rs@uunet.UU.NET (Rich Salz) (06/24/87)
Submitted by: Brian Harvey <bh@mit-amt>
Mod.Sources: Volume 10, Number 25
Archive-Name: logo/Part05
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# Contents: logoman.1
# Wrapped by rsalz@pineapple.bbn.com on Wed Jun 24 14:27:00 1987
PATH=/bin:/usr/bin:/usr/ucb ; export PATH
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echo shar: Will not over-write existing file \"logoman.1\"
else
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sed "s/^X//" >logoman.1 <<'END_OF_logoman.1'
X.ll 7.5i
X.lt 7.5i
X.po .2i
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X.de TL
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X.ce
X\\$1
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X.TL "LSRHS Logo Manual"
X.NB
X.de EX
X.ft 3
X.in +5
X.sp 1
X.nf
X..
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X.fi
X.PP
X..
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X.uf 3
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X..
X.PP
X.UB "Introduction."
XLogo is a computer programming language which was
Xdesigned to be both simple to use and extremely powerful. It was
Xdesigned by a group of computer scientists at MIT and at Bolt, Beranek, and
XNewman. Its structure is based largely on the LISP language, which is
Xwidely used in Artificial Intelligence research, but the notation has been
Xchanged to make Logo far easier for a beginner than LISP. This manual
Xdescribes a version of Logo for the PDP-11, originally written in C at the
XBoston Children's Museum and extensively modified at the Lincoln-Sudbury
XRegional High School.
X.PP
XThe power of Logo comes primarily from the idea of the \f2procedure\f1. A
Xprocedure is simply something the computer "knows" how to do; some
Xprocedures are built into Logo itself (these are called \f2primitive\f1
Xprocedures), while others are \f2defined\f1 by the programmer in terms of
Xthese simple procedures. Defined procedures can be used as part of the
Xdefinition of other procedures, so that a complicated program can be built
Xin "layers" of complexity. This layered structure is analogous to the
Xdescription of a complex machine in terms of building blocks: an automobile
Xis made of a chassis, a drive train, an electrical system, and so on. The
Xdrive train contains an engine, a transmission, a clutch, an axle, and so
Xon. The transmission contains a housing, gears, and levers. A lever may
Xinclude connecting joints at the ends and at a pivot point. Eventually the
Xdescription of the automobile reaches the level of bolts and washers; these
Xcorrespond to the primitive procedures in Logo.
X.PP
X.UB "Starting Logo."
XUse the shell command \f3logo\f1 to start the Logo
Xinterpreter. When Logo is running it will print a question mark (?) at the
Xbeginning of a line to indicate that it is ready for you to type in a Logo
Xinstruction. The instruction may print something on the terminal, or draw a
Xline on a graphics display screen, or move a turtle around the floor. Then
Xanother question mark is typed and you may give another instruction. (If Logo
Xprints a "greater than" sign (>) instead of a question mark, it is
Xin \f2procedure definition mode\f1,
Xwhich will be described later. Type your system quit character
X(control-G at Lincoln-Sudbury) to return to normal mode.)
X.PP
XIf an argument is used with the shell command \f3logo\f1, the argument is
Xtaken as the name of a procedure, which Logo runs before doing anything
Xelse. You can therefore create a shell script which will start Logo, run
Xa procedure automatically, and (if you say "goodbye" at the end of the
Xprocedure) exit.
X.PP
X.UB "Syntax."
XUnlike most computer languages, Logo has an almost
Xentirely uniform
Xsyntax. That is, all of the different commands Logo understands are
Xrepresented using the same notation: the name of a procedure is followed by
Xits \f2inputs\f1, which may be \f2constants\f1 (like numbers) or else may
Xbe the results of using other procedures. Here is a simple example:
X.EX
Xprint "hello
X.EC
XIn this Logo instruction, the primitive procedure \f3print\f1 is used with
Xa constant input, the word \f3hello\f1. The quotation mark indicates that
X\f3hello\f1 is being used to represent the word itself; without the
Xquotation mark it would have been interpreted as the name of a procedure,
Xjust as \f3print\f1 is the name of a procedure. In Logo, the \f3print\f1
Xprocedure always requires exactly one input, which is the thing to print. The
Xinput can be a \f2word\f1, as in this example, or a \f2list\f1, which will
Xbe explained later. (A \f2number\f1 is a special case of a word, and a
X\f2sentence\f1 is a special case of a list.) Here is another example:
X.EX
Xprint first "hello
X.EC
XIn this instruction, the primitive procedure \f3first\f1 is used. This
Xprocedure takes one input, a word, and has an \f2output\f1 which is the
Xfirst letter of the word. The output from \f3first\f1 is used as the input
Xto \f3print\f1, so what is printed is the letter \f3h\f1 rather than the
Xword \f3hello\f1 as in the earlier example.
