Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/unix/volume10/logo/part05/logoman.1

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.UB "Introduction."
Logo is a computer programming language which was
designed to be both simple to use and extremely powerful.  It was
designed by a group of computer scientists at MIT and at Bolt, Beranek, and
Newman.  Its structure is based largely on the LISP language, which is
widely used in Artificial Intelligence research, but the notation has been
changed to make Logo far easier for a beginner than LISP.  This manual
describes a version of Logo for the PDP-11, originally written in C at the
Boston Children's Museum and extensively modified at the Lincoln-Sudbury
Regional High School.
.PP
The power of Logo comes primarily from the idea of the \f2procedure\f1.  A
procedure is simply something the computer "knows" how to do; some
procedures are built into Logo itself (these are called \f2primitive\f1
procedures), while others are \f2defined\f1 by the programmer in terms of
these simple procedures.  Defined procedures can be used as part of the
definition of other procedures, so that a complicated program can be built
in "layers" of complexity.  This layered structure is analogous to the
description of a complex machine in terms of building blocks: an automobile
is made of a chassis, a drive train, an electrical system, and so on.  The
drive train contains an engine, a transmission, a clutch, an axle, and so
on.  The transmission contains a housing, gears, and levers.  A lever may
include connecting joints at the ends and at a pivot point.  Eventually the
description of the automobile reaches the level of bolts and washers; these
correspond to the primitive procedures in Logo.
.PP
.UB "Starting Logo."
Use the shell command \f3logo\f1 to start the Logo
interpreter.  When Logo is running it will print a question mark (?) at the
beginning of a line to indicate that it is ready for you to type in a Logo
instruction.  The instruction may print something on the terminal, or draw a
line on a graphics display screen, or move a turtle around the floor.  Then
another question mark is typed and you may give another instruction.  (If Logo
prints a "greater than" sign (>) instead of a question mark, it is
in \f2procedure definition mode\f1,
which will be described later.  Type your system quit character
(control-G at Lincoln-Sudbury) to return to normal mode.)
.PP
If an argument is used with the shell command \f3logo\f1, the argument is
taken as the name of a procedure, which Logo runs before doing anything
else.  You can therefore create a shell script which will start Logo, run
a procedure automatically, and (if you say "goodbye" at the end of the
procedure) exit.
.PP
.UB "Syntax."
Unlike most computer languages, Logo has an almost
entirely uniform
syntax.  That is, all of the different commands Logo understands are
represented using the same notation: the name of a procedure is followed by
its \f2inputs\f1, which may be \f2constants\f1 (like numbers) or else may
be the results of using other procedures.  Here is a simple example:
.EX
print "hello
.EC
In this Logo instruction, the primitive procedure \f3print\f1 is used with
a constant input, the word \f3hello\f1.  The quotation mark indicates that
\f3hello\f1 is being used to represent the word itself; without the
quotation mark it would have been interpreted as the name of a procedure,
just as \f3print\f1 is the name of a procedure.  In Logo, the \f3print\f1
procedure always requires exactly one input, which is the thing to print.  The
input can be a \f2word\f1, as in this example, or a \f2list\f1, which will
be explained later.  (A \f2number\f1 is a special case of a word, and a
\f2sentence\f1 is a special case of a list.)  Here is another example:
.EX
print first "hello
.EC
In this instruction, the primitive procedure \f3first\f1 is used.  This
procedure takes one input, a word, and has an \f2output\f1 which is the
first letter of the word.  The output from \f3first\f1 is used as the input
to \f3print\f1, so what is printed is the letter \f3h\f1 rather than the
word \f3hello\f1 as in the earlier example.
.PP
Don't confuse the \f2output\f1 from a procedure with what is \f2printed\f1
by the \f3print\f1 command.  In Logo, the word "output" is not used to
refer to what is printed by a program, just as the word "input" does not
mean something you type into the program.  Instead, these words refer to
objects (words or lists) which are given to a procedure (inputs) or
produced by a procedure (outputs).  A particular procedure has a fixed
number of inputs and outputs.  The number of inputs may be anything,
including zero, whereas the number of outputs is either zero or one.  A
procedure with an output (like \f3first\f1) is called an \f2operation\f1; one
without an output (like \f3print\f1) is called a \f2command\f1.
