book

Perl by Example

by Ellie Quigley

December 2014

Beginner

888 pages

25h 23m

English

Addison-Wesley Professional

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Acknowledgments
1.1 What Is Perl?1.2 What Is an Interpreted Language?1.3 Who Uses Perl?1.3.1 Which Perl?1.3.2 What Are Perl 6, Rakudo Perl, and Parrot?1.4 Where to Get Perl1.4.1 CPAN (cpan.org)1.4.2 Downloads and Other Resources for Perl (perl.org)1.4.3 ActivePerl (activestate.com)1.4.4 What Version Do I Have?1.5 Perl Documentation1.5.1 Where to Find the Most Complete Documentation from Perl1.5.2 Perl man Pages1.5.3 Online Documentation1.6 What You Should Know1.7 What’s Next?
2.1 Quick Start, Quick Reference2.1.1 A Note to Programmers2.1.2 A Note to Non-Programmers2.1.3 Perl Syntax and ConstructsRegular ExpressionsPassing Arguments at the Command LineReferences and PointersObjectsLibraries and ModulesDiagnostics2.2 Chapter Summary2.3 What’s Next?
3.1 Getting Started3.1.1 Finding a Text Editor3.1.2 Naming Perl Scripts3.1.3 Statements, Whitespace, and Linebreaks3.1.4 Strings and Numbers3.2 Filehandles3.3 Variables (Where to Put Data)3.3.1 What Is Context?3.3.2 Comments3.3.3 Perl Statements3.3.4 Using Perl Built-in Functions3.3.5 Script Execution3.4 Summing It Up3.4.1 What Kinds of Errors to Expect3.5 Perl Switches3.5.1 The -e Switch (Quick Test at the Command Line)3.5.2 The -c Switch (Check Syntax)3.5.3 The -w Switch (Warnings)3.6 What You Should Know3.7 What’s Next?Exercise 3: Getting with It Syntactically

