Home > Articles > Programming > Perl

Stacking Perl's Building Blocks: Lists and Arrays

  • Print
  • + Share This
Discover the numerous ways of manipulating lists and arrays in Perl and follow up with a game example.

See all Sams Teach Yourself on InformIT Programming Tutorials.

This chapter is from the book

Scalars are Perl's singular nouns. They can represent any one thing—a word, a record, a document, a line of text, or a character. Often, though, you need to talk about collections of things—many words, a few records, two documents, fifty lines of text, or a dozen characters.

When you need to talk about many things in Perl, you use list data. You can represent list data in three different ways: by using lists, arrays, and hashes.

Lists are the simplest representation of list data. A list is simply a group of scalars. Sometimes they're written with a set of parentheses encasing the scalars, which are separated by commas. For example, (2, 5, $a, "Bob") is a list that contains two numbers, a scalar variable $a, and the string "Bob". Each scalar in a list is called a list element. In keeping with the philosophy of Least Surprise (see Hour 2, "Perl's Building Blocks: Numbers and Strings"), Perl's lists can contain as many scalar elements as you like. Because scalars can also be arbitrarily large, a list can hold quite a lot of data.

To store list data so that you can refer to it throughout your program, you need an array variable. Array variables are represented in Perl with an "at" sign (@) as the type identifier followed by a valid variable name (as discussed in Hour 2, "Perl's Building Blocks: Numbers and Strings"). For example, @foo is a valid array variable in Perl. You can have the same name for an array variable as a scalar variable; for example, $names and @names refer to different things—$names to a scalar variable, and @names to an array. The two variables have nothing to do with each other.

Individual items in an array are called array elements. Individual array elements are referred to by their position within the array, called an index. That is, we can refer to the third array element of the array @foo, the fifth array element of the array @names, and so on.

The other list type, a hash, is similar to an array. Hashes will be discussed further in Hour 7, "Hashes."

In this hour you will learn

  • How to fill and empty arrays

  • How to examine arrays element by element

  • How to sort and print arrays

  • How to split scalars into arrays and join arrays back into scalars

Putting Things into Lists and Arrays

Putting things into a literal list is easy. As you just saw, the syntax for a literal list is a set of parentheses enclosing scalar values. The following is an example:

(5, 'apple', $x, 3.14159)

This example creates a four-element list containing the numbers 5, the string 'apple', whatever happens to be in the scalar variable $x, and pi.

If the list contains only simple strings, and putting single quotation marks around each string gets to be too much for you, Perl provides a shortcut—the qw operator. An example of qw follows:

qw( apples oranges 45.6 $x )

This example creates a four-element list. Each element of the list is separated from the others by whitespace (spaces, tabs, or newlines). If you have list elements that have embedded whitespace, you cannot use the qw operator. This code works just as though you had written the following:

('apples', 'oranges', '45.6', '$x')

Notice that the $x is encased in single quotation marks. The qw operator does not do variable interpolation on elements that look like variables; they are treated as though you wanted them that way literally. So '$x' is not converted to whatever the value of the scalar variable $x is; it's left alone as a string containing a dollar sign and the letter x.

Perl also has a useful operator that works in literal lists; it's called the range operator. The range operator is designated by a pair of periods (..). The following is an example of this operator:

(1..10)

The range operator takes the left operand (the 1) and the right operand (the 10) and constructs a list consisting of all the numbers between 1 and 10, inclusive. If you need several ranges in a list, you can simply use multiple operators:

(1..10, 20..30);

The preceding example creates a list of 21 elements: 1 through 10 and 20 through 30. Giving the range operator a right operand less than the left, such as (10..1), produces an empty list.

The range operator works on strings as well as numbers. The range (a..z) generates a list of all 26 lowercase letters. The range (aa..zz) generates a much larger list of 676 letter pairs starting with aa, ab, ac, ad and ending with zx, zy, zz.

Arrays

Literal lists are usually used to initialize some other structure: an array or a hash. To create an array in Perl, you can simply put something into it. With Perl, unlike other languages, you don't have to tell it ahead of time that you're creating an array or how large the array is going to be. To create a new array and populate it with a list of items, you could do the following:

@boys=qw( Greg Peter Bobby );

This example, called an array assignment, uses the array assignment operator—the equals sign, just as in a scalar assignment. After that code runs, the array @boys contains three elements: Greg, Peter, and Bobby. Notice also that the code uses the qw operator; using this operator saves you from having to type six quotation marks and two commas.

Array assignments can also involve other arrays or even empty lists, as shown in the following examples:

@copy=@original;
@clean=();

Here, all the elements of @original are copied into a new array called @copy. If @copy already had elements before the assignment, they are now lost. After the second statement is executed, @clean is empty. Assigning an empty list (or an empty array) to an array variable removes all the elements from the array.

If a literal list contains other lists, arrays, or hashes, these lists are all flattened into one large list. Observe this snippet of code:

@boys=qw( Greg Peter Bobby );
@girls=qw( Marcia Jan Cindy );
@kids=(@girls, @boys);
@family=(@kids, ('Mike', 'Carol'), 'Alice');

The list (@girls, @boys) is flattened by Perl to a simple list containing first all the girls' names and then all the boys' names before the values are assigned to @kids. On the next line, the array @kids is flattened, and the list ('Mike', 'Carol') is flattened into one long list; then that list is assigned to @family. The original structures of @boys, @girls, @kids, and the list ('Mike', 'Carol') are not preserved in @family—only the individual elements from Greg through Alice. In other words, the preceding snippet for building @family is equivalent to this assignment:

@family=qw(Marcia Jan Cindy Greg Peter Bobby Mike Carol Alice );

The left side of an array assignment can be a list if it contains only variable names. The array assignment initializes the variables on that list. Consider this example:

($a, $b, $c)=qw(apples oranges bananas);

Here, $a is initialized to 'apples', $b to 'oranges', and $c to 'bananas'.

If the list on the left contains an array, that array receives all the remaining values from the right side, no matter where it is in the list. The reason is that an array can contain an indefinite number of elements. Observe here:

Figure 4.1

In this example, $a is set to 'peaches'. The remaining fruits in the list on the right are assigned to @fruit on the left. No elements are left for $c to receive a value (because the array on the left side of an assignment absorbs all the remaining values from the right), so $c is set to undef.

It's also important to note that if the left side contains more variables than it has elements, the leftover variables receive the value undef. If the right side has more variables than the list on the left has elements, the extra elements on the right are simply ignored. The following figure shows another example to help understand that concept.

Figure 4.2

In the first line, $t, $u, and $v all receive a value from the right side. The extra right-side element ('quail') is simply not used for this expression. In the second line, $a, $b, and $c all receive a value from the right. $d, however, has nothing to get from the right ($c takes the last value, 'gopher'), so $d is set to undef.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020