.PP
Some operations only have two possible outputs: the word \f3true\f1 and
the word \f3false\f1.  Such a procedure is called a \f2predicate\f1.  Predicates
are used to allow a program to carry out some instruction only if a
particular condition is met.  By convention, predicates generally have
names ending with the letter "p".
.PP
.UB "Multiple inputs to operations."
Several Logo primitive procedures are
operations with two inputs.  The arithmetic operations, like \f3sum\f1, are
examples of this.  A special extension to Logo syntax allows such an operation
to have more than two inputs by enclosing the operation and its inputs in
parentheses or braces:
.EX
(sum 2 5 13 8.5)
.EC
Association is right to left.  At least two inputs must be given, except for
the \f3list\f1 operation, which can take one input if parenthesized.
.PP
.UB "Multi-instruction lines."
It is possible to put more than one instruction on the same line when you
are typing to Logo.  If you do this, it is recommended that you type a
semicolon (;) between the instructions for improved readability:
.EX
print "hello; print "goodbye
.EC
For compatibility with other versions of Logo, it is permitted to leave out
the semicolon.  Later in this manual, the phrase "instruction line" will
mean one or more instructions on a line.
.PP
.UB "Multi-line instructions."
It is possible to continue an instruction on
a second line.  To do this, end the first line with a backslash (\\),
immediately followed by the RETURN key.  If you are typing a quoted word,
you must end the word (with a space, for example) before using the
backslash-RETURN combination.  Inside a quoted word, backslash-RETURN
means to put an actual RETURN as part of the word.
.PP
.UB "Comments."
It is possible to include in an instruction line comments
which are meant for human readers of your program (including yourself, next
week), and which are not Logo instructions.  To do this, begin the comment
with an exclamation point (!).  Everything after the exclamation point on the
line will be ignored by Logo.  For example:
.EX
print [Hi, there.] ! A friendly greeting.
.EC
However, the exclamation point does not begin a comment if it is part of a
word or list (see below).  You should type a space before the exclamation
point, as in the example above, to make sure it will be interpreted as the
beginning of a comment.
.PP
.UB "Words."
Every computer language deals with particular kinds
of objects.  Most languages, like FORTRAN or BASIC or Pascal, are best at
dealing with numbers.  Logo is a \f2list processing\f1 language, which is
at its best with more complicated data structures.  The two main categories
of object are the \f2word\f1 and the \f2list\f1.
.PP
A \f2word\f1 is a string of characters.  (A \f2character\f1 is a letter,
digit, space, or punctuation mark.  Things like TAB and RETURN on the
terminal keyboard are also characters, although they are not usually used
as part of Logo words.)  A word can be any length, including zero.  The way
to indicate a word as part of a Logo program is to use the quotation mark
(") before the word.  The word begins with the character after the
quotation mark and continues until a space, a tab, the end of the
line, or one of these characters:
.EX
( ) [ ] { } " ;
.EC
A quotation mark immediately followed by a space or one of the other
word-terminating characters indicates the \f2empty word\f1, which is a word
of length zero.
.PP
Please notice that, unlike most programming languages, Logo does not use
quotation marks in pairs to delimit strings of characters.  The following
instruction is an error:
.EX
print "aardvark"
.EC
This is an error because the \f3print\f1 command is followed by \f2two\f1
words, the word \f3aardvark\f1 and an empty word which is indicated by the
second quotation mark.  Since \f3print\f1 only uses one input, the second
word has no purpose, and Logo gives the error message
.EX
You don't say what to do with "
.EE
In order to include one of the word-terminating characters in a word, you
must precede it with a backslash (\\).  Do not confuse backslash with the
regular slash (/) character.  For example, this instruction:
.EX
print "\\(boo\\)
.EC
will print the five characters \f3(boo)\f1 as its result.  The space
character may be included in a word by using a percent (%) instead of the
space.  Therefore, the following are equivalent:
.EX
print "Hello%there.
print "Hello\\ there.
