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diff --git a/js/games/nluqo.github.io/~bh/v2ch1/v2ch1.html b/js/games/nluqo.github.io/~bh/v2ch1/v2ch1.html new file mode 100644 index 0000000..c0e72da --- /dev/null +++ b/js/games/nluqo.github.io/~bh/v2ch1/v2ch1.html @@ -0,0 +1,777 @@ + +<P><HTML> +<HEAD> +<TITLE>Computer Science Logo Style vol 2 ch 1: Data Files</TITLE> +</HEAD> +<BODY> +<CITE>Computer Science Logo Style</CITE> volume 2: +<CITE>Advanced Techniques</CITE> 2/e Copyright (C) 1997 MIT +<H1>Data Files</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../csls2.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/v2ch01.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../v2-toc2.html">Back to Table of Contents</A> +<TR><TD align="right"><A HREF="../v2ch0/ack.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v2ch2/v2ch2.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="https://mitpress.mit.edu/books/computer-science-logo-style-second-edition-volume-2">MIT +Press web page for <CITE>Computer Science Logo Style</CITE></A> +</TABLE></TABLE> + +<HR><P>Program file for this chapter: <A HREF="format.lg"><CODE>format</CODE></A> + + +<P>The programming techniques that you learned in the first volume of this +series are all you need to express any computation. That is, given any +question that a computer program can answer, you can write the program in +Logo using those techniques. Also, those techniques can be used, with few +if any changes in notation, in any implementation of Logo. However, saying +that a problem can be solved using certain tools doesn't mean that it can be +solved in the most convenient way. In this volume the overall goal is to +expand your repertoire of Logo techniques, so that you'll find it easier to +deal with more difficult problems. Some of the techniques here are unique +to Berkeley Logo; others exist in other dialects, but in significantly +different forms. + +<P>Probably the most glaring omission in the first volume is that we made no +provision for saving information from one session to the next. (You do know +how to save a Logo workspace, but that's too all-or-nothing to be very +useful. You'd like to be able to save specific kinds of information, and +perhaps use that information in some program outside of Logo.) In this +chapter we'll explore the use of <EM>data files</EM> in Logo +programs. + +<P>There isn't much in the way of truly new ideas here. There are a few new +primitives and a few grubby details about how files are named in your +particular computer, but for the most part you won't have to change the way +you think about the programming process. My plan for this chapter is to +give a quick summary of the vocabulary you'll need, and spend most of the +chapter on a practical programming project that will show you the sort of +thing you can accomplish easily in Logo. + +<P><H2>Reader and Writer</H2> + +<P>We've been reading and writing data all along. We've been reading +from the keyboard, with operations like <CODE>readlist</CODE> and <CODE> +readchar</CODE>, and we've been writing to your screen, with commands +like <CODE>print</CODE> and <CODE>type</CODE>. + +<P>The goal now is to read and write the same data, but from and to other +devices. This includes files on a hard disk or a diskette, +but also things like printers or TV cameras if you have them. The +same procedures that read the keyboard and write the screen can be used for +these other devices as well. The trick is to divert the attention of those +procedures to someplace else. + +<P>The part of the Logo interpreter that reads characters for <CODE>readlist</CODE> +and <CODE>readchar</CODE> is called the <EM>reader;</EM> the part that handles +<CODE>print</CODE> and its friends is the <EM>writer.</EM> The commands +<CODE>setread</CODE> and <CODE>setwrite</CODE> tell the reader and the writer, +respectively, what file or device to use. The input to either command is +the name of a file or device. The format of that name will vary from one +operating system to another, so you should look it up in your computer's +reference manual. Generally it will be the same format that you (I assume) +have already been using as input to the <CODE>save</CODE> and <CODE>load</CODE> commands. + +<P>If you invoke <CODE>setread</CODE> with the empty list as input, it tells the +reader to read from the keyboard. If you give <CODE>setwrite</CODE> the +empty list as input, it tells the writer to write to the screen. In +other words the empty list "turns off" whatever file or device you +may have been using and returns to Logo's usual style of +interaction. + +<P>You can switch the attention of the reader or the writer among several files +in rotation without "losing your place" in each one. You must <EM>open</EM> a +file when you want to begin reading or writing it before you can use it as +input to <CODE>setread</CODE> or <CODE>setwrite</CODE>. You do this with the +<CODE>openread</CODE> or <CODE>openwrite</CODE> command.