X.PP
XDon't confuse the \f2output\f1 from a procedure with what is \f2printed\f1
Xby the \f3print\f1 command. In Logo, the word "output" is not used to
Xrefer to what is printed by a program, just as the word "input" does not
Xmean something you type into the program. Instead, these words refer to
Xobjects (words or lists) which are given to a procedure (inputs) or
Xproduced by a procedure (outputs). A particular procedure has a fixed
Xnumber of inputs and outputs. The number of inputs may be anything,
Xincluding zero, whereas the number of outputs is either zero or one. A
Xprocedure with an output (like \f3first\f1) is called an \f2operation\f1; one
Xwithout an output (like \f3print\f1) is called a \f2command\f1.
X.PP
XSome operations only have two possible outputs: the word \f3true\f1 and
Xthe word \f3false\f1. Such a procedure is called a \f2predicate\f1. Predicates
Xare used to allow a program to carry out some instruction only if a
Xparticular condition is met. By convention, predicates generally have
Xnames ending with the letter "p".
X.PP
X.UB "Multiple inputs to operations."
XSeveral Logo primitive procedures are
Xoperations with two inputs. The arithmetic operations, like \f3sum\f1, are
Xexamples of this. A special extension to Logo syntax allows such an operation
Xto have more than two inputs by enclosing the operation and its inputs in
Xparentheses or braces:
X.EX
X(sum 2 5 13 8.5)
X.EC
XAssociation is right to left. At least two inputs must be given, except for
Xthe \f3list\f1 operation, which can take one input if parenthesized.
X.PP
X.UB "Multi-instruction lines."
XIt is possible to put more than one instruction on the same line when you
Xare typing to Logo. If you do this, it is recommended that you type a
Xsemicolon (;) between the instructions for improved readability:
X.EX
Xprint "hello; print "goodbye
X.EC
XFor compatibility with other versions of Logo, it is permitted to leave out
Xthe semicolon. Later in this manual, the phrase "instruction line" will
Xmean one or more instructions on a line.
X.PP
X.UB "Multi-line instructions."
XIt is possible to continue an instruction on
Xa second line. To do this, end the first line with a backslash (\\),
Ximmediately followed by the RETURN key. If you are typing a quoted word,
Xyou must end the word (with a space, for example) before using the
Xbackslash-RETURN combination. Inside a quoted word, backslash-RETURN
Xmeans to put an actual RETURN as part of the word.
X.PP
X.UB "Comments."
XIt is possible to include in an instruction line comments
Xwhich are meant for human readers of your program (including yourself, next
Xweek), and which are not Logo instructions. To do this, begin the comment
Xwith an exclamation point (!). Everything after the exclamation point on the
Xline will be ignored by Logo. For example:
X.EX
Xprint [Hi, there.] ! A friendly greeting.
X.EC
XHowever, the exclamation point does not begin a comment if it is part of a
Xword or list (see below). You should type a space before the exclamation
Xpoint, as in the example above, to make sure it will be interpreted as the
Xbeginning of a comment.
X.PP
X.UB "Words."
XEvery computer language deals with particular kinds
Xof objects. Most languages, like FORTRAN or BASIC or Pascal, are best at
Xdealing with numbers. Logo is a \f2list processing\f1 language, which is
Xat its best with more complicated data structures. The two main categories
Xof object are the \f2word\f1 and the \f2list\f1.
X.PP
XA \f2word\f1 is a string of characters. (A \f2character\f1 is a letter,
Xdigit, space, or punctuation mark. Things like TAB and RETURN on the
Xterminal keyboard are also characters, although they are not usually used
Xas part of Logo words.) A word can be any length, including zero. The way
Xto indicate a word as part of a Logo program is to use the quotation mark
X(") before the word. The word begins with the character after the
Xquotation mark and continues until a space, a tab, the end of the
Xline, or one of these characters:
X.EX
X( ) [ ] { } " ;
X.EC
XA quotation mark immediately followed by a space or one of the other
Xword-terminating characters indicates the \f2empty word\f1, which is a word
Xof length zero.
