functional Programming Language . It is an easier to read adaptation and dialect of the Lisp language; some have called it Lisp without the Parentheses . It was created for Education al use, more so for Constructivist Teaching , by Daniel G. Bobrow , Wally Feurzeig and Seymour Papert . Today, it is known mainly for its "turtle graphics", but it also has significant facilities for handling lists, files, and I/O. Logo 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 . The goal was to create a "mathland" 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 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 February 2007 there were over 170 implementations and dialects of Logo, each with its own strengths. (Most of those 170 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, developed by LCSI for the Apple II computer and popular in 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 . 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 StarLogo and 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. An interface also exists for Cricket Robot s. 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 The idea is that a turtle with a pen strapped to it can be instructed to do simple things like move forward 100 spaces or turn around. From these building blocks you can build more complex shapes like squares, triangles, circles--using these to draw houses or sailboats. 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. The idea of turtle graphics is also useful for example in Lindenmayer System for generating Fractal s. Turtle geometry and 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 60s 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. Turtle geometry works somewhat differently from (x,y) addressed Cartesian Geometry , being primarily Vector -based (i.e. relative direction and distance from a starting point) in comparison to coordinate-addressed systems such as PostScript . As a practical matter, the use of turtle geometry instead of a more traditional model mimics the actual movement logic of the turtle robot. The following are examples of turtle code. While seemingly very simple, turtles can be given groups of instructions, essentially creating libraries of more complex commands. In practice short forms are used. For example, "LEFT 90" is written "LT 90". Key words are usually written in upper case for beginners, but more advanced texts use lower case. Example 1: a square FORWARD 100 LEFT 90 FORWARD 100 LEFT 90 FORWARD 100 LEFT 90 FORWARD 100 LEFT 90 This draws a square with sides 100 units long. Example 2: a triangle The commands may be written on one line, or more. In this example we'll use the short form for FORWARD and RIGHT, which are FD and RT respectively.FD 100 RT 120 FD 100 RT 120 FD 100 RT 120 Example 3: dotted line The turtle's pen can be lifted and lowered, thus drawing a rudimentary dotted line. Anything written after the ; (semicolon) is ignored, allowing the coder to insert comments. FD 20 ;(drawing a line and moving) PENUP ;(now we've lifted the pen so it won't draw anything even if we do move) FD 20 ;(not drawing but moving) PENDOWN ;(now we've lowered the pen so it draws a line wherever the turtle moves) FD 20 ;(drawing a line and moving) PENUP FD 20 ;(etc...) PENDOWN FD 20 Example 4: loops You could also use loop (repeat) commands. This would draw the exact same box as in the first example: REPEAT 4 100 LEFT 90 Which executes the command " FD 100 LEFT 90" four times.An approximation of a circle can be constructed easily with 360 small rotations and a step forward: REPEAT 360 1 RIGHT 1 . Example 5: new words You can teach the turtle new words, i.e. groups of instructions, or procedures. This can be done from the Logo prompt or an Editor, which is invoked by EDALL in many Logo dialects. Here we define the word CHAIR.EDALL TO CHAIR REPEAT 4 100 RT 90 FD 200 END Once one is finished with the editor, it must be exited. The new word is saved into the available vocabulary, but the definition will be lost once the Logo interpreter is stopped. In this case, any time CHAIR is entered, "REPEAT 4 100 LEFT 90 FD 200" will be executed. CHAIR can be used as a command; for example, REPEAT 4 [CHAIR] would compound the CHAIR operation four times.Example 6: erasing (in the UCBLogo dialect) The turtle can erase a line, using the command PENERASE (PE). The pen can be restored with the command PENPAINT (PPT). EDALL ;(to enter the editor mode, then the actual procedure)CS CHAIR WAIT 200 ERASECHAIR This example introduces two new instructions, which are best taught by running them through a Logo interpreter and observing the result. This typifies the general spirit of Logo. Example 7: parameters: giving the word changeable information Logo can pass extra information to its words, and return information. We must tell the word to expect something and give it a name. Notice the use of the colon. We are passing 'by value' and the colon is pronounced as 'the value of'. When the procedure is run with a command like CHAIR 200, the size takes the value 200 so we go 'FD the value of 200'.EDALL ;(to enter the editor mode, then the actual procedure)TO CHAIR :thesize REPEAT 4 :thesize RT 90 FD :thesize FD :thesize END Try CS REPEAT 9 50 RT 20 CHAIR 100 WAIT 50 RT 20 If you do need help, type HELP, or HELP "HOME (note the single quote mark.)Other uses of the turtle 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 Sprite s. Turtle geometry is also sometimes used in environments other than Logo as an alternative to a strictly coordinate-addressed graphics system. THE LANGUAGE Logo is generally known as an interpreted language, although recently there have been developed compiled Logo dialects - like 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 Subroutine s in programming terminology, of which there are two types:
A command is similar to a , where a "query" is an "operation" in Logo). A special subset of operations, called Predicate s, which just output the word true or false, are conventionally written with a final p. Examples include emptyp, wordp, and listp.
