Table of Contents
写在前面
羊陀书太厚，于是买了这本作为入门。说不清楚自己为什么要学习 Perl，版上 用的人似乎也不多，我知道的就只有超人。或许最主要的原因还是 Lighty，它 的配置文件是用这个的，当初配置它的时候看了下然后把事情搞定，于是就很牛 掰地把 Perl 写到自己会的语言清单里面去，现在想想这样的简历拿出来未免心 虚，于是决定学一下。照老规矩贴课后习题的作业。
Chapter 1 简介
Exercise 1 Hello world.
#!/usr/bin/perl -w print "hello, world!\n";
Exercise 3
The code is printed on the book already. So all I have to do is type it in and understand it. That is not that hard to me because I have been learning a little PHP and regular expressions for a while. The code just simply store the every line of the output of that shell command into array @lines, then use the foreach loop to work through it and change every word between '<' and '>' to upper case using regular expression.
As you may noticed, the print has no parameters. In perl, sometimes when the parameters are omitted, the magic scalar $_ will be used as the default parameter. So it would be @lines in this example.
#!/usr/bin/perl -w @lines = `perldoc -u -f atan2`; foreach (@lines) { s/\w<([^>]+)>/\U$1/g; print; }
Chapter 2 标量数据
Exercise 3
Previous exercises are the basis of this one, so getting this one posted only would be appropriate enough. This exercise works on I/O and operators.
#!/usr/bin/perl -w chomp($radius = <STDIN>); if ($radius <= 0 ) { print "0\n"; } else { $circumference = 2 * 3.141592654 * $radius; print $circumference . "\n"; }
Exercise 4 A not very robust one. :-/
#!/usr/bin/perl -w chomp($operand_1 = <STDIN>); chomp($operand_2 = <STDIN>); print $operand_1 * $operand_2 . "\n";
Exercise 5
Until the exercise problem prompts me use the 'x' operator, I was thinking some very ancient and traditional 'for' loop to solve this problem. So here are two solutions for this problem. The easier one is replace the '*' operator by 'x' in the solution of exercise four. My initial way is this one below.
#!/usr/bin/perl -w chomp($operand_1 = <STDIN>); chomp($operand_2 = <STDIN>); #print $operand_1 x $operand_2 . "\n"; while ($operand_2 > 0) { print $operand_1; $operand_2 -= 1; } print "\n";
Chapter 3 列表和数组
Exercise 1
A context problem was encountered when I was doing this chapter's homeworks, which is a good thing for learning Perl but an boresome thing to get the job done, that the 'print' just ignore the fact that I was intend to use a list context. I think the '.' operator should be the most suspected.
#!/usr/bin/perl -w chomp(@lines = <STDIN>); @lines = reverse @lines; print "@lines\n";
Exercise 2 mind qw.
#!/usr/bin/perl -w @names = qw/ fred betty barney dino wilma pebbles bamm-bamm /; chomp(@lines = <STDIN>); #@lines = reverse @lines; foreach $choice (@lines) { print $names[$choice** 1] . "\n"; }
Exercise 3
As I've mentioned in exercise one of this chapter, to avoid the issue
that print @lines . "\n"; would take the @lines as a scalar which was
not what I intended to, I use foreach.
#!/usr/bin/perl -w chomp(@lines = <STDIN>); @lines = sort @lines; print "@lines\n"; foreach $sorted (@lines) { print $sorted . "\n"; }
Chapter 4 子例程
Exercise 1
The main part of the code below is already written. My code, subroutine 'total', is derived from the better version of 'max' from this book.
#!/usr/bin/perl -w my @fred = qw/ 1 3 5 7 9/; my $fred_total = &total(@fred); print "The total of \@fred is $fred_total.\n"; print "Enter some numbers on seperate lines: "; my $user_total = &total(<STDIN>); print "The total of those numbers is $user_total.\n"; sub total { my ($total) = shift @_; foreach (@_) { $total += $_; } $total; }
Exercise 2
Well, this one is easy if we do it the book's way. Just create an array variable.
my @array = 1..1000; # Just add this one.
Exercise 3
Again the code of previous subroutine can be taken advantage.
#!/usr/bin/perl -w my @fred = &above_average(1..10); print "\@fred is @fred\n"; print "(Should be 6 7 8 9 10)\n"; my @barney = &above_average(100, 1..10); print "\@barney is @barney\n"; print "(Should be jsut 100)\n"; sub above_average { my @above; my $average = &average(@_); foreach (@_) { if ($_ > $average) { push (@above, $_); } } @above; } sub average { my ($total) = shift @_; foreach (@_) { $total += $_; } $total / ($#_ + 2); # We've shifted @_; }
Chapter 5 输入与输出
Exercise 1
As a matter of fact, the while loop could be written more briefly by
using Perl syntax sugar and its most loved variable $_. There is one
more thing which I think should be aware of that the difference
between using bare list and using list surrounded by quotation
marks. Perl will simply get every scalar of that list printed out if
the used in first way.
#!/usr/bin/perl -w #tac while (defined($line = <>)) { push(@tac, $line); } print reverse @tac;
Exercise 2
#!/usr/bin/perl -w while (defined($lines = <STDIN>)) { chomp($lines); push(@input, $lines); } print ((1..9, 0) x 7); print "\n"; foreach (@input) { printf "%20s\n", $_; }
Exercise 3
Whether "%$indents\n" will work or not has not been testified. Guess
not. Anyway, this separation of format parameter from function printf
does a lot of help to me. It is used in a wide range.
#!/usr/bin/perl -w while (defined($lines = <STDIN>)) { chomp($lines); push(@input, $lines); } $indent = shift @input; print ((1..9, 0) x ($indent / 10)); print "\n"; $format = "%" . $indent . "s\n"; foreach (@input) { printf $format, $_; }
Chapter 6 散列
Exercise 1
Just as what you see, I use the biref version of 'while' instead at
this time. I didn't try omit the $_ of push function, so I have no
idea about whether that would work out or not.
The special separator => is just the same as comma to
Perl.
#!/usr/bin/perl -w %family = ( "fred" => "flintstone", "barney" => "rubble", "wilma" => "flintstone", ); while (<STDIN>) { chomp; push(@first, $_); } $format = "%-15s" x 2 . "\n"; printf $format, "First Name", "Family Name"; foreach (@first) { if ($family{$_}) { printf $format, $_, $family{$_}; } else { printf $format, $_, "Unknown"; } }
Exercise 2
The if-else branch is not essential to accomplish the work proposed by
this exercise. Perl would transform undef to 0 if it encounters a data
operating context. If the key has not been inserted to the hash yet,
represents the key is still zero, the relevant value to this specific
key which is undef will be used as 0 to plus 1. That is, This line of
code will work.
#!/usr/bin/perl -w while (<STDIN>) { chomp; if (!exists $count{$_}) { $count{$_} = 1; } else { $count{$_} += 1; } # Actually the if-else branch is not essential, # just do this will work. # $count{$_} += 1; } $format = "%-10s%5d\n"; while (($key, $value) = each %count) { printf $format, $key, $value; } # sort names first. foreach $key (sort keys %count) { printf $format, $key, $count{$key}; }
Chapter 7 正则表达式
Exercise 1 & Exercise 2
Those examples provided in this book didn't introduce the
=~ operator because you are not required to specifically
using this operator to match the Perl's most loved variable to your
pattern.
#!/usr/bin/perl -w unless (open FILE, "<", "fred.txt") { die "Cannot open input file: $!"; } while (<FILE>) { # ($_ =~ /(f|F)red/) { if (/(f|F)red/) { print; } } close FILE;
Exercise 3 Change pattern to /\./.
Exercise 4 /[A-Z][a-z]+/
Exercise 5 /(wilma.*fred)|(fred.*wilma)/
Chapter 8 以正则表达式进行匹配
#!/usr/bin/perl -w # A script to examine pattern while (<>) { chomp; if (/YOUR_PATTERN_GOES_HERE/) { print "Matched: |$`<$&>$'|\n"; } else { print "No matched: |$_|\n"; } }
Exercise 1 /match/
Exercise 2 /\w+a\b/
The anchor \b which indicates boundary of a word should not be omitted. If 'a' is accepted, please change \w+ to \w*.
Exercise 3 & Exercise 4
#!/usr/bin/perl -w while (<>) { chomp; if (/(\w+a)(\b.{0,4})/) { print "Matched: '$1' '$2'\n"; } else { print "No matched: |$_|\n"; } }
Exercise 5
#!/usr/bin/perl -w while (<>) { chomp; if (/\s+$/) { print "$`|$&|\n"; } else { print "Sentence is not ended with white spaces.\n"; } }
Chapter 9 以正则表达式处理文本
Exercise 1
The source speaks. Wrap variables by parentheses to introduce scalar in pattern.
#!/usr/bin/perl -w chomp($what = <STDIN>); while(<>) { chomp; if (/($what){3}/) { print "Matched!\n"; } }
Exercise 2
$^I and the diamond command. Like Perl geeks always said, "there is
always more than one way." Anyway, the use of 'g' and 'i' parameters is
preferred.
#!/usr/bin/perl -w use strict; $^I = ".out"; while (<>) { s/(F|f)red/Larry/; print; }
Exercise 3
At my first thought, there is no need of use some anchor like \n or null(\0). That was wrong.
#!/usr/bin/perl -w use strict; $^I = ".out"; while (<>) { chomp; s/Fred/\n/gi; s/Wilma/Fred/gi; s/\n/Wilma/g; print; }
Exercise 4
This works. I can't figure out what's the difference between my solution and the key provided by this book. Please refer to the book or check exercise 5 to see what the literal difference is. Maybe actually they are the same thing. lol
#!/usr/bin/perl -w use strict; $^I = ".out"; while (<>) { s/(^#!.*)$/$1\n## Copyright (c) 2007 by Jake/; print; }
Exercise 5
Rewriting @ARGV is smart.
#!/usr/bin/perl -w use strict; my %done; foreach (@ARGV) { $done{$_} = 1; } while (<>) { if (/^## Copyright/) { delete $done{$ARGV}; # remove } } print %done; @ARGV = sort keys %done; # reset @ARGV $^I = ".out"; while (<>) { # s/(^#!.*)$/$1\n## Copyright (c) 2007 by Jake/; if (/^#!/) { $_ .= "## Copyright (c) 2007 by Jake\n"; } print; }
The 'official' answer
The $^I variable is not used in this solution. It replaces the file
extension by 'out' instead of adding something.
#!/usr/bin/perl -w my $in = $ARGV[0]; if (not defined $in) { die "Usage: $0 filename"; } my $out = $in; $out =~ s/(\.\w+)?$/.out/; # replace file extension with .out unless (open IN, "<$in") { die "Can't open '$in': $!"; } unless (open OUT, ">$out") { die "Can't write '$out': $!"; } while (<IN>) { s/Larry/Fred/gi; print OUT $_; }
Chapter 10 其他控制结构
Exercise 1
We can remove the use of $quit by using `last'.
#!/usr/bin/perl -w use strict; my $key = int(1 + rand 100); my $quit = 0; while (!$quit) { chomp(my $guess = <STDIN>); if ($guess =~ m/quit|exit/i) { print "Bye. The number is: $key\n"; $quit = 1; } elsif ($guess < $key) { print "Too low: $guess\n"; } elsif ($guess > $key) { print "Too high: $guess\n"; } else { # Hit the key print "get it!\n"; $quit = 1; } }
Chapter 11 文件测试
Exercise 1
Copied from appendix one...
#!/usr/bin/perl -w use strict; die "There is no file specified!\n" if @ARGV == 0; foreach my $file (@ARGV) { my $attributes = &get_attributes($file); print "$file $attributes.\n"; } sub get_attributes { my $file = shift @_; return 'does not exist.' unless -e $file; my @attr; push @attr, 'readable' if -r $file; push @attr, 'writable' if -w $file; push @attr, 'executable' if -x $file; return 'exits' unless @attr; 'is ' . join ' and ', @attr; }
Exercise 2
High-water-mark. (ACL, Access Control List) You may notice that this code is just the same as the code from appendix. Well, I write this after reading that.
#!/usr/bin/perl -w use strict; die "No arguments!" if @ARGV == 0; my $oldest_file = shift @ARGV; my $oldest_age = -M $oldest_file; foreach my $file (@ARGV) { my $age = -M $file; ($oldest_file, $oldest_age) = ($file, $age) if $age > $oldest_age; } printf "The oldest file is %s.\nIt is %.1f days old.\n", $oldest_file, $oldest_age;
chapter 12 目录操作
Exercise 1
The title asked whether globbing is better or directory handle is more efficient. But my first thought is directory handle because there is some example source could be used already. Anyway, the answer shows globbing is simpler. So the branches of if-else are written in different ways.
#!/usr/bin/perl -w use strict; my $target; if (@ARGV == 0) { print "No target, going home.\n"; chdir or die "Cannot switch directory: $!\n"; my @files = <*>; # glob foreach (@files) { print "$_\n"; } } else { # directory handle $target = shift @ARGV; print "Changing directory...\n"; opendir DIR, $target or die "Cannot switch directory: $!\n"; while (my $file = readdir DIR) { print "$file\n"; } }
Exercise 2 & 3
To get those special dot files, like Larry always said, there is more
than one way. If you intend to use glob, replace <*> with <.* *>. If
you want to do that with directory handle, the original code will just
do the job. To omit those files (they might be annoying), add this:
next if $name =~ /^\./;
Exercise 4
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-10 use strict; die "No arguments!\n" if @ARGV == 0; foreach (@ARGV) { unlink $_ or warn "Cannot remove '$_'! $!\n"; }
Exercise 5
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-10 use strict; use File::Basename qw/ basename/; use File::Spec::Functions qw/ catfile /; die "Usage: $0 <source file> <target file>\n" unless @ARGV == 2; my ($source, $target) = @ARGV; $target = catfile($target, basename $source) if (-d $target); # $target =~ /\/$/; Cross platform... if (-e $target) { warn "$target exits!\n"; } elsif (rename $source, $target) { } else { warn "$0 failed: $!\n"; }
Exercise 6
Like the question says itself, the difference between this question and the question above this one is so tiny that just do some string replacement will finish the job well.
Exercise 7
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; use File::Basename qw/ basename/; use File::Spec::Functions qw/ catfile /; die "Usage: $0 <-s> <source file> <target file>\n" unless @ARGV >= 2 && @ARGV <= 3; my $symlink = $ARGV[0] eq '-s'; shift @ARGV if $symlink; my ($source, $target) = @ARGV; $target = catfile($target, basename $source) if (-d $target); # $target =~ /\/$/; Cross platform... if (-e $target) { warn "$target exits!\n"; } elsif ($symlink) { symlink $source, $target or warn "Cannot make soft link: $!"; } else { link $source, $target or warn "Cannot make soft link: $!"; }
Exercise 8
Both using readlink first then check whether the variable is defined or not and using -l operator to check its existence then readlink are okay.
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; foreach (<*>) { printf "%s -> %s\n", $_, readlink $_ if -l $_; }
Chapter 13 字符串与排序
Exercise 1
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; my @numbers; open IN, "<numbers.txt"; push @numbers, split while (<IN>); foreach (sort { $a <=> $b } @numbers) { printf "%20g\n", $_; }
Exercise 2
The cmp operator and The space craft operator.
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; my %names = qw/ fred flintstone Wilma Flintstone Barney Rubble betty rubble Bamm-Bamm Rubble PEBBLES FLINTSTONE /; my @result = sort by_name keys %names; sub by_name { "\L$names{$a}" cmp "\L$names{$b}" or "\L$a" cmp "\L$b"; } foreach (@result) { printf "%15s %15s\n", $_, $names{$_}; }
Exercise 3
Getting used to using last is important to me right now because I'm thinking procedures in traditional C style.
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; chomp(my $string = <STDIN>); chomp(my $char = <STDIN>); my $position = 0; while (($position = index($string, $char, $position)) > 0) { print $position++ . " "; } print "\n";
Chapter 14 进程管理
Exercise 1
#!/usr/bin/perl -w use strict; system "ls -l \$HOME"; # $HOME is not a vriable in perl, but in shell. # This example shows how to use parent's environment variables. # It's the special hash %ENV. print "$ENV{'PATH'}\n";
Exercise 2
Maybe this will work. I have been reading some book about bash programming for a period. This question to me is just some file handle redirecting. Haven't figure out whether there are some Perl ways yet.
ls -l 1> ls.out 2> ls.err
Redirecting file handle in Perl way.
open STDOUT, ">ls.out" or die "Can't write to ls.out: $!\n"; open STDERR, ">ls.err" or die "Can't write to ls.err: $!\n";
Exercise 3
At the beginning, I thought this task as some sort of string split work. But I've got a really bad habit which makes me go check the key before doing some real work. So when the answer points out to me that only saturday and sunday which consists of our weekly leisure time are begin with letter `S'. So, here is the code.
#!/usr/bin/perl -w ## Copyright (c) Jake Chen 2007-11-11 use strict; if (`date` =~ /^(Sat|Sun)/) { print "Go play.\n"; } else { print "Work...\n"; }
Chapter 15 Perl 模块
Exercise 1
The book recommands me refer to `the big camel' to acquire more knowledge about hash. It seems it's just the right thing to do.
#!/usr/bin/perl -w use strict; use Module::CoreList; my %modules = %{ $Module::CoreList::version{5.006} }; print join "\n", keys %modules; print "\n";
Exercise 2
After Perl version 5.6, module File::Spec provides procedural
functions rather than its original object oriented style.
#!/usr/bin/perl -w use strict; use Cwd; use File::Spec::Functions; my $directory = cwd; my @files = glob "*"; foreach my $file (@files) { print catfile($directory, $file) . "\n"; }
Exercise 3
File::Basename has some examples in the bok already, so I'll just
write some specific things about modules in Perl. To import specific
functions only, add import list when you import them.
use File::Basename qw/ basename /
If you leave the list to a single space or some spaces (\s+), Perl would import no functions. Type the whole function `path' to use it.
