Home > Articles

This chapter is from the book

An Example: Simple Statistics

Here's an example called stats.pl, which prompts you for numbers, one at a time. When you're done entering numbers, it gives you a count of the numbers, the sum, and the average. It's a rather silly kind of statistics script, but it'll demonstrate tests, variable assignment, and pattern matching for input verification (and we'll be building on this script later on). Here's an example of what it looks like when run (including what happened when I accidentally typed an r in the middle of entering the numbers):

% stats.pl
Enter a number: 3
Enter a number: 9
Enter a number: 3
Enter a number: r
Digits only, please.
Enter a number: 7
Enter a number: 4
Enter a number: 7
Enter a number: 3
Enter a number:

Total count of numbers: 7
Total sum of numbers: 36
Average: 5.14
%

Listing 3.1 shows the code behind the statistics script.

Listing 3.1 The stats.pl Script

1: #!/usr/local/bin/perl -w
2: 
3: $input = ''; # temporary input
4: $count = 0; # count of numbers
5: $sum = 0;  # sum of numbers
6: $avg = 0;  # average
7: 
8: while () {
9:  print 'Enter a number: ';
10:  chomp ($input = <STDIN>);
11:  if ($input eq '') { last; }
12: 
13:  if ($input =~ /\D/) {
14:    print "Digits only, please.\n";
15:    next;
16:  }
17: 
18:  $count++;
19:  $sum += $input;
20: }
21: 
22: $avg = $sum / $count;
23: 
24: print "\nTotal count of numbers: $count\n";
25: print "Total sum of numbers: $sum\n";
26: printf("Average (mean): %.2f\n", $avg);

This script has three main sections: an initialization section, a section for getting and storing the input, and a section for computing the average and printing out the results.

Here's the initialization section (with line numbers in place):

3: $input = ''; # temporary input
4: $count = 0; # count of numbers
5: $sum = 0; # sum of numbers
6: $avg = 0; # average

We're using four scalar variables here: one to store the input as it comes in, one to keep track of the count of numbers, one to hold the sum, and one to hold the average.

The next section is where you prompt for the data and store it:

8: while () {
9:  print 'Enter a number: ';
10:  chomp ($input = <STDIN>);
11:  if ($input eq '') { last; }
12: 
13:  if ($input =~ /\D/) {
14:    print "Digits only, please.\n";
15:    next;
16:  }
17: 
18:  $count++;
19:  $sum += $input;
20: }

This second part of the script uses a while loop and a couple if conditionals to read the input repeatedly until we get a blank line. And also to test the input to make sure that we didn't get anything that wasn't a number. I still haven't discussed how loops and conditionals are defined in Perl (and we won't get around to it until Day 6). So, I'm going to pause here and give you a very basic introduction so you will not be totally lost for the next few days.

A while loop says "while this thing is true, execute this stuff." With a while loop Perl executes a test, and if the test is true it executes everything inside the curly braces (here, everything in between lines 9 and 20). Then, it'll go back and try the test again, and if it's true again, it'll execute all that code again, and so on. The loop means it goes around and around and around until the test is false.

Usually, the test is contained inside the parentheses (line 8), and can be any of the tests you learned about yesterday. Here, there is no test, so this is an infinite loop; it never stops, at least not here. We'll find a way to break out of it from inside the loop.

An if conditional is simpler than a loop. An if conditional has a test, and if the test is true, Perl executes some code. If the test is false, sometimes Perl executes some other code (if the if conditional has a second part, called an else), and sometimes it just goes onto the next part of the script. So, for example, in the if conditional in line 11, the test is if the input is equal to the empty string ''. If the test is true, last is executed. The last keyword is used to immediately break out of a while loop and stop looping. If the test is false, Perl skips over line 11 altogether and continues onto line 13.

In the if conditional, lines 13 through 16, the test is a pattern match. Here we're testing the $input to see if it contains any nondigit characters. If it does, we execute the print statement in line 14, and then call next. The next keyword skips to the end of the while loop (in this case, skipping lines 18 and 19), and restarts the next loop at the top of the while again. Just as with line 11, if the test in line 13 was false, Perl skips over everything in line 13 to 16 and continues onto line 18 instead.

Now that you know about if and while, let's start at the top and figure out what this bit of code actually does. It's a while loop with no test, so it'll keep going forever until something breaks you out of it. Inside the body of the while, we use line 10 to grab the actual input (and I know you're still waiting to learn what chomp and <STDIN> do; it's coming up soon). Line 11, as I mentioned, tests for an empty string, and if we got one, breaks out of the loop. The empty string in the input will only occur if the user hit return without typing anything; that is the signal to the stats program that the end of input has been reached. Note the string test (ne) here; a number test would convert the empty string to 0, which is not what we want. 0 is valid input for the stats program.

When you get to line 13 we know we have something in $input, but we want to make sure that you have valid input, that is, numeric data. You're going to be performing arithmetic on this data in lines 18 and 19, and if you end up with nonnumeric data in the input, and warnings turned on, Perl is going to complain about that data. By verifying and rejecting invalid input you can make sure your scripts do not do unfriendly things like spew errors, or crash when your users are running them.

Lines 13 through 15 are the input validation test. If the input did contain nonnumeric data, we print an error and the loop restarts by prompting for new data.

By the time we get to line 18 we now know that we have data to be handled in $input and that data does not contain nonnumeric characters. Now we can add that new data to our current store of data. In lines 18 and 19 we increment the $count variable, and modify the $sum variable to add the new value that was input. With these two lines we can keep a running total of the count and the sum as each new bit of input comes along, but we'll wait until all the data has been entered to calculate the average.

And, finally, we finish up by calculating the average and printing the results:

22: $avg = $sum / $count;
23: 
24: print "\nTotal count of numbers: $count\n";
25: print "Total sum of numbers: $sum\n";
26: printf("Average (mean): %.2f\n", $avg);

Line 22 is a straightforward calculation, and lines 24 through 26 print the count, the sum, and the average. Note the \n at the beginning of the first print statement; this will print an extra blank line before the summary. Remember that \n can appear anywhere in a string, not just at the end.

In the third summary line, you'll note we're using printf again to format the number output. This time, we used a printf format for a floating-point number that prints 2 decimal places of that value (%.2f). You get more information about printf in the next section.

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