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. Example 8: A spiral drawn using recursion to spiral :size if :size > 30 {Link without Title} ; an exit condition fd :size rt 15 ; many lines of action
end spiral 10 Data There are three datatypes in UCBLogo,
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 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 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 its subprocedures (a.k.a. dynamic Scope ). Calling a 'procedure' with 'inputs', creates 'local variables' which hold the contents of the parameters.Lists Logo inherits lists from Lisp , and they are its primary method of storing "vectors". Arrays are also provided.
Example 9: using list primitives to extract the first five members of a list One way would be to use iteration. ifelse lessp count :alist 6 op :alist "olist [ ] repeat 5 make "olist lput first :alist :olist make "alist bf :alist output :olist end show firstfive 2 3 4 5 6 7 8 9 2 3 4 5 foreach firstfive 2 3 4 5 6 7 8 9 10 - ? 9 8 7 6 5 Another, more elegant way would be if :num = 0 [ ] 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 if or not numberp :num not listp :alist [Error: Bad argument(s) stop] if not equalp 0 :num - int :num "num int :num if :num=0 [ ] if equalp count :alist abs :num :alist if lessp count :alist abs :num :alist if :num>0 tail.left :num :alist if :num<0 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 2 3 4 5 6 7 8 9 2 3 4 5 show tail -5 2 3 4 5 6 7 8 9 6 7 8 9 Control structure commands Logo provides several common control structures.
There are iteration commands Recursion is Logo's preferred processing paradigm. Template iteration 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 repesent four flavours of template iteration, known as explicit-slot, named-procedure, named-slot (or Lambda) and procedure-text. show filter (count ?) > 4 the quick brown fox jumps over the lazy dog brown jumps
20 30 40 50 RUN list of commands ;run a list of commands (or programs) from in a program. 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 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.openread {Link without Title} setread {Link without Title} setreadpos nn readchar setread {Link without Title} close {Link without Title} . There are equivalent commands to change the output stream, openwrite, openappend, setwrite, setwritepos nn. dribble {Link without Title} 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 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 10: calculating and drawing a Sundial for a given latitude This is a typical garden dial. The graphic can be printed and transferred to wood or brass to make an accurate garden timepiece. cs show in your latitude as a whole number make "latitude readword ;uses keyboard input for 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 A sundial plate must be adjusted by latitude using the formula
The Gnomon Angle = latitude. This dial above is set for 38N, the latitude of Berkeley, California. A small correction should also be made for longitude. 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. GIF animations Simple . Below is an example of code to generate a simple animation. to squareani :number setactivearea -5 105 105 ;we do not want to save the entire screen local "col setpensize 3 ;make it a noticeable square make "append "false ;signify that we want to start a new animation repeat :number [ cs
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): HELLO WORLD TO HELLO PRINT world! END HELLO BIBLIOGRAPHY
EXTERNAL LINKS
|
|
|