Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :

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Chapter 3: Perl Data Structures

Version 0.95

Contents

3.1. String operations

3.2 Numeric values and arithmetic operators

3.3. Operations on Arrays

3.4. Operations on Hashes

3.5. Typical errors

3.6. Summary

3.7. Exercises

Initially you can assume that like shell Perl has variables global to the script. this is OK assumptions if you scripts hover around 100 lines or so, which  is true for many Unix sysadmin.

If you need to write larger script the issue of namespaces became critical and  you should always use pragma strict in your scripts which help to segment  global namespace and detect many error that otherwise might go unnoticed into production version of the script. Actually you can benefit from pragma strict in small scripts too if they use nested loops and generally have complex logic.

Somewhat weaker but still useful version of pragma strict is

strict 'subs';

Which is highly recommended for all your scripts. Again, this is highly recommend option and you can omit it only at your own (and unnecessary) risk.

Uninitialized variables in Perl have a special value undef which is converted to zero in numeric comparisons and empty string in string comparisons. Memory allocation is automatic and unused variables can be garbage collected dynamically.

There are two major types of variables in Perl:

• Scalar variables that have a single assigned to them value and . Scalar variables are prefixed with $, arrays with @ and hashes with % sign. Generally they can be of two distinct types string values and numeric values. Conversion from string value to numeric value and vise versa performed automatically based on the type of operation used
  • Numeric values, represented internally a floating numbers with double precision. If string does not represent a valid number and you attempt to use numeric operation on it a zero will be used. This is a perfectly legitimate case in Perl. No errors will be given.
  • String values. Strings are allocated dynamically and you do not need to specify the length of the string.  Perl provides fast access to substring, slower access to a random character. Rather slow search of substring especially if length is big (in kilobytes or megabytes). Good access to sequential characters in both direction. Any change of the string (or most to be exact) involves freeing and reallocating memory.
• Composite variables that are collection of several values indexed with numeric keys or string keys
  • Array and lists are indexed with numeric key. Arrays (actually this not an array in C language style -- it is a list that behaves much like text editor buffer. You can get lines from the buffer using line numbers, delete lines, exchanges lines, etc. The first line has index (line number) 0(zero); there is no principal differences between arrays and associative array (hashes, see below) in Perl -- both are indexed lists, but arrays are more close to text editor buffer paradigm, while hashes are closer to symbol table paradigm). Array in Perl are sparse so it does not matter what is the value of the max index for memory allocation. Only existing elements are stored. Index of array is more like a key then offset from the beginning of the array in the memory as is the case with compiled languages. Perl provides fast access and assignment of value to the element if index is known. Attempt to retrieve element that was never assigned results in retrieving special value called undef . Fast insertion of element but slow insertion of a new element between existing elements with consecutive indexes as you need to shift elements and correct indexes of existing elements. Deletion is fast as you can assign the element the value of undef. Slow search. Built-in sort operation exist, so ordered array provides the possibility to speed search using binary search or other algorithms.
  • Hashes (associative arrays) are indexed with string keys.  An hash is like an array in that it stores strings in sequence but uses another string as an index to address individual items instead of a number. In essence this an abstraction of compiler symbol table. Implementation-wise binary search or hashing functions are involved. They provide fast retrieval of a value corresponding to specific key, fast insertion of new key-value pairs and fast deletion (key should be known). Hashes can be converted to lists with key and value for each pair represented by two sequential elements of the list. Typically hashes are converted to lists in foreach loops.

Each variable has several attributes, for example "is defined" attribute (which can be retrieved with defined built-in function) and as such internally is represented by a data structuresin interpreter identifier dictionary.

Perl provide a rich set of built-in operation for lists and associative arrays (hashes). It also provides some connections between these two data structures. Here Perl is really one of the top if not the top language in existence. If you task manipulating text files and  creating some forms of summaries or reports (or if the task in hand can be re-structured along these lines) you can benefit from using Perl.

At the same time multidimensional arrays are not natural to Perl and if your task is centered along the usage of multidimensional arrays you probably will be in disadvantage and should consider either using references or switch to other language that supports this abstraction better.

Like most programming language Perl also have several operating system specific data types. For example Perl supports the following data structures:

• Files and named pipes. We will discuss them later in a separate chapter.
• Pointers. Explicit pointers was a new, revolutionary data type which were first introduced into high level programming languages in PL/1,  but in full bloom appeared in C, in which it became kind of feature that define that language. In languages like C pointer is address of particular variable. Scripting languages  introduce one level of indirection here. Pointer here is pointer to the corresponding entry of identifier table that interpreter maintains and which has entries for all variables. So it is more like inode concept in Unix. In this sense all Perl variable are similar to files. So pointers carry the type of the  variable (array, or hash or scalar variable) with them. And you can introduce typical file attributes on them such  as read only. This is a really innovating and extremely useful feature of Perl as no other popular scripting language includes pointers explicitly. Common wisdom suggests that pointers should be excluded from high level language and, for example, Java does not contain them explicitly. They are part of OO framework. similar situation exists in Python. It is a wrong approach. We will cover them in a separate chapter.

Overview of literature

Good introduction to Perl variables can be found in

• InformIT Perl's Building Blocks Numbers and Strings Literals which is a reprint of a chapter from Sams Teach Yourself Perl in 24 Hours, 2nd Edition
• The Perl Data Structures Cookbook by Tom Christiansen (October 1995 )

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Etc

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotes :  Somerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose Bierce :  Bernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOS :  Programming Languages History : PL/1 : Simula 67 : C : History of GCC development :  Scripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-Month :  How to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Hater�s Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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Last modified: March 12, 2019