<SUP>*</SUP> Once a file is opened, you can <CODE>setread</CODE> or <CODE> +setwrite</CODE> to it, read or write some data, then switch to a different file +for a while, and then continue where you left off. When you're finished +using the file, you must <CODE>close</CODE> it. + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP><CODE>Openwrite</CODE> creates +a new, empty file, replacing any file that might previously have existed +with the same name. Berkeley Logo also provides <CODE>openupdate</CODE>, which +opens an existing file for both reading and writing simultaneously, and <CODE> +openappend</CODE>, which opens an existing file for writing, putting the newly +written data after the old contents of the file. I won't use those in this +book, though.</SMALL></BLOCKQUOTE></SMALL><P>Some operating systems allow access to devices like printers using the same +programming interface that works for files. In those systems, +you can <CODE>setwrite</CODE> to a printer just as you can to a disk file. The +format of the input to <CODE>setwrite</CODE> may be different (a device name +instead of a file name), but there is no conceptual difference. + +<P> + +<H2>End of File</H2> + +<P>When reading information from a file, the problem arises of what +happens when there is no more left to read. How does a program +know it's reached the end of the file? + +<P>Berkeley Logo provides two ways to answer this question. If the structure +of your program makes it convenient to test for the end of the file <EM> +before</EM> attempting to read more information from the file, you can use the +predicate <CODE>eofp</CODE>, which takes no inputs, and returns <CODE>true</CODE> if the +file currently being read is at its end. (If Logo is reading from the +keyboard, then <CODE>eofp</CODE> always returns <CODE>false</CODE>.) + +<P>In some cases it may be more convenient to try to read from the file, and +then later test whether there was really any information available to read. +To make this possible, the reading operations output an empty datum +when there is nothing left to read, but of the opposite type from +their usual output. In other words <CODE>readlist</CODE>, which usually +outputs a list, outputs an empty <EM>word</EM> to indicate the end of a +file. <CODE>Readchar</CODE>, which normally outputs a word, outputs an empty +<EM>list</EM> when there are no more characters to be read. You can +use <CODE>wordp</CODE> or <CODE>listp</CODE>, therefore, to check for the end of the +file. + +<P>Here's an example. <CODE>Extract</CODE> is a command that takes two inputs, a +word and a filename. Its effect is to print every line in that file +that contains the chosen word. For example, you might have a file in +which each line contains someone's name and telephone number; you +could use this procedure to find a particular person in the file. + +<P><PRE>to extract :word :file +openread :file +setread :file +extract1 :word +setread [] +close :file +end + +to extract1 :word +local "line +if eofp [stop] +make "line readlist +if memberp :word :line [print :line] +extract1 :word +end + +? <U>extract "brian "phonelist</U> +Brian Harvey 555-2368 +Brian Silverman 555-5274 +</PRE> + +<P>Notice that the program restores reading from the keyboard +when it's done reading the file. In the example I'm assuming that +<CODE>phonelist</CODE> is the name of a file you've created earlier, +with a Logo program or with your favorite text editor outside +of Logo. + +<P><H2>Case Sensitivity</H2> + +<P>In this example, I used the word <CODE>brian</CODE>, in all lower case +letters, as the input to <CODE>extract</CODE>, whereas the data file contained +the word <CODE>Brian</CODE> with an initial upper case or +capital letter. You can control whether or not Logo considers +those two words equal by changing the value of the variable +<CODE>caseignoredp</CODE>. If this variable has the value <CODE>true</CODE>, as it does +by default, then <CODE>equalp</CODE> and <CODE>memberp</CODE> consider upper and lower +case letters the same. But if you say + +<P><PRE>make "caseignoredp "false +</PRE> + +<P>then upper and lower case letters will not be equal. (This +variable does <EM>not</EM> affect Logo's understanding of the names of +procedures and variables, in which case is always ignored. The words +<CODE>print</CODE> and <CODE>PRINT</CODE> always name the same procedure, for example.) + +<P><H2>Dribble Files</H2> + +<P>Not everything Logo prints goes through the