UCBLogo
UCBLogo, also termed Berkeley Logo, is a programming language, a dialect of the language Lisp, and its subdialect, the language Logo. It is closest to a de facto standard for the language Logo, with its facilities for handling lists, files, input/output, and recursion in scripts, and can be used to teach most computer science concepts, as University of California, Berkeley lecturer Brian Harvey did in his Computer Science Logo Style trilogy. It is free and open-source software released under a GNU General Public License.
Graphical user interface
UCBLogo has a rudimentary graphical user interface, so several projects exist that provide a better interface. MSWLogo and its successor FMSLogo, for Microsoft Windows, are commonly used in schools in the United Kingdom and Australia.Design
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 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. The turtle can be set to erase anything below it, using the command PENERASE, while the pen can be set to start drawing again with the command PENPAINT, in UCBLogo.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.An example code:
FD 20 ; draw a line and move
PENUP ; lift the pen so it draws nothing
FD 20 ; move and not draw
PENDOWN ; lower the pen so it draws again
FD 20 ; draw a line and move
PENUP ; lift the pen so it draws nothing
FD 40 ; move and not draw
PENDOWN ; lower the pen so it draws again
RT 20 ; rotate right 20 degrees
Data
There are three datatypes in UCBLogo:- the word
- the list
- the array
There is no static typing. The interpreter detects the datatype by context.
Two important symbols are:
- The colon means the contents of. This is an extremely useful symbol that keeps reminding students that a variable is really a place in memory.
- The doublequote means the word is evaluated as itself, or its value after evaluation is the same as it was before. This is important. For users from other programming languages: the doublequote is not paired as opening and closing quotes.
Variable assignment is handled in Logo with the
make
command, as exemplified by these two equivalent statements:make "x sum :y 3
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 do not 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, which is termed dynamic scope. Calling a procedure with inputs also creates local variables that hold the argument values.Lists
Logo inherits lists from Lisp, and they are its main method to store 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
, anditem
. Data elements are added using sentencefput
andlput
. - 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
There is one conditional structure.
- ifelse test
- while condition
- until condition
- repeat number
Template iteration
Logo also provides list-based control structures. The basic idea is of two lists:OPERATION
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, 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 from the list
show gprop :listname :name ;to get the matching value from the list
Input, output
For input/output, text may be written to the command window usingprint
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.Syntax
Commands may be written on one line, or more. Many commands have mnemonic short forms; for exampleFORWARD
and RIGHT
are coded FD
and RT
respectively. This makes the input less onerous. Anything written after the ; is ignored, allowing the coder to insert comments.; draws a square with sides 100 units long
FORWARD 100
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:
Loops
There are three loop commands; is one. This draws a square.REPEAT 4
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:. Loops may be nested, giving results with little effort.
REPEAT 36 RT 10]
Functions and procedures
Each line is made up of function calls, of which there are two types:- commands like
print
. - operations like
sum
,first
orreadlist
.
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.
TO CHAIR REPEAT 4 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 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 200
will be executed. The word CHAIR
can be used as a command; for example, REPEAT 4
would repeat the CHAIR
operation four times.EDALL ;
TO ERASECHAIR
PE
BK 200 REPEAT 4
PPT
END
CS CHAIR WAIT 200 ERASECHAIR
A WAIT delay between the drawing and the erasing can introduce the illusion of motion:
CS REPEAT 20
Arguments, parameters
Logo can pass extra information to its words, and return information. The procedure 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 CHAIR :thesize
REPEAT 4
FD :thesize
END
CS
REPEAT 9
Other notes
Mathematics in Logo uses prefix or Polish notation, like:help "keyword ;.
Logo allows for recursion, a process where a procedure calls itself.
to spiral :size
if :size > 30 ; an exit condition
fd :size rt 15 ; many lines of action
spiral :size *1.02 ; the tailend recursive call
end
spiral 10
Symbolic computing code examples
Filter, map and reduce examples
? print filter3 4
? print map
1 4 9 16
? print reduce
999
?
Max can be implemented as
to max :a :boutput ifelse :a > :b
end
Define and use a procedure which calculates the average of its numeric arguments
? to average 2> op /
> end
? print average 1 5
3
3
? print apply "average
3
?
Define and use a procedure to compute plurals
? to plural :word> if equalp last :word "y
> if equalp last :word "s
> output word :word "s
> end
? print plural "body
bodies
? print map "plural
books bodies viruses
?
Transcribe a sentence into Pig latin example
? to pigl :word> if punctuationp last :word
> op pigl.real :word
> end
? to pigl.real :word
> if vowelp first :word
> op pigl.real word bf :word first :word
> end
? to vowelp :letter
> op memberp :letter
> end
? to punctuationp :letter
> op memberp :letter
> end
? print map "pigl
igpay atinlay isay unfay, anday ardhay otay astermay atay ethay amesay imetay!
?