.EC
To include a percent character or a backslash character in a word, you must
precede it with a backslash.
.PP
.UB "Numbers."
A number is a special case of a word, in which the
characters are all digits.  (That definition isn't quite complete, and will
be expanded in the next paragraph.)  A number need not be preceded with a
quotation mark.  (This rule is possible because normally Logo interprets
words without quotation marks as the names of procedures, but there are no
procedures whose names begin with digits.)  If a quotation mark is not
used, then any nondigit terminates the word.
.PP
Actually, numbers may be written in scientific notation.  That is, they can
include signs, decimal points, and a power of 10 by which the number is
multiplied.  This \f2exponent\f1 is indicated by the letter \f3e\f1
followed by the integer power of 10.  The following numbers have the
same value:
.EX
1000
"1000
1000.00
1e3
10.0e+2
"+1.0e3
10000e-1
.EC
Notice that if the number begins with a sign it must be quoted.  A quoted
number still must begin with a digit or a sign, not with a
decimal point or a letter \f3e\f1.  (The
letter may be a capital \f3E\f1, by the way.)  If a number is quoted, it
must end with one of the normal word-terminating characters.  A number
which contains only digits (no decimal point or exponent) is called
an \f2integer\f1.  Note that a number with a decimal point is not
considered an integer even if the digits after the decimal point are
all zero.
.PP
Since a number is a word, the usual character-manipulating procedures may
be applied to it.  For example,
.EX
print first 1024
.EC
prints the digit \f31\f1 which is the first character of the number.  In
addition, there are arithmetic procedures which apply specifically to numbers:
.EX
print sum 3 2
.EC
prints the number 5.  These procedures will be listed later.
.PP
.UB "Lists."
A word can be thought of as a list of characters; for
example, the word \f3hello\f1 is a list of five letters.  In Logo it is
possible to manipulate not only lists of characters but also lists of
words, lists of lists of words, and so on.  This is a very powerful capability
which allows very complicated data structures to be manipulated easily.  To
indicate a list in a program, you put square brackets ([ and ]) around the
list, and separate the list elements with spaces.  For example:
.EX
print [This is a list of seven words.]
.EC
A list all of whose elements are words is called a \f2sentence\f1.  Here
is an example of a list which is not a sentence:
.EX
print [[This is a list][of two sentences.]]
.EE
Within a bracketed list, square brackets delimit sub-lists (lists which are
elements of the main list).  The quotation mark, parentheses, and braces
are not considered special within a bracketed list, unlike the rules for
quoted words.  A list may extend over more than one line; that is, if you
have typed an open square bracket ([) and have not yet typed the matching
close square bracket, the Logo instruction is not ended by typing the RETURN
key.
.PP
.UB "Variables."
A variable is an entity which has a \f2name\f1, which
is a word, and a \f2thing\f1 (also called a \f2value\f1), which
can be any Logo object.  Variables
are used to "remember" a computed object for repeated or delayed use
in a program.  In Logo, the most common way that a variable acquires
a value is that it is associated with an input to a user-written
procedure.  In the following example, don't worry about the details
of the format of the procedure, which will be explained later:
.EX
to pff :sentence
print first first :sentence
end
.EC
This is the definition of a command with one input.  The name of the
command is \f3pff\f1.  It has one input because in the "title line" (the
one starting \f3to pff\f1) there is one variable name after the command
name.  The variable whose name is \f3sentence\f1 is associated with the
first (and only, in this case) input to \f3pff\f1.  In the line starting
with the word \f3print\f1, the notation \f3:sentence\f1 means "the \f2thing\f1
of the variable whose \f2name\f1 is \f3sentence\f1".  (To refer to the
name itself, quote it as you would any word.)  If this procedure is used
in a Logo instruction like this:
.EX
pff [This is the poop.]