X.PP
XPlease notice that, unlike most programming languages, Logo does not use
Xquotation marks in pairs to delimit strings of characters. The following
Xinstruction is an error:
X.EX
Xprint "aardvark"
X.EC
XThis is an error because the \f3print\f1 command is followed by \f2two\f1
Xwords, the word \f3aardvark\f1 and an empty word which is indicated by the
Xsecond quotation mark. Since \f3print\f1 only uses one input, the second
Xword has no purpose, and Logo gives the error message
X.EX
XYou don't say what to do with "
X.EE
XIn order to include one of the word-terminating characters in a word, you
Xmust precede it with a backslash (\\). Do not confuse backslash with the
Xregular slash (/) character. For example, this instruction:
X.EX
Xprint "\\(boo\\)
X.EC
Xwill print the five characters \f3(boo)\f1 as its result. The space
Xcharacter may be included in a word by using a percent (%) instead of the
Xspace. Therefore, the following are equivalent:
X.EX
Xprint "Hello%there.
Xprint "Hello\\ there.
X.EC
XTo include a percent character or a backslash character in a word, you must
Xprecede it with a backslash.
X.PP
X.UB "Numbers."
XA number is a special case of a word, in which the
Xcharacters are all digits. (That definition isn't quite complete, and will
Xbe expanded in the next paragraph.) A number need not be preceded with a
Xquotation mark. (This rule is possible because normally Logo interprets
Xwords without quotation marks as the names of procedures, but there are no
Xprocedures whose names begin with digits.) If a quotation mark is not
Xused, then any nondigit terminates the word.
X.PP
XActually, numbers may be written in scientific notation. That is, they can
Xinclude signs, decimal points, and a power of 10 by which the number is
Xmultiplied. This \f2exponent\f1 is indicated by the letter \f3e\f1
Xfollowed by the integer power of 10. The following numbers have the
Xsame value:
X.EX
X1000
X"1000
X1000.00
X1e3
X10.0e+2
X"+1.0e3
X10000e-1
X.EC
XNotice that if the number begins with a sign it must be quoted. A quoted
Xnumber still must begin with a digit or a sign, not with a
Xdecimal point or a letter \f3e\f1. (The
Xletter may be a capital \f3E\f1, by the way.) If a number is quoted, it
Xmust end with one of the normal word-terminating characters. A number
Xwhich contains only digits (no decimal point or exponent) is called
Xan \f2integer\f1. Note that a number with a decimal point is not
Xconsidered an integer even if the digits after the decimal point are
Xall zero.
X.PP
XSince a number is a word, the usual character-manipulating procedures may
Xbe applied to it. For example,
X.EX
Xprint first 1024
X.EC
Xprints the digit \f31\f1 which is the first character of the number. In
Xaddition, there are arithmetic procedures which apply specifically to numbers:
X.EX
Xprint sum 3 2
X.EC
Xprints the number 5. These procedures will be listed later.
X.PP
X.UB "Lists."
XA word can be thought of as a list of characters; for
Xexample, the word \f3hello\f1 is a list of five letters. In Logo it is
Xpossible to manipulate not only lists of characters but also lists of
Xwords, lists of lists of words, and so on. This is a very powerful capability
Xwhich allows very complicated data structures to be manipulated easily. To
Xindicate a list in a program, you put square brackets ([ and ]) around the
Xlist, and separate the list elements with spaces. For example:
X.EX
Xprint [This is a list of seven words.]
X.EC
XA list all of whose elements are words is called a \f2sentence\f1. Here
Xis an example of a list which is not a sentence:
X.EX
Xprint [[This is a list][of two sentences.]]
X.EE
XWithin a bracketed list, square brackets delimit sub-lists (lists which are
Xelements of the main list). The quotation mark, parentheses, and braces
Xare not considered special within a bracketed list, unlike the rules for
Xquoted words. A list may extend over more than one line; that is, if you
Xhave typed an open square bracket ([) and have not yet typed the matching
Xclose square bracket, the Logo instruction is not ended by typing the RETURN
Xkey.
X.PP
X.UB "Variables."
XA variable is an entity which has a \f2name\f1, which
Xis a word, and a \f2thing\f1 (also called a \f2value\f1), which
Xcan be any Logo object. Variables
Xare used to "remember" a computed object for repeated or delayed use
Xin a program. In Logo, the most common way that a variable acquires
Xa value is that it is associated with an input to a user-written
Xprocedure. In the following example, don't worry about the details
Xof the format of the procedure, which will be explained later:
X.EX
Xto pff :sentence
Xprint first first :sentence
Xend
X.EC
XThis is the definition of a command with one input. The name of the
Xcommand is \f3pff\f1. It has one input because in the "title line" (the
Xone starting \f3to pff\f1) there is one variable name after the command
Xname. The variable whose name is \f3sentence\f1 is associated with the
Xfirst (and only, in this case) input to \f3pff\f1. In the line starting
Xwith the word \f3print\f1, the notation \f3:sentence\f1 means "the \f2thing\f1
Xof the variable whose \f2name\f1 is \f3sentence\f1". (To refer to the
Xname itself, quote it as you would any word.) If this procedure is used
Xin a Logo instruction like this:
X.EX
Xpff [This is the poop.]
X.EC
Xthen the variable \f3sentence\f1 has the value \f3[This is the poop.]\f1.
X.PP
XIt is also possible to assign a value to a variable by an explicit
XLogo instruction. There is a primitive procedure to do this:
X.sp 1
X\f3make\f1 \(em Command, two inputs.
X.IN
XThe first input is the name of a variable (that is, it must be a word); the
Xsecond is any Logo object. The effect of the command is to assign the
Xsecond input as the value of the variable named by the first input.
X.OU
XIf you are accustomed to programming in a non-procedural language like
XBASIC, you should strenuously avoid the temptation to overuse \f3make\f1;
Xexplicit assignment is almost always the wrong thing to do in Logo. Total
Xabstention is the best policy for a Logo beginner.
X.PP
XIn Logo, variables are \f2dynamically scoped\f1. That means that a
Xvariable can "belong to" a particular procedure; such a variable can
Xbe used by that procedure and by any procedure which is used by an
Xinstruction within the procedure, but is not available to the procedure
Xwhich invoked the owning procedure. In other words, such a \f2local\f1
Xvariable comes into being when the owning procedure starts running, and
Xdisappears when that procedure is finished. It is possible for a
Xprocedure with a local variable to use another procedure with a local
Xvariable of the same name. In that case, the variable belonging to the
X"inner" procedure is the one which is associated with the name as long
Xas it exists; when the inner procedure is finished, the "hidden"
Xvariable belonging to the outer procedure is again available.
X.PP
XA variable which is associated with the input to a procedure is
Xautomatically local to that procedure. Other variables are normally
X\f2global\f1: they are "permanent" and do not disappear when the
Xprocedure in which they get their values finish. It is both possible
Xand desirable to make such variables local, by an explicit instruction
Xto that effect:
X.sp 1
X\f3local\f1 \(em Command, one input.
X.IN
XThe input must be a word. A variable with that word as its name
Xis created, local to the currently running procedure (that is,
Xlocal to the procedure in which the \f3local\f1 command is used).
X.OU
XThe virtue of local variables is that they make procedures more
Xindependent of one another than they would be if global variables
Xwere used. In other words, if you use local variables consistently,
Xthen nothing that happens in one procedure will change the values
Xof variables used in another procedure. This makes it very much
Xeasier to find program errors.
X.PP
X.UB "Case insensitivity."
XNames of procedures (primitive or user-defined) and names of variables
Xmay be typed in upper or lower case. They are converted internally to
Xlower case. That is, the variables \f3foo\f1 and \f3FOO\f1 are the
Xsame variable. Letters in other contexts are not converted to lower
Xcase. For example, \f3equalp\f1 will report that the words \f3foo\f1
Xand \f3FOO\f1 are not equal.
X.PP
XNames of procedures and names of variables may include only letters,
Xdigits, and the special characters period (.) and underscore (_). Also,
Xnames of procedures are limited to 11 characters in some versions of Unix.
X.PP
X.UB "Primitive procedures to define user procedures."
XThere are
Xtwo ways to define your own procedure. The first way, using the \f3to\f1
Xcommand, is simple to learn but limited in flexibility. The second way,
Xusing the \f3edit\f1 command, is more complicated to learn, but makes it
Xeasy to make changes in your procedures. The \f3edit\f1 command uses the
Xtext editing program \f3edt\f1, just as you might use it outside of Logo
Xto edit a document you want to print. Once you've learned the special
Xediting commands in \f3edt\f1, it's easy to use. The \f3to\f1 command makes
Xit possible to begin programming in Logo without having learned how to use
X\f3edt\f1. It just lets you type in your procedure definition, without any
Xprovision for correcting errors or changing the definition of the procedure.
XIt is fast and convenient for short procedures, but limited.
X.sp 1
XThe \f3to\f1 command is unique, in Logo, in that its inputs are interpreted
Xin a special way. The inputs aren't \f2evaluated\f1: Logo doesn't run any
Xprocedures you name, or look up the values of any variables, before carrying
Xout the \f3to\f1 command. The example below should make this clearer.
X.sp 1
X\f3to\f1 \(em Command, special form, see below.
X.IN
XThis command takes a variable number of inputs. The first is the name
Xof a procedure to be defined. The rest, if any, must be preceded by
Xcolons, and are the names of variables to be used as inputs to the
Xprocedure. Logo responds to the \f3to\f1 command by printing a
X"greater than" sign (>) prompt, to show you that you are defining a procedure
Xrather than entering commands to be executed immediately. You type
Xthe instruction lines which make up the definition. When
Xyou are done with the definition, type the special word \f3end\f1 on
Xa line by itself. For example:
X.sp 1
X.nf
X\f3\z_?to twoprint :thing
X\z_>print :thing
X\z_>print :thing
X\z_>end
X\z_?\f1
X.fi
X.sp 1
XThis example shows the definition of a procedure named \f3twoprint\f1,
Xwhich has one input, named \f3thing\f1. The procedure you are defining
Xwith the \f3to\f1 command must not already be defined.
X.OU
X\f3edit\f1 \(em Command, zero or one input. Abbreviation: \f3ed\f1
X.IN
XThe input to this command must be a word, which is the name of a procedure,
Xor a list of words, each of which is the name of a procedure.
X(Unlike the \f3to\f1 command, but like all other Logo procedures, the
X\f3edit\f1 command evaluates its input, so you must use a quotation mark
Xbefore the procedure name, if only one is given, to indicate
Xthat it is the name itself which is
Xthe input to \f3edit\f1; otherwise Logo would actually run the procedure
Xto calculate the input to \f3edit\f1.) The procedure you name may or may
Xnot already be defined. Logo responds to the \f3edit\f1 command by running
Xthe text editor \f3edt\f1, editing the definition of the procedure(s) named in
Xits input. (If a procedure was not previously defined, Logo creates an
Xinitial definition for it which contains only a title line and the end line.)
XYou then edit the definition(s) with \f3edt\f1. When you write the file and
Xleave \f3edt\f1, Logo will use the edited file as the definition(s) of the
Xprocedure(s). You must not put anything in the file except procedure
Xdefinitions; in other words, every nonempty line in the file must be between
Xa "to" line and an "end" line.
X.sp 1
XIf the \f3edit\f1 command is given with no input, \f3edt\f1 is given the same
Xfile as from the last time you used the \f3edit\f1 command. This is a
Xconvenience for editing the same procedure(s) repeatedly.
X.sp 1
XIf, while editing, you change your mind and want to leave \f3edt\f1
Xwithout redefining anything, use the command \f3ESC ^Z\f1 instead of
Xthe normal \f3^Z\f1. This special way of leaving \f3edt\f1 tells Logo
Xnot to redefine your procedures. You have the choice, before exiting
X\f3edt\f1, of writing or not writing the temporary file which contains
Xthe definitions. If you don't write the file, another \f3edit\f1 command
Xwith no input will re-read the previous contents of the temporary file;
Xif you do, another \f3edit\f1 will re-read the new version.
X.sp 1
XIf your Unix environment contains a variable named EDITOR, the contents of
Xthat variable is used as the name of the text editor program instead of
Xthe standard \f3edt\f1. The variable can contain a full pathname, or just
Xa program name if the program can be found in /bin or /usr/bin. Your favorite
Xeditor may not have a facility like \f3edt\f1's ESC ^Z to abort redefinition.
X.OU
X\f3show\f1 \(em Command, one input. Abbreviation: \f3po\f1
X.IN
XThe input to this command is a word or a list of words. Each word must be
Xthe name of a procedure. The command prints out the definition(s) of the
Xprocedure(s) on your terminal. (The abbreviation \f3po\f1 stands for
X\f3printout\f1, which is the name used for this command in some other versions
Xof Logo.)
X.OU
X\f3pots\f1 \(em Command, no inputs.
X.IN
XThis command types at your terminal the title lines of all procedures
Xyou've defined. The name is an abbreviation for "print out titles".
X.OU
X\f3erase\f1 \(em Command, one input. Abbreviation: \f3er\f1
X.IN
XAs for the \f3show\f1 command, the input is either a word, naming one
Xprocedure, or a list of words, naming more than one. The named procedures
Xare erased, so that they are no longer defined.
X.OU
X.ti 5
X.UB "Primitive procedures to manipulate words and lists."
XThere are
Xprimitive procedures to print text objects on the terminal, to read
Xthem from the terminal, to combine them into larger objects, to split
Xthem into smaller objects, and to determine their size and nature:
X.sp 1
X\f3print\f1 \(em Command, one input. Abbreviation: \f3pr\f1
X.IN
XThe input, which may be a word or a list, is printed on the terminal,
Xfollowed by a new line character. (That is, the terminal is positioned
Xat the beginning of a new line after printing the object.) If the
Xobject is a list, any sub-lists are delimited by square brackets, but
Xthe entire object is not delimited by brackets.
X.OU
X\f3type\f1 \(em Command, one input.
X.IN
XThe input, which may be a word or a list, is printed on the terminal,
X\f2without\f1 a new line character. (That is, the terminal remains
Xpositioned at the end of the object after printing it.) Brackets
Xare used as with the \f3print\f1 command.
X.OU
X\f3fprint\f1 \(em Command, one input. Abbreviation: \f3fp\f1
X.IN
XThe input is printed as by the \f3print\f1 command, except that if it
Xis a list (as opposed to a word) it is enclosed in square brackets. The
Xname of the command is short for "full print".
X.OU
X\f3ftype\f1 \(em Command, one input. Abbreviation: \f3fty\f1
X.IN
XThe input is printed as by the \f3type\f1 command, except that if it
Xis a list, it is enclosed in square brackets.
X.OU
X\f3readlist\f1 \(em Operation, no inputs. Abbreviation: \f3rl\f1
X.IN
XLogo waits
Xfor a line to be typed by the user. The contents of the line are made
Xinto a list, as though typed within square brackets as part of a Logo
Xinstruction. (The user should not actually type brackets around the
Xline, unless s/he desires a list of one element, which is a list
Xitself.) That list is the output from the operation.
X.OU
X\f3request\f1 \(em Operation, no inputs.
X.IN
XA question mark is printed on the terminal as a prompt. Then Logo waits
Xfor a line to be typed by the user, as for \f3readlist\f1.
X.OU
X\f3word\f1 \(em Operation, two inputs.
X.IN
XThe two inputs must be words. The output is a word which is the
Xconcatenation of the two inputs. There is no space or other
Xseparation of the two inputs in the output.
X.OU
X\f3sentence\f1 \(em Operation, two inputs. Abbreviation: \f3se\f1
X.IN
XThe two inputs may be words or lists. The output is a list formed
Xfrom the two inputs in this way: if either input is a word, that
Xword becomes a member of the output list; if either input is a
Xlist, the \f2members\f1 of that input become members of the
Xoutput. Here are some examples:
X.EX
X.ta 2i 4i
Xfirst input second input output
X"hello "test [hello test]
X"goodbye [cruel world] [goodbye cruel world]
X[a b] [c d] [a b c d]
X[] "garply [garply]
X.EC
XIf an input is the empty list, as in the last example above, it
Xcontributes nothing to the output.
X.OU
X\f3list\f1 \(em Operation, two inputs.
X.IN
XThe output is a list of two elements, namely, the two inputs. The
Xinputs may be words or lists.
X.OU
X\f3fput\f1 \(em Operation, two inputs.
X.IN
XThe first input may be any Logo object; the second must be a list. The
Xoutput is a list which is identical with the second input except that
Xit has an extra first member, namely, the first input.
X.OU
X\f3lput\f1 \(em Operation, two inputs.
X.IN
XThe first input may be any Logo object; the second must be a list. The
Xoutput is a list which is identical with the second input except that
Xit has an extra last member, namely, the first input.
X.OU
X\f3first\f1 \(em Operation, one input.
X.IN
XThe input may be any non-empty Logo object. If the input is a list,
Xthe output is its first member. If the input is a word, the output is
Xa single-letter word, namely the first letter of the input. If the
Xinput is empty (a word or list of length zero) an error results.
X.OU
X\f3last\f1 \(em Operation, one input.
X.IN
XThe input may be any non-empty Logo object. If the input is a list,
Xthe output is its last member. If the input is a word, the output is
Xa single-letter word, namely the last letter of the input. If the
Xinput is empty (a word or list of length zero) an error results.
X.OU
X\f3butfirst\f1 \(em Operation, one input. Abbreviation: \f3bf\f1
X.IN
XThe input may be any non-empty Logo object. If the input is a list,
Xthe output is a list equal to the input list with the first member
Xremoved. (If the input list has only one member, the output is
Xthe \f2empty list\f1, a list of zero members.) If the input is
Xa word, the output is a word equal to the input word with the
Xfirst letter removed. (If the input is a single-letter word, the
Xoutput is the \f2empty word\f1.) If the input is empty, an
Xerror results.
X.OU
X\f3butlast\f1 \(em Operation, one input. Abbreviation: \f3bl\f1
X.IN
XThe input may be any non-empty Logo object. If the input is a list,
Xthe output is a list equal to the input list with the last member
Xremoved. (If the input list has only one member, the output is
Xthe \f2empty list\f1, a list of zero members.) If the input is
Xa word, the output is a word equal to the input word with the
Xlast letter removed. (If the input is a single-letter word, the
Xoutput is the \f2empty word\f1.) If the input is empty, an
Xerror results.
X.OU
X\f3count\f1 \(em Operation, one input.
X.IN
XThe input may be any Logo object. If the input is a list, the
Xoutput is a number indicating the number of members in the list. (Note:
Xonly top-level members are counted, not members of members. The count
Xof the list
X.EX
X[[This is] [a list]]
X.EC
Xis 2, not 4.) If the input is a word, the output is the number of
Xletters (or other characters) in the word. Remember that in Logo a
Xnumber is just a particular kind of word, so the output from \f3count\f1 can
Xbe manipulated like any other Logo word.
X.OU
X\f3emptyp\f1 \(em Operation (predicate), one input.
X.IN
XThe input can be any Logo object. The output is the word \f3true\f1 if
Xthe input is of length zero (i.e., it is the empty word or the empty
Xlist). The output is the word \f3false\f1 otherwise.
X.OU
X\f3wordp\f1 \(em Operation (predicate), one input.
X.IN
XThe input can be any Logo object. The output is the word \f3true\f1 if
Xthe input is a word. The output is the word \f3false\f1 if the input
Xis a list.
X.OU
X\f3sentencep\f1 \(em Operation (predicate), one input.
X.IN
XThe input can be any Logo object. The output is the word \f3true\f1 if
Xthe input is a sentence, i.e., a list of words. The output is the word
X\f3false\f1 if the input is a word, or if any member of the input is a
Xlist.
X.OU
X\f3is\f1 \(em Operation (predicate), two inputs.
X.IN
XThe inputs can be any Logo objects. The output is the word \f3true\f1 if
Xthe two inputs are identical. That is, they must be of the same type
X(both words or both lists), they must have the same count, and their
Xmembers (if lists) or their characters (if words) must be identical. The
Xoutput is the word \f3false\f1 otherwise. (Note: this is an exception
Xto the convention that names of predicates end with the letter "p".)
X.OU
X\f3memberp\f1 \(em Operation (predicate), two inputs.
X.IN
XIf the second input is a word, the first input must be a word of
Xlength one (a single character), and the output is \f3true\f1 if and
Xonly if the first input is contained in the second as a character. If
Xthe second input is a list, the first input can be any Logo object,
Xand the output is \f3true\f1 if and only if the first input is a member
Xof the second input. (Note that this is member, not subset.)
X.OU
X\f3item\f1 \(em Operation, two inputs. Abbreviation: \f3nth\f1
X.IN
XThe first input must be a positive integer less than or equal to
Xthe \f3count\f1 of the second input. If the second input is a word,
Xthe output is a word of length one containing the selected character
Xfrom the word. (Items are numbered from 1, not 0.) If the second input
Xis a list, the output is the selected member of the list.
X.OU
X.ti 5
X.UB "Primitive procedures for turtles and graphics."
XAn important
Xpart of the Logo environment is a rich set of applications to which the
Xcomputer can be directed. The most important of these is \f2turtle
Xgeometry\f1, a way of describing paths of motion in a plane which is
Xwell-suited to computer programming. There are two ways to use the
Xturtle procedures. First, you can control a \f2floor turtle\f1, a small
Xrobot which can move one the floor or on a table under computer
Xcontrol. Second, you can use a \f2display turtle\f1 to draw pictures
Xon the TV screen of a graphics terminal.
X.PP
XEach computer center has a different variety of graphics hardware
Xavailable. Floor turtles are very different from display turtles, but also
Xeach kind of display terminal has different characteristics. For example,
Xsome terminals can draw in several colors; others can't. The following
Xdescriptions of graphics primitives explain the "best" case for each one
Xand mention restrictions on some graphics devices.
X.PP
XThe floor turtle can draw pictures on paper, because it has a pen
Xattached to its "belly": the underside of the turtle. Since it is
Xa mechanical device, however, it is not very precise; the pictures
Xyou get may not be exactly like what your program specifies. A more
Xinteresting way to use the floor turtle is to take advantage of its
X\f2touch sensors\f1. Switches under the turtle's dome allow the computer
Xto know when the turtle bumps into an obstacle. You can use this
Xinformation to write programs to get around obstacles or to follow
Xa maze.
X.PP
XThe display turtle lives on the surface of a TV screen. It can draw
Xpictures more precisely than the floor turtle, since it does not
Xmeasure distances and angles mechanically. It is also faster than
Xthe floor turtle. When using the display turtle, remember that
Xit interprets commands relative to its own position and direction,
Xjust as the floor turtle does. The command \f3left\f1, for example,
Xturns the turtle to its own left, which may or may not be toward
Xthe left side of the TV screen.
X.sp 1
X\f3turtle\f1 \(em Command, one input. Abbreviation: \f3tur\f1
X.IN
XThe input is the name of a turtle. You can only control one turtle
Xat a time, so using this command a second time releases the turtle
Xpreviously selected. The names of floor turtles are numbers like
X\f30\f1 and \f31\f1. If you are using
Xa graphics display terminal (not just a text display trminal), you can
Xcontrol the display turtle by using the word \f3display\f1 (or the
Xabbreviation \f3dpy\f1) as the turtle name. (As usual, the word
Xmust be preceded by a quotation mark.) If you use a graphics primitive
Xwithout selecting a turtle, Logo assumes that you want to use the
Xdisplay turtle. But once you select a floor turtle, you must say
X\f3turtle "display\f1 explicitly to switch to the display.
X.sp 1
XThe word \f3off\f1 as input to the \f3turtle\f1 command releases a floor
Xturtle, if you have one, or turns off the graphics display if you have
Xbeen using the display turtle. This also happens when you leave Logo.
X.OU
X\f3forward\f1 \(em Command, one input. Abbreviation: \f3fd\f1
X.IN
XThe input is a number, the distance you would like the turtle
Xto move. For a floor turtle, the unit of distance is however
Xfar the turtle can travel in 1/30 second. For a display turtle,
Xthe unit is one dot on the TV screen. (Note: on some displays, one
Xdot horizontally may not be the same length as one dot vertically. The
X\f3setscrunch\f1 command allows you to control the
Xrelative sizes so that squares come out
Xsquare.) The turtle moves in whatever direction it is pointing
Xwhen you use the command.
X.OU
X\f3back\f1 \(em Command, one input. Abbreviation: \f3bk\f1
X.IN
XThe input is a number, a distance to move, as in the \f3forward\f1
Xcommand. The difference is that the turtle moves backward, i.e.,
Xin the direction exactly opposite to the way it's pointing.
X.OU
X\f3left\f1 \(em Command, one input. Abbreviation: \f3lt\f1
X.IN
XThe input is a number, the number of degrees of angle through
Xwhich the turtle should turn counterclockwise. This command
Xdoes not change the \f2position\f1 of the turtle, but merely
Xits \f2heading\f1 (the direction in which it points). The
Xturn will be only approximately correct for the floor turtle,
Xbecause of mechanical errors. For the display turtle, the
Xangle will be perfectly reproducible, although it may not look
Xquite right on the screen because of the difference in size
Xbetween horizontal and vertical dots. Nevertheless, a display
Xturtle program will work in the sense that when the turtle is
Xsupposed to return to its starting point, it will do so.
X.OU
X\f3right\f1 \(em Command, one input. Abbreviation: \f3rt\f1
X.IN
XThe input is a number; the turtle turns through the specified
Xnumber of degrees clockwise.
X.OU
X\f3penup\f1 \(em Command, no inputs. Abbreviation: \f3pu\f1
X.IN
XThis command tells the turtle to raise its pen from the paper,
Xso that it does not leave a trace when it moves. In the case
Xof the display turtle, there is no physical pen to move
Xmechanically, but the effect is the same: any \f3forward\f1 or
X\f3back\f1 commands after this point do not draw a line. The
Xfloor turtle starts with its pen up; the display turtle starts
Xwith its pen down. Note: the floor turtle will not move on
Xthe carpet correctly with its pen down; put it on a smooth
Xsurface if you want to draw pictures.
X.OU
END_OF_logoman.1
if test 35585 -ne `wc -c <logoman.1`; then
echo shar: \"logoman.1\" unpacked with wrong size!
fi
# end of overwriting check
fi
echo shar: End of archive 5 \(of 6\).
cp /dev/null ark5isdone
MISSING=""
for I in 1 2 3 4 5 6 ; do
if test ! -f ark${I}isdone ; then
MISSING="${MISSING} ${I}"
fi
done
if test "${MISSING}" = "" ; then
echo You have unpacked all 6 archives.
echo "Now see the README"
rm -f ark[1-9]isdone
else
echo You still need to unpack the following archives:
echo " " ${MISSING}
fi
## End of shell archive.
exit 0