Teaching Perl - Week 3 - Textual Data

So last week you should have covered variables, conditionals, flow control statements, calling functions, stdin/out/err, and using new perl features.

This week we're covering text processing, scalars, external libraries, and reinforcing what we learned last week. Text Processing is one of Perl's specialty's, mostly due to it's powerful regular expression engine.

First install LWP with cpan. cpanp -i Bundle::LWP will do it.

So this is almost completely new... what does it do? and what does it have to do with text?

LWP is short for lib www perl. LWP::Simple is a simple procedural interface to it. getprint is a method which is roughly equivilant to fetch uri and print the content that you got back. So this isn't very useful... (ok it's actually quite useful if you're on arch and you want a PKGBUILD) what we want to do in this case is cut the junk and print the pkgname. As a side note, this code does the exact same thing as the python example in Head First Programming: A Learner's Guide to Programming Using the Python Language but is shorter if you cut the boilerplate (#! ... use strict; use warnings ), and the same length if you don't.

Now let's have a first attempt at getting the package name shall we? We need to read the contents into a variable first. note: we've changed from getprint just to get because it returns the contents to a variable. (note: still shorter than the python.) At this point the book goes into how a string is an array of characters. Perl is contextual so at this point our scalar is essentially a string but just because it's a scalar does not mean it's currently a string. In our previous examples we were handling it in a numeric context.
So here we're looking at the 11 character substring past character 281. The output should be modern-perl. This code is horrible, what should happen if I were to change this PKGBUILD? (if this ever doesn't output modern-perl that's probably exactly what happened). Remember say appends a newline character to it's output print doesn't. We used print previously because the output already had a newline at the end.

There are thousands of PKGBUILD's on AUR let's see if we can get the name from another one by changing the URI.The output at the time of this writing is

dirs')
depe

and will probably change. A newline is just another character in the scalar so nothing prevents one from being printed within the substring. Our URI got a little long here so we moved split that peace of code up to be on 2 lines for readability. Your code lines shouldn't go over 78 characters long if you can help it. A URI would be an acceptable exception though.

So we use the index function to search the string for some text and return the position we can then use that position to get our substring. The position returned is 1 less than where the string we searched for starts. So we added the 9 to get to the string we actually want. Our output is now

perl-moose'

why the '? well "perl-moose" is shorter than "modern-perl" so we've got extra characters in there.

Let's use a regular expression.
regular expressions are really their own mini language, very terse, and several dialects. Have your students go through the Regular Expression Tutorial and the Regular Expression page. In short this regex looks for pkgname='something' in $content and takes something and puts in in $pkgname. the ( ) in the / / tell which part of the regex to assign to $pkgname. [\w-]+ says to match 1 or more alphanumeric or _ or - characters. Regular expressions is a huge topic in and of itself. So don't spend a huge amount of time on it, they are important to know, but you don't have to master them right now.

I'd suggest having a copy of Regular Expression Pocket Reference: Regular Expressions for Perl, Ruby, PHP, Python, C, Java and .NET (Pocket Reference (O'Reilly)) and possibly Mastering Regular Expressions and if you want some common recipes Regular Expressions Cookbook. You should also warn your students that although you can use regular expressions to match any kind of text, that in the case of well defined formats, such as xml, html, csv, ini and more that there are better ways of extracting data. In the cases of things like email and phone numbers they should not write their own regex's but use modules to validate such things as they will undoubtedly make mistakes doing so, and a module has been well tested.

The example in Head First Programming: A Learner's Guide to Programming Using the Python Language goes on to explain how to check the page periodically for updates, and only output when a value is less than a certain amount. Since I have no idea when I'll be updating these PKGBUILDs I don't want to get into that code. It would make a good homework assignment. You can use sleep to make sure you only fetch the code ever so often. I can guarantee barring a buggy release I won't update any PKGBUILD more than once every 86400 seconds (1 day). You'll have to check pkgver and pkgrel to see if either of them are newer than the previously fetched PKGBUILD.

Teaching Perl - Week 3 - Prelude Commentary

So I think that Head First Programming: A Learner's Guide to Programming Using the Python Language is a great guideline to teach any programming course, esp a 10 week course as it has 10 chapters. We're one week behind. Arguably you could merge week 2 into week 1 and I may do just that in a rewrite of this series. In fact I'd suggest you do just that. But for now I'm going to continue on and we'll see what people think once I reach the end. No reason I can't rewrite this series, especially since it's not really planned I'm just writing it on the fly.

Although this book is a good guideline (note: I'm also concerned that if I follow it's examples to the letter all the time I might violate fair use), chapter 2 annoys me, it's on textual data and uses html scraping as an example. However, it completely ignores html parsers and any other sane way of doing it. So I had to find a better example and I think I have with parsing PKGBUILD's, which are sort of a microformat bash script.

Teaching Perl - Week 2 ( part2 )

This part2 was prompted by Chromatic's post on state. I'd never heard of state before, and it's documentation is poor. Let's take a look at how state could affect our game.As you can see this allows us to make our $guess variable lexically scoped, meaning it's contained within the loop. This however generates some warnings: Use of uninitialized value $guess in numeric eq (==) at ./game.pl line 14. and if you enter non-numeric input Argument "l\n" isn't numeric in numeric eq (==) at ./game.pl line 14, line 1.. Both are completely fixable, but lets fix the uninitialized first.