writer. Error messages and +trace output always go to the screen, not into a file. The idea is that +even when you're using files, you're still programming interactively, and +those messages are part of the programming process rather than +part of the result of your program. + +<P>Sometimes, though, you want to capture in a file <EM>everything</EM> +that happens while you're using Logo. Some programming teachers, for +instance, like to look over their students' shoulders but can't look +at everyone at once. If you record everything you do, your teacher +can print out the record, take it home, and study it overnight. The +formal name for this kind of record is a <EM>transcript file,</EM> but +it's more popularly known as a <EM>dribble file.</EM> (The metaphor is +that there's a leak in the pipe between the computer and the screen +and some of the data dribbles out into the file.) + +<P>The <CODE>dribble</CODE> command takes a file name as input and starts +dribbling into that file. The <CODE>nodribble</CODE> command, with no input, +turns off dribbling. Information is sent to the dribble file <EM> +in addition to</EM> being printed on your screen, or written in a file by +the writer. Compare this with the effect of <CODE>setwrite</CODE>, which +tells Logo to print into a file <EM>instead of</EM> onto the screen. + +<P>If you want to keep a transcript of a programming session, remember +that much of your interaction with Logo happens in the Logo editor +and that that kind of interaction can't be recorded in a dribble +file. So you might want to make it a habit to <CODE>po</CODE> the procedures +you've edited, each time you leave the editor. + +<P><H2>A Text Formatter</H2> + +<P>Okay, it's time for the practical project I promised you. Probably +the most useful "real" program you can find for a home computer is a +word processor. There are +two parts to a word processing package: a text editor and a +formatter. The editor is the part of the system that lets you type in +your document, correct errors, and make additions and deletions +later. The formatter is the part that takes what you type and turns +it into beautiful printed pages with even margins and so on. +(In most word processors, these two parts are integrated, so that every +character you type makes an immediate change in the beautifully formatted +document. But in principle the two tasks are separable.) + +<P>I'm going to write a text formatter. I assume that you have some way +to put text into a file. (In some versions of Logo the same +editor that you use for procedures can also edit text files. +Otherwise you probably have a separate program that edits files, or +else you can write one in Logo!) The formatter will read lines from a +file, fill and justify paragraphs, and print the result. (To <EM> +fill</EM> text means to fit as many words as possible into each printed +line. To <EM>justify</EM> the text is to insert extra spaces between +words so that both margins line up.) You can see how the +formatter will work by examining the example on the following pages. +I've shown both what's in the file and what my program prints. + +<HR> + +<P>Formatter input file: + +<P><PRE><SMALL> +When I wrote the first edition of this book in 1984, I said that the study of +computer programming was intellectually rewarding for young children in +elementary school, and for computer science majors in college, but that high +school students and adults studying on their own generally had an +intellectually barren diet, full of technical details of some particular +computer brand. + +At about the same time I wrote those words, the College Board was introducing +an Advanced Placement exam in computer science. Since then, the AP course has +become popular, and similar official or semi-official computer science +curricula have been adopted in other countries as well. Meanwhile, the +computers available to ordinary people have become large enough and powerful +enough to run serious programming languages, breaking the monopoly of BASIC. +* nofill +I think that there shall never exist +a poem as lovely as a tree-structured list. +* yesfill +So, the good news is that intellectually serious computer science is within +the reach of just about everyone. The bad news is that the curricula tend to +be imitations of what is taught to beginning undergraduate computer science +majors, and I think that's too rigid a starting point for independent +learners, and especially for teenagers. + +See, the wonderful thing about computer programming is that it's fun, perhaps +not for everyone, but for very many people. There aren't many mathematical +activities that appeal so spontaneously. Kids get caught up in the +excitement of programming, in the same way that other kids (or maybe the +same ones) get caught up in acting, in sports, in journalism (provided the +paper isn't run by teachers), or in ham radio. If schools get too serious +about computer science, that spontaneous excitement can be lost. I once +heard a high school teacher say proudly that kids used to hang out in his +computer lab at all hours, but since they introduced the computer science +curriculum, the kids don't want to program so much because they've learned +that programming is just a means to the end of understanding the +curriculum. No! The ideas of computer science are a means to the end of +getting computers to do what you want. +*skip 4 +*make "nofilltab 15 +*nofill +Computer +Science +Apprenticeship +*yesfill +*make "spacing 2 +My goal in this series of books is to make the goals and methods of a serious +computer scientist accessible, at an introductory level, to people who are +interested in computer programming but are not computer science majors. If +you're an adult or teenaged hobbyist, or a teacher who wants to use the +computer as an educational tool, you're definitely part of this audience. +I've taught these ideas to teachers and to high school students. What I enjoy +most is teaching high school freshmen who bring a love of programming into the +class with them--the ones who are always tugging at my arm to tell me what they +found in the latest Byte. +</SMALL></PRE><P> + +<HR> + +<P>Formatted output: + +<PRE><SMALL><SMALL><SMALL><SMALL> + + + + + + + When I wrote the first edition of this book in 1984, I said + that the study of computer programming was intellectually + rewarding for young children in elementary school, and for + computer science majors in college, but that high school students + and adults studying on their own generally had an intellectually + barren diet, full of technical details of some particular + computer brand. + + At about the same time I wrote those words, the College + Board was introducing an Advanced Placement exam in computer + science. Since then, the AP course has become popular, and + similar official or semi-official computer science curricula have + been adopted in other countries as well. Meanwhile, the computers + available to ordinary people have become large enough and + powerful enough to run serious programming languages, breaking + the monopoly of BASIC. + + I think that there shall never exist + a poem as lovely as a tree-structured list. + + So, the good news is that intellectually serious computer + science is within the reach of just about everyone. The bad news + is that the curricula tend to be imitations of what is taught to + beginning undergraduate computer science majors, and I think + that's too rigid a starting point for independent learners, and + especially for teenagers. + + See, the wonderful thing about computer programming is that + it's fun, perhaps not for everyone, but for very many people. + There aren't many mathematical activities that appeal so + spontaneously. Kids get caught up in the excitement of + programming, in the same way that other kids (or maybe the same + ones) get caught up in acting, in sports, in journalism (provided + the paper isn't run by teachers), or in ham radio. If schools get + too serious about computer science, that spontaneous excitement + can be lost. I once heard a high school teacher say proudly that + kids used to hang out in his computer lab at all hours, but since + they introduced the computer science curriculum, the kids don't + want to program so much because they've learned that programming + is just a means to the end of understanding the curriculum. No! + The ideas of computer science are a means to the end of getting + computers to do what you want. + + + + + + Computer + Science + Apprenticeship + + My goal in this series of books is to make the goals and + + methods of a serious computer scientist accessible, at an + + + + + + +<CENTER><HR width="50%"></CENTER> + + + + + + introductory level, to people who are interested in computer + + programming but are not computer science majors. If you're an + + adult or teenaged hobbyist, or a teacher who wants to use the + + computer as an educational tool, you're definitely part of this + + audience. I've taught these ideas to teachers and to high school + + students. What I enjoy most is teaching high school freshmen who + + bring a love of programming into the class with them--the ones + + who are always tugging at my arm to tell me what they found in + + the latest Byte. + +</SMALL></SMALL></SMALL></SMALL></PRE> +<HR> + +<P>For the most part the formatter just copies words from one file to +another, filling and justifying as it goes. A blank line in the file +indicates a break between paragraphs. The program skips a line +between paragraphs and indents the first line of the new paragraph. +It's possible to control the formatter's work by including <EM> +formatting commands</EM> in the file. These are the lines that start +with asterisks in the example. For example, the line that says + +<P><PRE>* nofill +</PRE> + +<P>means, "From now on, stop filling paragraphs. Instead, +each line in the input file should be one line in the printed result." The +<CODE>yesfill</CODE> command returns to normal paragraph style.<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>I'd +have liked to call the command <CODE>fill</CODE>, as it would be in a +commercial word processing program, but unfortunately that's the name +of a primitive procedure in Logo.</SMALL></BLOCKQUOTE></SMALL><P>To run the program, invoke the <CODE>format</CODE> command. This command +takes two inputs: the name of a file to read and the name of a file +to write. The latter might be the name of the printer if your operating +system allows it. + +<P><H2>Page Geometry</H2> + +<P>The program uses several global variables to determine the layout of a +printed page. Vertical measurements are in vertical lines (6 per inch for +most computer printers); horizontal measurements are in characters (10 per +inch is common, although there is more variation in this unit). The program +assumes fixed-width characters; a more professional program would handle +variable-width character fonts, but the added complexity wouldn't help you +learn the things I'm most interested in now. + +<P><CENTER><IMG SRC="formatter.jpg" ALT="figure: formatter"></CENTER> + +<P><P> + +<TABLE> +<TR><TH align="left">pageheight<TD> Height of the entire sheet of paper, including margins. +<TR><TH align="left">topmar<TD> Number of lines of margin at the top of each page. +<TR><TH align="left">lines<TD> Number of lines to be printed on each page. +<TR><TH align="left">parskip<TD> Number of blank lines between paragraphs. +<TR><TH align="left">spacing<TD> 1 for single spaced printing, 2 for double spaced, etc. +<TR><TH align="left">leftmar<TD> Number of characters of margin at the left of the page. +<TR><TH align="left">width<TD> Number of characters to print on each line. +<TR><TH align="left">filltab<TD> Number of characters to indent the first line of a paragraph. +<TR><TH align="left">nofilltab<TD> Number of characters to indent each nofill line. +</TABLE> + +<P>The formatter recognizes formatting commands, in the file it's +reading, to change the values of these variables. By a strange +coincidence these formatting commands look similar to the Logo +commands to set a variable. In the sample file, for instance, the +formatting command + +<P><PRE>*make "spacing 2 +</PRE> + +<P>is used to start double spacing. + +<P><H2>The Program</H2> + +<P>Here are the procedures that make up the formatter. + +<P><PRE> +to format :from :to +openread :from +openwrite :to +setread :from +setwrite :to +init.vars +loop +setread [] +setwrite [] +close :from +close :to +end + +to init.vars +make "pageheight 66 +make "topmar 6 +make "lines 54 +make "leftmar 7 +make "width 65 +make "filltab 5 +make "nofilltab 0 +make "parskip 1 +make "spacing 1 +make "started "false +make "filling "true +make "printed 0 +make "inline [] +end + +to <A NAME="loop">loop +forever [if process nextword [stop]] +end + +;; Add a word to the output file, starting a new line if necessary + +to <A NAME="process">process</A> :word +if listp :word [output "true] +if not :started [start] +if (:linecount+1+count :word) > :width [putline] +addword :word +output "false +end + +to <A NAME="addword">addword :word</A> +if not emptyp :line [make "linecount :linecount+1] +make "line lput :word :line +make "linecount :linecount+count :word +end + +to <A NAME="putline">putline</A> +repeat :leftmar+:indent [type "| |] +putwords :line ((count :line)-1) (:width-:linecount) +newline +skip :spacing +end + +to <A NAME="putwords">putwords</A> :line :spaces :filler +local "perword +if emptyp :line [stop] +type first :line +make "perword ifelse :spaces > 0 [int ((:filler+:spaces-1)/:spaces)] [0] +if :filler > 0 [repeat :perword [type "| |]] +type "| | +putwords (butfirst :line) (:spaces-1) (:filler-:perword) +end + +;; Get the next input word, reading a new line if necessary + +to <A NAME="nextword">nextword</A> +if not emptyp :inline [output extract.word] +if not :filling [break] +make "inline readword +if listp :inline [break output []] +if emptyp :inline [break output nextword] +if equalp first :inline "|*| ~ + [run butfirst :inline + make "inline "] +make "inline skipspaces :inline +output nextword +end + +to <A NAME="extract.word">extract.word</A> +local "result +make "result firstword :inline +make "inline skipfirst :inline +output :result +end + +to firstword :word +if emptyp :word [output "] +if equalp first :word "| | [output "] +output word (first :word) (firstword butfirst :word) +end + +to skipfirst :word +if emptyp :word [output "] +if equalp first :word "| | [output skipspaces :word] +output skipfirst butfirst :word +end + +to skipspaces :word +if emptyp :word [output "] +if equalp first :word "| | [output skipspaces butfirst :word] +output :word +end + +;; Formatting helpers + +to start +make "started "true +repeat :topmar [print []] +newindent +end + +to newindent +newline +make "indent ifelse :filling [:filltab] [:nofilltab] +make "linecount :indent +end + +to newline +make "line [] +make "indent 0 +make "linecount 0 +end + +to <A NAME="break">break</A> +if emptyp :line [stop] +make "linecount :width +putline +newindent +if :filling [skip :parskip] +end + +;; Formatting commands to be invoked by the user + +to <A NAME="skip">skip</A> :howmany +break +repeat :howmany [print []] +make "printed :printed+:howmany +if :printed < :lines [stop] +repeat :pageheight-:printed [print []] +make "printed 0 +end + +to nofill +break +make "filling "false +newindent +end + +to yesfill +break +if not :filling [skip :parskip] +make "filling "true +newindent +end +</PRE> + +<P>To help you understand this program, you should start by imagining that the +text file contains one big paragraph with no formatting commands. For each +word in the file, <A HREF="v2ch1.html#loop"><CODE>loop</CODE></A> +invokes <A HREF="v2ch1.html#nextword"><CODE>nextword</CODE></A> to read the word and +process to process it. Just take <CODE>nextword</CODE> on faith for now +and look at <A HREF="v2ch1.html#process"><CODE>process</CODE></A>. +The third and fourth instruction lines are the +interesting ones. The third line asks whether adding this word to the +partially filled print line will overflow its width. If so, <CODE>process</CODE> +invokes <A HREF="v2ch1.html#putline"><CODE>putline</CODE></A> to print +that line and start a new one. Then, in +either case, <CODE>process</CODE> invokes <A HREF="v2ch1.html#addword"><CODE>addword</CODE></A> +to add the word to the print +line it's accumulating. <CODE>Addword</CODE> puts the word at the end of the line +and also adds its length to <CODE>:linecount</CODE>, the number of characters in +the line. If this isn't the first word of a new line, then it must also add +another character to <CODE>:linecount</CODE> to take account of the space between +words. + +<P><A HREF="v2ch1.html#putline"><CODE>Putline</CODE></A> is +essentially just a fancy <CODE>print</CODE> command. The +complication comes in because the program is trying to justify the line by +adding spaces where needed between words. To do this, it has to <CODE>type</CODE> +the line a word at a time; that's the task of +<A HREF="v2ch1.html#putwords"><CODE>putwords</CODE></A>. In that +procedure, <CODE>:spaces</CODE> is the number of spaces between words not yet +printed; in other words it's the number of positions into which extra spaces +can be shoved. (The idea is to spread out the necessary spaces as evenly as +possible.) <CODE>:Filler</CODE> is the total number of extra spaces we need to +insert; <CODE>:perword</CODE> is the number that should be inserted after the +particular word we're typing right now. (When I started writing <CODE> +putline</CODE> and <CODE>putwords</CODE>, I thought that I could just calculate +<CODE>:perword</CODE> once for each line. But if the number of extra spaces we want +to insert is not a multiple of the number of positions available, then the +number of extra spaces may not be equal for every word in the line.) + +<P>That's pretty much the whole story about the printing part of the +program. The reading part is handled by +<A HREF="v2ch1.html#nextword"><CODE>nextword</CODE></A>. It reads a +line at a time into the variable +<CODE>inline</CODE>. <CODE>Nextword</CODE> uses the Logo +primitive <CODE>readword</CODE> to read a line, rather than the usual +<CODE>readlist</CODE>, to avoid Logo's usual special handling of parentheses and +brackets. <CODE>Readword</CODE> outputs a word containing all of the characters on +the line that it reads, even if the line includes spaces, which would +ordinarily separate words. Therefore, the program must divide the +long word output by <CODE>readword</CODE> into ordinary words; that's the job of +<A HREF="v2ch1.html#extract.word"><CODE>extract.word</CODE></A> +and its subprocedures <CODE>firstword</CODE>, +<CODE>skipword</CODE>, and <CODE>skipspaces</CODE>. + +<P>Each time <A HREF="v2ch1.html#nextword"><CODE>nextword</CODE></A> +is invoked, it removes one word from the line and +outputs that word. When <CODE>:inline</CODE> is empty, <CODE>nextword</CODE> reads a new +line from the file. There are four possibilities: First, the end of the +file may be reached. <CODE>Listp</CODE> tests for this; if so, <CODE>nextword</CODE> +outputs an empty list. Second, the new line can be empty, indicating a +paragraph break. In this case <CODE>nextword</CODE> invokes +<A HREF="v2ch1.html#break"><CODE>break</CODE></A> and reads +another line. Third, the new line can be a formatting command, starting +with an asterisk. In this case <CODE>nextword</CODE> just <CODE>run</CODE>s the line, +minus the asterisk, and reads another line. Fourth, the line can be an +ordinary text line, in which case <CODE>nextword</CODE> goes back to extracting +words from the line. + +<P>In most programming languages, most of the effort in writing a +formatter like this would be in recognizing and evaluating the +formatting commands. I hope you appreciate how much Logo's ability to +<CODE>run</CODE> instructions found in a file simplifies this task! The danger +in this technique is that an invalid instruction in the input file will +crash the formatting program, giving a Logo error message. (This is +especially bad because after the error message we are left with a +half-written output file still open.) I'd like to "catch" errors +while running the user's instructions; you'll see how to do that in +Chapter 3. + +<P>The rest of the program is just a bunch of detail. The +<A HREF="v2ch1.html#skip"><CODE>skip</CODE></A> +command is written to be used both by the formatting program itself +and as a formatting command, as in the example I showed earlier. As +an exercise in understanding program structure, notice that <CODE>skip</CODE> +invokes <A HREF="v2ch1.html#break"><CODE>break</CODE></A> +and <CODE>break</CODE> invokes <CODE>skip</CODE>; then explain +why they don't just keep invoking each other forever, like a recursive +procedure without a stop rule. + +<P>Another slightly tricky part to understand is the variable +<CODE>started</CODE> and the procedure <CODE>start</CODE>. +<CODE>Start</CODE> is invoked by +<CODE>process</CODE>, but only once, before processing the very first word of +the text. Ensuring the "only once" is the sole purpose of +<CODE>started</CODE>, a variable that initially contains <CODE>false</CODE> and is +changed to <CODE>true</CODE> by <CODE>start</CODE>. Instead, why don't I just +invoke <CODE>start</CODE> from <CODE>format</CODE> before calling <CODE>loop</CODE>? The +answer is that this technique allows the file to start with an +instruction like + +<P><PRE>*make "topmar 10 +</PRE> + +<P>Any such instructions will be evaluated <EM>before</EM> +processing the first text word. If <CODE>start</CODE> were invoked by +<CODE>format</CODE>, the top margin would be skipped before this instruction had a +chance to set <CODE>:topmar</CODE>. + +<P><H2>Improving the Formatter</H2> + +<P>Actually, using <CODE>make</CODE> as a formatting command is a little +schlock--not what I'd call good "human engineering." If you wanted +to make a million dollars selling this program, you'd add several +little procedures like this: + +<P><PRE>to topmar :lines +make "topmar :lines +end +</PRE> + +<P>Like <CODE>nofill</CODE> and <CODE>yesfill</CODE>, these procedures would be +used only as formatting commands, not as part of the formatter itself. + +<P>The program leaves out a lot of things you'd like to be able to do. +You should be able to number pages automatically in the top or bottom +margins. (That's a pretty easy modification; most of the work would +be in <CODE>skip</CODE>.) You'd like to be able to center lines on the page +for chapter headings. If your printer can underline or use different +type faces, you'll want a way to control those things with formatting +commands.<SUP>*</SUP> + +<P><SMALL><BLOCKQUOTE><SMALL><SUP>*</SUP>If you're <EM>really</EM> ambitious, you could try +teaching the program about footnotes!</SMALL></BLOCKQUOTE></SMALL> + +<P>Still, this is a usable program carrying out a real task. It takes +19 Logo procedures averaging 7 lines each. This would be a +much harder project in most languages. What makes it so manageable in +Logo? First, <EM>modularity.</EM> A small procedure for each task makes +the overall program easier to understand than it would be if it were +all in one piece. Second, Logo's data types, words and lists, are +well suited to this problem. Third, Logo's control mechanisms, especially +recursive operations and <CODE>run</CODE>, have the needed flexibility. + +<P> + +<P><A HREF="../v2-toc2.html">(back to Table of Contents)</A> +<P><A HREF="../v2ch0/ack.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../v2ch2/v2ch2.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> |