4.1 The Special Filehandles STDOUT, STDIN, STDERR4.2 Words4.3 The print Function4.3.1 Quotes Matter!Double QuotesSingle QuotesBackquotesPerl’s Alternative Quotes4.3.2 Literals (Numeric, String, and Special)Numeric LiteralsString LiteralsSpecial Literals4.3.3 Printing Without Quotes—The here documenthere documents and CGI4.4 Fancy Formatting with the printf Function4.4.1 Saving Formatting with the sprintf Function4.4.2 The No Newline say Function4.5 What Are Pragmas?4.5.1 The feature Pragma4.5.2 The warnings Pragma4.5.3 The diagnostics Pragma4.5.4 The strict Pragma and Words4.6 What You Should Know4.7 What’s Next?Exercise 4: A String of Perls
5.1 More About Data Types5.1.1 Basic Data Types (Scalar, Array, Hash)5.1.2 Package, Scope, Privacy, and StrictnessPackage and Scope5.1.3 Naming Conventions5.1.4 Assignment Statements5.2 Scalars, Arrays, and Hashes5.2.1 Scalar VariablesAssignmentThe defined FunctionThe undef FunctionThe $_ Scalar Variable5.2.2 ArraysAssignmentOutput and Input Special Variables ($, and $“)Array SizeThe Range Operator and Array AssignmentAccessing ElementsLooping Through an Array with the foreach LoopArray Copy and SlicesMultidimensional Arrays—Lists of Lists5.2.3 Hashes—Unordered ListsAssignmentAccessing Hash ValuesHash SlicesRemoving Duplicates from a List Using a Hash5.2.4 Complex Data Structures5.3 Array Functions5.3.1 Adding Elements to an ArrayThe push FunctionThe unshift Function5.3.2 Removing and Replacing ElementsThe delete FunctionThe splice FunctionThe pop FunctionThe shift Function5.3.3 Deleting NewlinesThe chop and chomp Functions (with Lists)5.3.4 Searching for Elements and Index ValuesThe grep Function5.3.5 Creating a List from a ScalarThe split Function5.3.6 Creating a Scalar from a ListThe join Function5.3.7 Transforming an ArrayThe map Function5.3.8 Sorting an ArrayThe sort Function5.3.9 Checking the Existence of an Array Index ValueThe exists Function5.3.10 Reversing an ArrayThe reverse Function5.4 Hash (Associative Array) Functions5.4.1 The keys Function5.4.2 The values Function5.4.3 The each Function5.4.4 Removing Duplicates from a List with a Hash5.4.5 Sorting a Hash by Keys and ValuesSort Hash by Keys in Ascending OrderSort Hash by Keys in Reverse OrderSort Hash by Keys NumericallyNumerically Sort a Hash by Values in Ascending OrderNumerically Sort a Hash by Values in Descending Order5.4.6 The delete Function5.4.7 The exists Function5.4.8 Special HashesThe %ENV HashThe %SIG HashThe %INC Hash5.4.9 Context Revisited5.5 What You Should Know5.6 What’s Next?Exercise 5: The Funny Characters
6.1 About Perl Operators—More Context6.1.1 Evaluating an Expression6.2 Mixing Types6.3 Precedence and Associativity6.3.1 Assignment Operators6.3.2 Boolean6.3.3 Relational OperatorsNumericString6.3.4 Conditional Operators6.3.5 Equality OperatorsNumericString6.3.6 The Smartmatch Operator6.3.7 Logical Operators (Short-Circuit Operators)6.3.8 Logical Word Operators6.3.9 Arithmetic Operators and FunctionsArithmetic OperatorsArithmetic Functions6.3.10 Autoincrement and Autodecrement Operators6.3.11 Bitwise Logical OperatorsA Little Bit About BitsBitwise Operators6.3.12 Range Operator6.3.13 Special String Operators and Functions6.4 What You Should Know6.5 What’s Next?Exercise 6: Operator, Operator
7.1 Control Structures, Blocks, and Compound Statements7.1.1 Decision Making—Conditional Constructsif and unless StatementsThe if ConstructThe if/else ConstructThe if/elsif/else ConstructThe unless Construct7.2 Statement Modifiers and Simple Statements7.2.1 The if Modifier7.2.2 The unless Modifier7.3 Repetition with Loops7.3.1 The while Loop7.3.2 The until Loop7.3.3 The do/while and do/until Loops7.3.4 The for Loop (The Three-Part Loop)7.3.5 The foreach (for) Loop7.4 Looping Modifiers7.4.1 The while Modifier7.4.2 The foreach Modifier7.4.3 Loop ControlLabelsThe redo and goto StatementsNested Loops and LabelsThe continue Statement7.4.4 The switch Statement (given/when)The switch Feature (given/when/say)7.5 What You Should Know7.6 What’s Next?Exercise 7: What Are Your Conditions?
8.1 What Is a Regular Expression?8.1.1 Why Do We Need Regular Expressions?8.2 Modifiers and Simple Statements with Regular Expressions8.2.1 Pattern Binding Operators8.2.2 The DATA Filehandle8.3 Regular Expression Operators8.3.1 The m Operator and Pattern MatchingThe g Modifier—Global MatchThe i Modifier—Case InsensitivitySpecial Scalars for Saving PatternsThe x Modifier—The Expressive Modifier8.3.2 The s Operator and Substitution8.3.3 The Pattern Binding Operators with SubstitutionChanging the Substitution DelimitersSubstitution ModifiersUsing the Special $& Variable in a SubstitutionPattern Matching with a Real File8.4 What You Should Know8.5 What’s Next?Exercise 8: A Match Made in Heaven
9.1 The RegExLib.com Library9.2 Regular Expression Metacharacters9.2.1 Metacharacters for Single CharactersThe Dot MetacharacterThe s Modifier—The Dot Metacharacter and the NewlineThe Character ClassThe POSIX Bracket Expressions9.2.2 Whitespace Metacharacters9.2.3 Metacharacters to Repeat Pattern MatchesThe Greed FactorMetacharacters That Turn off GreedinessAnchoring MetacharactersThe m ModifierAlternationGrouping or ClusteringRemembering or CapturingTurning off GreedTurning off CapturingMetacharacters That Look Ahead and Behind9.2.4 The tr or y OperatorsThe d Delete OptionThe c Complement OptionThe s Squeeze Option9.3 Unicode9.3.1 Perl and Unicode9.4 What You Should Know9.5 What’s Next?Exercise 9: And the Search Goes On . . .
10.1 The User-Defined Filehandle10.1.1 Opening Files—The open Function10.1.2 Opening for ReadingClosing the FilehandleThe die Function10.1.3 Reading from a File and Scalar AssignmentThe Filehandle and $_The Filehandle and a User-Defined Scalar Variable“Slurping” a File into an ArrayUsing map to Create Fields from a FileSlurping a File into a String with the read Function10.1.4 Loading a Hash from a File10.2 Reading from STDIN10.2.1 Assigning Input to a Scalar Variable10.2.2 The chop and chomp Functions10.2.3 The read Function10.2.4 The getc Function10.2.5 Assigning Input to an Array10.2.6 Assigning Input to a Hash10.2.7 Opening for Writing10.2.8 Win32 Binary Files10.2.9 Opening for Appending10.2.10 The select Function10.2.11 File Locking with flock10.2.12 The seek and tell FunctionsThe seek FunctionThe tell Function10.2.13 Opening for Reading and Writing10.2.14 Opening for Anonymous PipesThe Output FilterSending the Output of a Filter to a FileInput Filter10.3 Passing Arguments10.3.1 The @ARGV Array10.3.2 ARGV and the Null Filehandle10.3.3 The eof Function10.3.4 The -i Switch—Editing Files in Place10.4 File Testing10.5 What You Should Know10.6 What’s Next?Exercise 10: Getting a Handle on Things
11.1 Subroutines/Functions11.1.1 Defining and Calling a SubroutineForward DeclarationScope of Variables11.2 Passing Arguments and the @_ Array11.2.1 Call-by-Reference and the @_ Array11.2.2 Assigning Values from @_Passing a Hash to a Subroutine11.2.3 Returning a Value11.2.4 Scoping Operators: local, my, our, and stateThe local OperatorThe my Operator11.2.5 Using the strict Pragma (my and our)The state Feature11.2.6 Putting It All Together11.2.7 Prototypes11.2.8 Context and SubroutinesThe wantarray Function and User-Defined Subroutines11.2.9 Autoloading11.2.10 BEGIN and END Blocks (Startup and Finish)11.2.11 The subs Function11.3 What You Should Know11.4 What’s Next?Exercise 11: I Can’t Seem to Function Without Subroutines
12.1 What Is a Reference?12.1.1 Hard ReferencesThe Backslash OperatorDereferencing the Pointer12.1.2 References and Anonymous VariablesAnonymous ArraysAnonymous Hashes12.1.3 Nested Data StructuresUsing Data::DumperArray of ListsArray of HashesHash of Hashes12.1.4 More Nested Structures12.1.5 References and SubroutinesAnonymous SubroutinesSubroutines and Passing by Reference12.1.6 The ref Function12.1.7 Symbolic ReferencesThe strict Pragma12.1.8 Typeglobs (Aliases)Filehandle References and Typeglobs12.2 What You Should Know12.3 What’s Next?Exercise 12: It’s Not Polite to Point!
13.1 Before Getting Started13.1.1 An Analogy13.1.2 What Is a Package?Referencing Package Variables and Subroutines from Another Package13.1.3 What Is a Module?13.1.4 The Symbol Table13.2 The Standard Perl Library13.2.1 The @INC ArraySetting the PERL5LIB Environment VariableThe lib Pragma13.2.2 Packages and .pm FilesThe require FunctionThe use Function (Modules and Pragmas)Using Perl to Include Your Own Library13.2.3 Exporting and ImportingThe Exporter.pm Module13.2.4 Finding Modules and Documentation from the Standard Perl LibraryViewing the Contents of the Carp.pm Module13.2.5 How to “Use” a Module from the Standard Perl Library13.2.6 Using Perl to Create Your Own ModuleCreating an Import Method Without Exporter13.3 Modules from CPAN13.3.1 The CPAN.pm ModuleRetrieving a Module from CPAN with the cpan Shell13.3.2 Using Perl Program Manager13.4 Using Perlbrew and CPAN Minus13.5 What You Should Know13.6 What’s Next?Exercise 13: I Hid All My Perls in a Package
14.1 The OOP Paradigm14.1.1 What Are Objects?14.1.2 What Is a Class?14.1.3 Some Object-Oriented Lingo14.2 Perl Classes, Objects, and Methods—Relating to the Real World14.2.1 The Steps14.2.2 A Complete Object-Oriented Perl ProgramA Perl Package Is a ClassA Perl Class14.2.3 Perl ObjectsReferencesThe Blessing14.2.4 Methods Are Perl SubroutinesDefinitionTypes of MethodsInvoking MethodsCreating the Object with a ConstructorCreating the Instance MethodsInvoking the Methods (User Interaction)14.2.5 Creating an Object-Oriented ModulePassing Arguments to MethodsPassing Parameters to Instance MethodsNamed Parameters and Data Checking14.2.6 Polymorphism and Runtime Binding14.2.7 Destructors and Garbage Collection14.3 Anonymous Subroutines, Closures, and Privacy14.3.1 What Is a Closure?14.3.2 Closures and Objects14.4 Inheritance14.4.1 The @ISA Array and Calling Methods14.4.2 $AUTOLOAD, sub AUTOLOAD, and UNIVERSAL14.4.3 Derived Classes14.4.4 Multiple Inheritance and Roles with Moose14.4.5 Overriding a Parent Method and the SUPER Pseudo Class14.5 Plain Old Documentation—Documenting a Module14.5.1 pod Files14.5.2 pod CommandsChecking Your pod Commands14.5.3 How to Use the pod Interpreters14.5.4 Translating pod Documentation into Text14.5.5 Translating pod Documentation into HTML14.6 Using Objects from the Perl Library14.6.1 An Object-Oriented Module from the Standard Perl Library14.6.2 Using a Module with Objects from the Standard Perl Library14.7 What You Should Know14.8 What’s Next?Exercise 14: What’s the Object of This Lesson?
15.1 Introduction15.2 What Is a Relational Database?15.2.1 Client/Server Databases15.2.2 Components of a Relational DatabaseThe Database ServerThe DatabaseTablesRecords and FieldsThe Database Schema15.2.3 Talking to the Database with SQLEnglish-like GrammarSemicolons Terminate SQL StatementsNaming ConventionsReserved WordsCase SensitivityThe Result Set15.3 Getting Started with MySQL15.3.1 Installing MySQL15.3.2 Connecting to MySQLEditing Keys at the MySQL ConsoleSetting a Password15.3.3 Graphical User ToolsThe MySQL Query BrowserThe MySQL Privilege System15.3.4 Finding the DatabasesCreating and Dropping a Database15.3.5 Getting Started with Basic CommandsCreating a Database with MySQLSelecting a Database with MySQLCreating a Table in the DatabaseData TypesAdding Another Table with a Primary KeyInserting Data into TablesSelecting Data from Tables—The SELECT CommandSelecting by ColumnsSelecting All ColumnsThe WHERE ClauseSorting TablesJoining TablesDeleting RowsUpdating Data in a TableAltering a TableDropping a TableDropping a Database15.4 What Is the Perl DBI?15.4.1 Installing the DBD DriverWithout the DBD-MySQL with PPMUsing PPM with LinuxInstalling the DBD::mysql Driver from CPAN15.4.2 The DBI Class Methods15.4.3 How to Use DBI15.4.4 Connecting to and Disconnecting from the DatabaseThe connect() MethodThe disconnect() Method15.4.5 Preparing a Statement Handle and Fetching ResultsSelect, Execute, and Dump the ResultsSelect, Execute, and Fetch a Row As an ArraySelect, Execute, and Fetch a Row As a Hash15.4.6 Getting Error MessagesAutomatic Error HandlingManual Error HandlingBinding Columns and Fetching Values15.4.7 The ? Placeholder and Parameter BindingBinding Parameters in the execute StatementBinding Parameters and the bind_param() Method15.4.8 Handling Quotes15.4.9 Cached Queries15.5 Statements That Don’t Return Anything15.5.1 The do() MethodAdding EntriesDeleting EntriesUpdating Entries15.6 Transactions15.6.1 Commit and Rollback15.6.2 Perl DBI, the Web, and the Dancer Framework15.7 What’s Left?15.8 What You Should Know15.9 What’s Next?Exercise 15: Practicing Queries and Using DBI
16.1 System Calls16.1.1 Directories and FilesBackslash IssuesThe File::Spec Module16.1.2 Directory and File AttributesUNIXWindows16.1.3 Finding Directories and Files16.1.4 Creating a Directory—The mkdir FunctionUNIXWindows16.1.5 Removing a Directory—The rmdir Function16.1.6 Changing Directories—The chdir Function16.1.7 Accessing a Directory via the Directory FilehandleThe opendir FunctionThe readdir FunctionThe closedir FunctionThe telldir FunctionThe rewinddir FunctionThe seekdir Function16.1.8 Permissions and OwnershipUNIXWindowsThe chmod Function (UNIX)The chmod Function (Windows)The chown Function (UNIX)The umask Function (UNIX)16.1.9 Hard and Soft LinksUNIXWindowsThe link and unlink Functions (UNIX)The symlink and readlink Functions (UNIX)16.1.10 Renaming FilesThe rename Function (UNIX and Windows)16.1.11 Changing Access and Modification TimesThe utime Function16.1.12 File StatisticsThe stat and lstat Functions16.1.13 Packing and Unpacking Data16.2 Processes16.2.1 UNIX Processes16.2.2 Win32 Processes16.2.3 The Environment (UNIX and Windows)16.2.4 Processes and FilehandlesLogin Information—The getlogin FunctionSpecial Process Variables (pid, uid, euid, gid, egid)The Parent Process ID—The getppid Function and the $$ VariableThe Process Group ID—The pgrp Function16.2.5 Process Priorities and NicenessThe getpriority FunctionThe setpriority Function (nice)16.2.6 Password InformationUNIXWindowsGetting a Password Entry (UNIX)—The getpwent FunctionGetting a Password Entry by Username—The getpwnam FunctionGetting a Password Entry by uid—The getpwuid Function16.2.7 Time and ProcessesThe Time::Piece ModuleThe times FunctionThe time Function (UNIX and Windows)The gmtime FunctionThe localtime Function16.2.8 Process Creation UNIXThe fork FunctionThe exec FunctionThe wait and waitpid FunctionsThe exit Function16.2.9 Process Creation Win32The start CommandThe Win32::Spawn FunctionThe Win32::Process Module16.3 Other Ways to Interface with the Operating System16.3.1 The syscall Function and the h2ph Script16.3.2 Command Substitution—The Backquotes16.3.3 The Shell.pm Module16.3.4 The system Function16.3.5 Globbing (Filename Expansion and Wildcards)The glob Function16.4 Error Handling16.4.1 The Carp ModuleThe die FunctionThe warn Function16.4.2 The eval Function16.5 Signals and the %SIG Hash16.5.1 Catching Signals16.5.2 Sending Signals to ProcessesThe kill FunctionThe alarm FunctionThe sleep Function16.5.3 Attention, Windows Users!16.6 What You Should KnowExercise 16: Interfacing with the System
A.1 Perl FunctionsA.2 Special VariablesA.3 Perl PragmasA.4 Perl ModulesA.5 Command-Line SwitchesA.6 DebuggerA.6.1 Getting Information About the DebuggerA.6.2 The Perl DebuggerA.6.3 Entering and Exiting the DebuggerA.6.4 Debugger Commands
B.1 What Is SQL?B.1.1 Standarizing SQLB.1.2 Executing SQL StatementsThe MySQL Query BrowserB.1.3 About SQL Commands/QueriesEnglish-like GrammarSemicolons Terminate SQL StatementsNaming ConventionsReserved WordsCase SenstivityThe Result SetB.1.4 SQL and the DatabaseThe show databases CommandThe USE CommandB.1.5 SQL Database TablesThe SHOW and DESCRIBE CommandsB.2 SQL Data Manipulation Language (DML)B.2.1 The SELECT CommandSelect Specified ColumnsSelect All ColumnsThe SELECT DISTINCT StatementLimiting the Number of Lines in the Result Set with LIMITThe WHERE ClauseUsing QuotesUsing the = and <> OperatorsWhat Is NULL?The > and < OperatorsThe AND and OR OperatorsThe LIKE and NOT LIKE ConditionsPattern Matching and the % WildcardThe _ WildcardThe BETWEEN StatementSorting Results with ORDER BYB.2.2 The INSERT CommandB.2.3 The UPDATE CommandB.2.4 The DELETE StatementB.3 SQL Data Definition LanguageB.3.1 Creating the DatabaseB.3.2 SQL Data TypesB.3.3 Creating a TableB.3.4 Creating a KeyPrimary KeysForeign KeysB.3.5 RelationsTwo Tables with a Common KeyUsing a Fully Qualified Name and a Dot to Join the TablesAliasesB.3.6 Altering a TableB.3.7 Dropping a TableB.3.8 Dropping a DatabaseB.4 SQL FunctionsB.4.1 Numeric FunctionsUsing GROUP BYB.4.2 String FunctionsB.4.3 Date and Time FunctionsFormatting the Date and TimeThe MySQL EXTRACT CommandB.5 Appendix SummaryB.6 What You Should KnowExercise B: Do You Speak My Language?
C.1 Getting StartedC.2 The ConstructorC.3 The AttributesC.3.1 The has FunctionC.3.2 Before and After Moose ExamplesC.3.3 Moose TypesC.3.4 Example Using Moose and ExtensionsC.3.5 Example Using Inheritance with MooseC.4 What About Moo?C.5 Appendix SummaryC.6 References
D.1 CPAN and @INCD.1.1 Finding ModulesD.1.2 Using ModulesI Already Have It!D.1.3 Package ManagerD.1.4 Manually: CPANlocal::libD.2 cpanmD.3 PerlbrewD.4 Caveats: C DependenciesD.5 Windows
E.1 A New Dancer AppE.1.1 VerbsE.1.2 TemplatingE.1.3 ParametersE.1.4 POSTExercise E: May I Have This Dance?

Content preview from Perl by Example

Chapter 11. How Do Subroutines Function?

In computer programming, a subroutine is a sequence of program instructions that perform a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed. Subprograms may be defined within programs, or separately in libraries that can be used by multiple programs. In different programming languages, a subroutine may be called a procedure, a function, a routine, a method, or a subprogram. The generic term callable unit is also sometimes used.¹

1. Wikipedia, “Subroutine,” http://en.wikipedia.org/wiki/Subroutine.

Perl allows you to collect a sequence ...