So all we've had to change is adding parenthesis around state and assign it to a value. If we had used a my declaration it would reassign to 0 every time and we'd never match the $winning_num. But since state will only ever initialize once we don't have that problem. We still have our problem if we enter a character though, let's sanitize that input.
So in order to fix the 'not a numerical comparison' issue we check $guess to see if it's a digit with a regular expression. If it isn't a digit we set it to 0 which is an invalid guess anyways, but fixes the warning. This actually fixes two bugs. Before if you entered a letter it would tell you that it was too low (which I don't actually understand, because aren't characters > numbers? if someone on Iron Man could explain it in a comment.)

Teaching Perl - Week 2

• Week 0
• Week 1 (part 1)
• Week 1 (part 2)

Guess the Number

So our little game is exceptionally mediocre and not very fun. Head First Programming: A Learner's Guide to Programming Using the Python Language suggests we give the game player some help by telling them if the number is lower or Higher.

This game is a lot less challenging, and still not very good. So one one of the problems is it exits instead of letting it try till we get it right. So lets change that.

This one is going to show your students something they likely haven't seen before. An until statement. until is logically equivalent to while not or in this case you could write while ( $guess != 5 ) {...} and have it do exactly the same thing.

We've got some bad practices in this code it lacks useful comments and uses a magic number. Also the instructions aren't clear if you don't know how the program is written. Let's add that stuff next.
So this code is too simple for comments, but I've tried to show what I think some good ones would be. Usually 'novice' books say use meaningful comments but don't show any. Students then end up writing comments like # initialize local variable guess to 0. That's not useful the code obviously says that. Instead comments should say, what a larger chunk of is trying to do, or why it's doing it that way.

In Head First Programming: A Learner's Guide to Programming Using the Python Language the author uses the variable secret where I've used $winning_num because he ends up using a random number generator (we will in a moment), however, this is not a very descriptive name. $winning_num is a better name because it's more descriptive of what the number actually is. He also skips removing the 'magic numbers' step which is probably why the variable name used is poor.

So let's make the winning number random.

There we go... Now are number is random. ... ... ... notice the logic error yet? no? go read the rand documentation. Got it now? ok simply put this generates 10 total possible random numbers starting at 0, e.g 0-9. Our code isn't broken but it's not doing what we want. But the docs don't say how to generate a range... how can I do that? run perldoc -q random this will return perlfaq4 we're looking for How do I get a random number between X and Y?. That tells us how to fix it... our code should now look something like.

we could just put 10 in rand in this case but that's not as flexible. You'll also note I commented on why we did it that way. Kind of a way of making sure no one goes back to the other way, it also tells people why we did it this way.

If you've used another language you might ask, "Why not use a switch case?". Most instructors and books say "perl doesn't have a switch case". Even perl intro still falsely states that

"Perl has most of the usual conditional and looping constructs except for case/switch (but if you really want it, there is a Switch module in Perl 5.8 and newer, and on CPAN."

. That's even worse! Switch.pm has been in core for a while now... but is deprecated an you should NOT use it. since perl 5.10.0 perl has had a Switch statement, so let's rewrite our code using that.

Note the use of use feature 'switch'; make sure you use this instead of use Switch; which will use the now deprecated Switch.pm module.

You can shorten use feature 'say'; use feature 'switch'; to use feature qw(say switch);.

Beside's introducing switch/case this introduces the default argument $_ and smart matching.

You'll also note that I changed the algorithm but my comments didn't change.

As you can see TIMTOWTDI.

Homework

What else is wrong with our game? how could we fix it? How could we prevent the problem from being reintroduced? How can we be sure that our random problem is actually fixed? and make sure that it's not reintroduced? Can you think of any other ways we can write this program? or any other improvements? write the program so that it has 3 difficulty levels easy, medium, hard with ranges 1-10,1-100,1-1000 respectively. Blog these to Iron Man.

Optional: if you have Programming: Principles and Practice Using C++ have your students do Drill 1 from chapter 3 (obviously in perl not c++).

Teaching Perl - Week 1 - ( Part2 )

• Week 0
• Week 1 (part 1)

Padre - Perl IDE

Now let's install Padre the Perl IDE to give the students a nice development environment. There are instructions on the website, however, cpanp -i Padre should get the trick done since it is on CPAN In the future Strawberry will ship with Padre but that future isn't quite here.

First Program

Ok so we want to make sure that we have Perl installed correctly and since you're taking my advice I'm assuming that you've given your students access to 5.10.1 and may have 5.8 as well... So I'm gonna do this one twice. Have them create a text file hello_world.pl and add the following lines

#!/usr/bin/env perl

print "Hello, World\n";

Since I'm sure this is not your students first programming class this will be familliar to them. Explain the shebang (#!) line and note that it has no effect on windows. Now let's modernize it, using the say feature from perl 5.10.

#!/usr/bin/env perl
use feature 'say';

say "Hello, World";

note the removal of the \n and that use feature is a pragma that allows you to enable features that are newer in perl that weren't available in the original perl 5. