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<html><head><title>Answers to Chapter 11 Exercises (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="appa_09.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="appa_11.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">A.10. Answers to Chapter 11 Exercises</h2><ol><li><p>Here's one way to do it:</p><blockquote><pre class="code">sub get_line { # prompts for, reads, chomps, and returns a line of input print $_[0]; chomp(my $line = <STDIN>); $line;}my $source = &get_line("Which source file? ");open IN, $source or die "Can't open '$source' for input: $!";my $dest = &get_line("What destination file? ");die "Won't overwrite existing file" if -e $dest; # optional safety testopen OUT, ">$dest" or die "Can't open '$dest' for output: $!";my $pattern = &get_line("What search pattern: ");my $replace = &get_line("What replacement string: ");while (<IN>) { s/$pattern/$replace/g; print OUT $_;}</pre></blockquote><p>This one needs to ask the user for several things, so we decided tomake a subroutine to take care of some of the work. The subroutineprints out the prompt, which is the first (and only) parameter to thesubroutine. Then it reads a line of input, chomps it, and returns it.That makes it easy to ask for each parameter, one after the other.</p><p>Once we know what the user wants for the source file, we try openingit. An earlier version of this program asked for all of theparameters first, but if the source file name is incorrect,there's no point in having the user type more parameters. Thisway can save the user some time and trouble. Note that the diemessage reports the file name inside quote marks; this can be helpfulin diagnosing a problem when a string has whitespace characters. Ifyou opened <tt class="literal">"<$source"</tt> instead of just plain<tt class="literal">$source</tt>, that's fine, too. (There's noreason to worry that the user of this program will do somethingnefarious, since anything they can do with this program, they couldaccomplish just as well without it. If this program were made to runover the web, to give one example, we'd need to be<em class="emphasis">much</em> more cautious about opening theuser's choice of file.)</p><p>As we hope you discovered when you tried it, it's easy tooverwrite an existing file simply by opening it for output. So we putin a safety test using the <tt class="literal">-e</tt> file test. Thecorresponding die message doesn't include <tt class="literal">$!</tt>because we're not reporting a failed request of the system. Bythe way, this test for overwrite is fine here, but it would beinsufficient in an environment where many copies of the same program(or different programs all working with the same files) might berunning at once. This typically happens with programs on the web: Twoprocesses check the same filename for existence at approximately thesame time, and both see that it doesn't exist. So one of themcreates the file, and an instant later the other one overwrites itwith a file of its own. This kind of concurrency problem can'tbe solved with the <tt class="literal">-e</tt> file test; some kind of filelocking (which is beyond the scope of this book) is needed.</p><p>With that safety test, the user won't accidentally overwrite anexisting file. Is that test a good idea? Well, if the user comes tosee you next week and says, "Golly, I'm glad you put inthat safety test. It kept me from accidentally overwriting myfile!", then you know that it was the right thing to do. But ifthe user says, "Dagnabit, your program is hard to use! I toldit what filename I wanted to use for output, and it wouldn'tlet me use it until I first deleted that file!", then it wasthe wrong thing to do. Making decisions like this is often thetoughest part of a programmer's job. Perhaps we should make theprogram ask something like, "Are you sure you want to overwritethe existing file `barney'?" by default, but have acommand-line option for the power user that says to overwrite withoutasking. Next version.</p><p>Once we've asked for everything and opened the files, the restof the program is pretty simple. The heart of the program is the loopat the end, which reads lines, updates them, and prints them out.Note that the substitution uses the <tt class="literal">/g</tt>option -- if you left that out, your program is broken, since theexercise asked that the program replace <em class="emphasis">every</em>occurrence of the search pattern, not just the first one on eachline.</p><p>Were you able to use regular expression metacharacters in the searchpattern? Sure; the substitution interpolates<tt class="literal">$pattern</tt> to make the search pattern. Were you ableto use memory variables and backslash escapes in the replacementstring? Nope; <tt class="literal">$replace</tt> is interpolated to make thereplacement string, but it's not<em class="emphasis">re-</em>interpolated to interpret any magicalcharacters. If <tt class="literal">$replace</tt> holds<tt class="literal">$1</tt>, that's a dollar sign and a numeral onein the replacement string. If Perl always kept re-interpolating, youcould never put a dollar sign or backslash into the replacementstring, since they'd always make something magical happen.(Actually, though, if you need one additional level of interpolation,it is possible. See the <tt class="literal">perlfaq</tt> manpages for somesuggestions on how to do this.)</p></li><li><p>Here's one way to do it:</p><blockquote><pre class="code">foreach my $file (@ARGV) { my $attribs = &attributes($file); print "'$file' $attribs.\n";}sub attributes { # report the attributes of a given file my $file = shift @_; return "does not exist" unless -e $file; my @attrib; push @attrib, "readable" if -r $file; push @attrib, "writable" if -w $file; push @attrib, "executable" if -x $file; return "exists" unless @attrib; 'is ' . join " and ", @attrib; # return value}</pre></blockquote><p>In this one, once again it's convenient to use a subroutine.The main loop prints one line of attributes for each file, perhapstelling us that <tt class="literal">'cereal-killer' is executable</tt> orthat <tt class="literal">'sasquatch' does not exist</tt>.</p><p>The subroutine tells us the attributes of the given filename. Ofcourse, if the file doesn't even exist, there's no needfor the other tests, so we test for that first. If there's nofile, we'll return early.</p><p>If the file does exist, we'll build a list of attributes. (Giveyourself extra credit points if you used the special<tt class="literal">_</tt> filehandle instead of <tt class="literal">$file</tt>on these tests, to keep from calling the system separately for eachnew attribute.) It would be easy to add additional tests like thethree we show here. But what happens if none of the attributes istrue? Well, if we can't say anything else, at least we can saythat the file exists, so we do. The <tt class="literal">unless</tt> clauseuses the fact that <tt class="literal">@attrib</tt> will be true (in aBoolean context, which is a special case of a scalar context) ifit's got any elements.</p><p>But if we've got some attributes, we'll join them with<tt class="literal">" and "</tt> and put <tt class="literal">"is "</tt> in front,to make a description like <tt class="literal">is readable andwritable</tt>. This isn't perfect however; if there arethree attributes, it says that the file <tt class="literal">is readable andwritable and executable</tt>, which has too many<tt class="literal">and</tt>s, but we can get away with it. If you wantedto add more attributes to the ones this program checks for, youshould probably fix it to say something like <tt class="literal">is readable,writable, executable, and nonempty</tt>. If that matters to you.</p><p>Note that if you somehow didn't put any filenames on thecommand line, this produces no output. This makes sense; if you askfor information on zero files, you should get zero lines of output.But let's compare that to what the next program does in asimilar case, in the discussion below.</p></li><li><p>Here's one way to do it:</p><blockquote><pre class="code">die "No file names supplied!\n" unless @ARGV;my $oldest_name = shift @ARGV;my $oldest_age = -M $oldest_name;foreach (@ARGV) { my $age = -M; ($oldest_name, $oldest_age) = ($_, $age) if $age > $oldest_age;}printf "The oldest file was %s, and it was %.1f days old.\n", $oldest_name, $oldest_age;</pre></blockquote><p>This one starts right out by complaining if it didn't get anyfilenames on the command line. That's because it'ssupposed to tell us the oldest filename -- and there ain'tone if there aren't any files to check.</p><p>Once again, we're using the "high-water-mark"algorithm. The first file is certainly the oldest one seen so far. Wehave to keep track of its age as well, so that's in<tt class="literal">$oldest_age</tt>.</p><p>For each of the remaining files, we'll determine the age withthe <tt class="literal">-M</tt> file test, just as we did for the first one(except that here, we'll use the default argument of<tt class="literal">$_</tt> for the file test). The last-modified time isgenerally what people mean by the "age" of a file,although you could make a case for using a different one. If the ageis more than <tt class="literal">$oldest_age</tt>, we'll use a listassignment to update both the name and age. We didn't have touse a list assignment, but it's a convenient way to updateseveral variables at once.</p><p>We stored the age from <tt class="literal">-M</tt> into the temporaryvariable <tt class="literal">$age</tt>. What would have happened if we hadsimply used <tt class="literal">-M</tt> each time, rather than using avariable? Well, first, unless we used the special<tt class="literal">_</tt> filehandle, we would have been asking theoperating system for the age of the file each time, a potentiallyslow operation (not that you'd notice unless you have hundredsor thousands of files, and maybe not even then). More importantly,though, we should consider what would happen if someone updated afile while we're checking it. That is, first we see the age ofsome file, and it's the oldest one seen so far. But before wecan get back to use <tt class="literal">-M</tt> a second time, someonemodifies the file and resets the timestamp to the current time. Nowthe age that we save into <tt class="literal">$oldest_age</tt> is actuallythe <em class="emphasis">youngest</em> age possible. The result would bethat we'd get the oldest file among the files tested from thatpoint on, rather than the oldest overall; this would be a toughproblem to debug!</p><p>Finally, at the end of the program, we use<tt class="literal">printf</tt> to print out the name and age, with theage rounded off to the nearest tenth of a day. Give yourself extracredit if you went to the trouble to convert the age to a number ofdays, hours, and minutes.</p></li></ol><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="appa_09.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="appa_11.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">A.9. Answer to Chapter 10 Exercise</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">A.11. Answers to Chapter 12 Exercises</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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