.EC
then the variable \f3sentence\f1 has the value \f3[This is the poop.]\f1.
.PP
It is also possible to assign a value to a variable by an explicit
Logo instruction.  There is a primitive procedure to do this:
.sp 1
\f3make\f1 \(em Command, two inputs.
.IN
The first input is the name of a variable (that is, it must be a word); the
second is any Logo object.  The effect of the command is to assign the
second input as the value of the variable named by the first input.
.OU
If you are accustomed to programming in a non-procedural language like
BASIC, you should strenuously avoid the temptation to overuse \f3make\f1;
explicit assignment is almost always the wrong thing to do in Logo.  Total
abstention is the best policy for a Logo beginner.
.PP
In Logo, variables are \f2dynamically scoped\f1.  That means that a
variable can "belong to" a particular procedure; such a variable can
be used by that procedure and by any procedure which is used by an
instruction within the procedure, but is not available to the procedure
which invoked the owning procedure.  In other words, such a \f2local\f1
variable comes into being when the owning procedure starts running, and
disappears when that procedure is finished.  It is possible for a
procedure with a local variable to use another procedure with a local
variable of the same name.  In that case, the variable belonging to the
"inner" procedure is the one which is associated with the name as long
as it exists; when the inner procedure is finished, the "hidden"
variable belonging to the outer procedure is again available.
.PP
A variable which is associated with the input to a procedure is
automatically local to that procedure.  Other variables are normally
\f2global\f1: they are "permanent" and do not disappear when the
procedure in which they get their values finish.  It is both possible
and desirable to make such variables local, by an explicit instruction
to that effect:
.sp 1
\f3local\f1 \(em Command, one input.
.IN
The input must be a word.  A variable with that word as its name
is created, local to the currently running procedure (that is,
local to the procedure in which the \f3local\f1 command is used).
.OU
The virtue of local variables is that they make procedures more
independent of one another than they would be if global variables
were used.  In other words, if you use local variables consistently,
then nothing that happens in one procedure will change the values
of variables used in another procedure.  This makes it very much
easier to find program errors.
.PP
.UB "Case insensitivity."
Names of procedures (primitive or user-defined) and names of variables
may be typed in upper or lower case.  They are converted internally to
lower case.  That is, the variables \f3foo\f1 and \f3FOO\f1 are the
same variable.  Letters in other contexts are not converted to lower
case.  For example, \f3equalp\f1 will report that the words \f3foo\f1
and \f3FOO\f1 are not equal.
.PP
Names of procedures and names of variables may include only letters,
digits, and the special characters period (.) and underscore (_).  Also,
names of procedures are limited to 11 characters in some versions of Unix.
.PP
.UB "Primitive procedures to define user procedures."
There are
two ways to define your own procedure.  The first way, using the \f3to\f1
command, is simple to learn but limited in flexibility.  The second way,
using the \f3edit\f1 command, is more complicated to learn, but makes it
easy to make changes in your procedures.  The \f3edit\f1 command uses the
text editing program \f3edt\f1, just as you might use it outside of Logo
to edit a document you want to print.  Once you've learned the special
editing commands in \f3edt\f1, it's easy to use.  The \f3to\f1 command makes
it possible to begin programming in Logo without having learned how to use
\f3edt\f1.  It just lets you type in your procedure definition, without any
provision for correcting errors or changing the definition of the procedure.
It is fast and convenient for short procedures, but limited.
.sp 1
The \f3to\f1 command is unique, in Logo, in that its inputs are interpreted
in a special way.  The inputs aren't \f2evaluated\f1: Logo doesn't run any
procedures you name, or look up the values of any variables, before carrying
out the \f3to\f1 command.  The example below should make this clearer.
.sp 1
\f3to\f1 \(em Command, special form, see below.
.IN
This command takes a variable number of inputs.  The first is the name
of a procedure to be defined.  The rest, if any, must be preceded by
colons, and are the names of variables to be used as inputs to the
procedure.  Logo responds to the \f3to\f1 command by printing a
"greater than" sign (>) prompt, to show you that you are defining a procedure
rather than entering commands to be executed immediately.  You type
the instruction lines which make up the definition.  When
you are done with the definition, type the special word \f3end\f1 on
a line by itself.  For example:
.sp 1
.nf
\f3\z_?to twoprint :thing
\z_>print :thing
\z_>print :thing
\z_>end
\z_?\f1
.fi
.sp 1
This example shows the definition of a procedure named \f3twoprint\f1,
which has one input, named \f3thing\f1.  The procedure you are defining
with the \f3to\f1 command must not already be defined.
.OU
\f3edit\f1 \(em Command, zero or one input.  Abbreviation: \f3ed\f1
.IN
The input to this command must be a word, which is the name of a procedure,
or a list of words, each of which is the name of a procedure.
(Unlike the \f3to\f1 command, but like all other Logo procedures, the
\f3edit\f1 command evaluates its input, so you must use a quotation mark
before the procedure name, if only one is given, to indicate
that it is the name itself which is
the input to \f3edit\f1; otherwise Logo would actually run the procedure
to calculate the input to \f3edit\f1.)  The procedure you name may or may
not already be defined.  Logo responds to the \f3edit\f1 command by running
the text editor \f3edt\f1, editing the definition of the procedure(s) named in
its input.  (If a procedure was not previously defined, Logo creates an
initial definition for it which contains only a title line and the end line.)
You then edit the definition(s) with \f3edt\f1.  When you write the file and
leave \f3edt\f1, Logo will use the edited file as the definition(s) of the
procedure(s).  You must not put anything in the file except procedure
definitions; in other words, every nonempty line in the file must be between
a "to" line and an "end" line.
.sp 1
If the \f3edit\f1 command is given with no input, \f3edt\f1 is given the same
file as from the last time you used the \f3edit\f1 command.  This is a
convenience for editing the same procedure(s) repeatedly.
.sp 1
If, while editing, you change your mind and want to leave \f3edt\f1
without redefining anything, use the command \f3ESC ^Z\f1 instead of
the normal \f3^Z\f1.  This special way of leaving \f3edt\f1 tells Logo
not to redefine your procedures.  You have the choice, before exiting
\f3edt\f1, of writing or not writing the temporary file which contains
the definitions.  If you don't write the file, another \f3edit\f1 command
with no input will re-read the previous contents of the temporary file;
if you do, another \f3edit\f1 will re-read the new version.
.sp 1
If your Unix environment contains a variable named EDITOR, the contents of
that variable is used as the name of the text editor program instead of
the standard \f3edt\f1.  The variable can contain a full pathname, or just
a program name if the program can be found in /bin or /usr/bin.  Your favorite
editor may not have a facility like \f3edt\f1's ESC ^Z to abort redefinition.
.OU
\f3show\f1 \(em Command, one input.  Abbreviation: \f3po\f1
.IN
The input to this command is a word or a list of words.  Each word must be
the name of a procedure.  The command prints out the definition(s) of the
procedure(s) on your terminal.  (The abbreviation \f3po\f1 stands for
\f3printout\f1, which is the name used for this command in some other versions
of Logo.)
.OU
\f3pots\f1 \(em Command, no inputs.
.IN
This command types at your terminal the title lines of all procedures
you've defined.  The name is an abbreviation for "print out titles".
.OU
\f3erase\f1 \(em Command, one input.  Abbreviation: \f3er\f1
.IN
As for the \f3show\f1 command, the input is either a word, naming one
procedure, or a list of words, naming more than one.  The named procedures
are erased, so that they are no longer defined.
.OU
.ti 5
.UB "Primitive procedures to manipulate words and lists."
There are
primitive procedures to print text objects on the terminal, to read
them from the terminal, to combine them into larger objects, to split
them into smaller objects, and to determine their size and nature:
.sp 1
\f3print\f1 \(em Command, one input.  Abbreviation: \f3pr\f1
.IN
The input, which may be a word or a list, is printed on the terminal,
followed by a new line character.  (That is, the terminal is positioned
at the beginning of a new line after printing the object.)  If the
object is a list, any sub-lists are delimited by square brackets, but
the entire object is not delimited by brackets.
.OU
\f3type\f1 \(em Command, one input.
.IN
The input, which may be a word or a list, is printed on the terminal,
\f2without\f1 a new line character.  (That is, the terminal remains
positioned at the end of the object after printing it.)  Brackets
are used as with the \f3print\f1 command.
.OU
\f3fprint\f1 \(em Command, one input.  Abbreviation: \f3fp\f1
.IN
The input is printed as by the \f3print\f1 command, except that if it
is a list (as opposed to a word) it is enclosed in square brackets.  The
name of the command is short for "full print".
.OU
\f3ftype\f1 \(em Command, one input.  Abbreviation: \f3fty\f1
.IN
The input is printed as by the \f3type\f1 command, except that if it
is a list, it is enclosed in square brackets.
.OU
\f3readlist\f1 \(em Operation, no inputs.  Abbreviation: \f3rl\f1
.IN
Logo waits
for a line to be typed by the user.  The contents of the line are made
into a list, as though typed within square brackets as part of a Logo
instruction.  (The user should not actually type brackets around the
line, unless s/he desires a list of one element, which is a list
itself.)  That list is the output from the operation.
.OU
\f3request\f1 \(em Operation, no inputs.
.IN
A question mark is printed on the terminal as a prompt.  Then Logo waits
for a line to be typed by the user, as for \f3readlist\f1.
.OU
\f3word\f1 \(em Operation, two inputs.
.IN
The two inputs must be words.  The output is a word which is the
concatenation of the two inputs.  There is no space or other
separation of the two inputs in the output.
.OU
\f3sentence\f1 \(em Operation, two inputs.  Abbreviation: \f3se\f1
.IN
The two inputs may be words or lists.  The output is a list formed
from the two inputs in this way: if either input is a word, that
word becomes a member of the output list; if either input is a
list, the \f2members\f1 of that input become members of the
output.  Here are some examples:
.EX
.ta 2i 4i
first input    second input    output
"hello    "test    [hello test]
"goodbye    [cruel world]    [goodbye cruel world]
[a b]    [c d]    [a b c d]
[]    "garply    [garply]
.EC
If an input is the empty list, as in the last example above, it
contributes nothing to the output.
.OU
\f3list\f1 \(em Operation, two inputs.
.IN
The output is a list of two elements, namely, the two inputs.  The
inputs may be words or lists.
.OU
\f3fput\f1 \(em Operation, two inputs.
.IN
The first input may be any Logo object; the second must be a list.  The
output is a list which is identical with the second input except that
it has an extra first member, namely, the first input.
.OU
\f3lput\f1 \(em Operation, two inputs.
.IN
The first input may be any Logo object; the second must be a list.  The
output is a list which is identical with the second input except that
it has an extra last member, namely, the first input.
.OU
\f3first\f1 \(em Operation, one input.
.IN
The input may be any non-empty Logo object.  If the input is a list,
the output is its first member.  If the input is a word, the output is
a single-letter word, namely the first letter of the input.  If the
input is empty (a word or list of length zero) an error results.
.OU
\f3last\f1 \(em Operation, one input.
.IN
The input may be any non-empty Logo object.  If the input is a list,
the output is its last member.  If the input is a word, the output is
a single-letter word, namely the last letter of the input.  If the
input is empty (a word or list of length zero) an error results.
.OU
\f3butfirst\f1 \(em Operation, one input.  Abbreviation: \f3bf\f1
.IN
The input may be any non-empty Logo object.  If the input is a list,
the output is a list equal to the input list with the first member
removed.  (If the input list has only one member, the output is
the \f2empty list\f1, a list of zero members.)  If the input is
a word, the output is a word equal to the input word with the
first letter removed.  (If the input is a single-letter word, the
output is the \f2empty word\f1.)  If the input is empty, an
error results.
.OU
\f3butlast\f1 \(em Operation, one input.  Abbreviation: \f3bl\f1
.IN
The input may be any non-empty Logo object.  If the input is a list,
the output is a list equal to the input list with the last member
removed.  (If the input list has only one member, the output is
the \f2empty list\f1, a list of zero members.)  If the input is
a word, the output is a word equal to the input word with the
last letter removed.  (If the input is a single-letter word, the
output is the \f2empty word\f1.)  If the input is empty, an
error results.
.OU
\f3count\f1 \(em Operation, one input.
.IN
The input may be any Logo object.  If the input is a list, the
output is a number indicating the number of members in the list.  (Note:
only top-level members are counted, not members of members.  The count
of the list
.EX
[[This is] [a list]]
.EC
is 2, not 4.)  If the input is a word, the output is the number of
letters (or other characters) in the word.  Remember that in Logo a
number is just a particular kind of word, so the output from \f3count\f1 can
be manipulated like any other Logo word.
.OU
\f3emptyp\f1 \(em Operation (predicate), one input.
.IN
The input can be any Logo object.  The output is the word \f3true\f1 if
the input is of length zero (i.e., it is the empty word or the empty
list).  The output is the word \f3false\f1 otherwise.
.OU
\f3wordp\f1 \(em Operation (predicate), one input.
.IN
The input can be any Logo object.  The output is the word \f3true\f1 if
the input is a word.  The output is the word \f3false\f1 if the input
is a list.
.OU
\f3sentencep\f1 \(em Operation (predicate), one input.
.IN
The input can be any Logo object.  The output is the word \f3true\f1 if
the input is a sentence, i.e., a list of words.  The output is the word
\f3false\f1 if the input is a word, or if any member of the input is a
list.
.OU
\f3is\f1 \(em Operation (predicate), two inputs.
.IN
The inputs can be any Logo objects.  The output is the word \f3true\f1 if
the two inputs are identical.  That is, they must be of the same type
(both words or both lists), they must have the same count, and their
members (if lists) or their characters (if words) must be identical.  The
output is the word \f3false\f1 otherwise.  (Note: this is an exception
to the convention that names of predicates end with the letter "p".)
.OU
\f3memberp\f1 \(em Operation (predicate), two inputs.
.IN
If the second input is a word, the first input must be a word of
length one (a single character), and the output is \f3true\f1 if and
only if the first input is contained in the second as a character.  If
the second input is a list, the first input can be any Logo object,
and the output is \f3true\f1 if and only if the first input is a member
of the second input.  (Note that this is member, not subset.)
.OU
\f3item\f1 \(em Operation, two inputs.  Abbreviation: \f3nth\f1
.IN
The first input must be a positive integer less than or equal to
the \f3count\f1 of the second input.  If the second input is a word,
the output is a word of length one containing the selected character
from the word.  (Items are numbered from 1, not 0.)  If the second input
is a list, the output is the selected member of the list.
.OU
.ti 5
.UB "Primitive procedures for turtles and graphics."
An important
part of the Logo environment is a rich set of applications to which the
computer can be directed.  The most important of these is \f2turtle
geometry\f1, a way of describing paths of motion in a plane which is
well-suited to computer programming.  There are two ways to use the
turtle procedures.  First, you can control a \f2floor turtle\f1, a small
robot which can move one the floor or on a table under computer
control.  Second, you can use a \f2display turtle\f1 to draw pictures
on the TV screen of a graphics terminal.
.PP
Each computer center has a different variety of graphics hardware
available.  Floor turtles are very different from display turtles, but also
each kind of display terminal has different characteristics.  For example,
some terminals can draw in several colors; others can't.  The following
descriptions of graphics primitives explain the "best" case for each one
and mention restrictions on some graphics devices.
.PP
The floor turtle can draw pictures on paper, because it has a pen
attached to its "belly": the underside of the turtle.  Since it is
a mechanical device, however, it is not very precise; the pictures
you get may not be exactly like what your program specifies.  A more
interesting way to use the floor turtle is to take advantage of its
\f2touch sensors\f1.  Switches under the turtle's dome allow the computer
to know when the turtle bumps into an obstacle.  You can use this
information to write programs to get around obstacles or to follow
a maze.
.PP
The display turtle lives on the surface of a TV screen.  It can draw
pictures more precisely than the floor turtle, since it does not
measure distances and angles mechanically.  It is also faster than
the floor turtle.  When using the display turtle, remember that
it interprets commands relative to its own position and direction,
just as the floor turtle does.  The command \f3left\f1, for example,
turns the turtle to its own left, which may or may not be toward
the left side of the TV screen.
.sp 1
\f3turtle\f1 \(em Command, one input.  Abbreviation: \f3tur\f1
.IN
The input is the name of a turtle.  You can only control one turtle
at a time, so using this command a second time releases the turtle
previously selected.  The names of floor turtles are numbers like
\f30\f1 and \f31\f1.  If you are using
a graphics display terminal (not just a text display trminal), you can
control the display turtle by using the word \f3display\f1 (or the
abbreviation \f3dpy\f1) as the turtle name.  (As usual, the word
must be preceded by a quotation mark.)  If you use a graphics primitive
without selecting a turtle, Logo assumes that you want to use the
display turtle.  But once you select a floor turtle, you must say
\f3turtle "display\f1 explicitly to switch to the display.
.sp 1
The word \f3off\f1 as input to the \f3turtle\f1 command releases a floor
turtle, if you have one, or turns off the graphics display if you have
been using the display turtle.  This also happens when you leave Logo.
.OU
\f3forward\f1 \(em Command, one input.  Abbreviation: \f3fd\f1
.IN
The input is a number, the distance you would like the turtle
to move.  For a floor turtle, the unit of distance is however
far the turtle can travel in 1/30 second.  For a display turtle,
the unit is one dot on the TV screen.  (Note: on some displays, one
dot horizontally may not be the same length as one dot vertically.  The
\f3setscrunch\f1 command allows you to control the
relative sizes so that squares come out
square.)  The turtle moves in whatever direction it is pointing
when you use the command.
.OU
\f3back\f1 \(em Command, one input.  Abbreviation: \f3bk\f1
.IN
The input is a number, a distance to move, as in the \f3forward\f1
command.  The difference is that the turtle moves backward, i.e.,
in the direction exactly opposite to the way it's pointing.
.OU
\f3left\f1 \(em Command, one input.  Abbreviation: \f3lt\f1
.IN
The input is a number, the number of degrees of angle through
which the turtle should turn counterclockwise.  This command
does not change the \f2position\f1 of the turtle, but merely
its \f2heading\f1 (the direction in which it points).  The
turn will be only approximately correct for the floor turtle,
because of mechanical errors.  For the display turtle, the
angle will be perfectly reproducible, although it may not look
quite right on the screen because of the difference in size
between horizontal and vertical dots.  Nevertheless, a display
turtle program will work in the sense that when the turtle is
supposed to return to its starting point, it will do so.
.OU
\f3right\f1 \(em Command, one input.  Abbreviation: \f3rt\f1
.IN
The input is a number; the turtle turns through the specified
number of degrees clockwise.
.OU
\f3penup\f1 \(em Command, no inputs.  Abbreviation: \f3pu\f1
.IN
This command tells the turtle to raise its pen from the paper,
so that it does not leave a trace when it moves.  In the case
of the display turtle, there is no physical pen to move
mechanically, but the effect is the same: any \f3forward\f1 or
\f3back\f1 commands after this point do not draw a line.  The
floor turtle starts with its pen up; the display turtle starts
with its pen down.  Note: the floor turtle will not move on
the carpet correctly with its pen down; put it on a smooth
surface if you want to draw pictures.
.OU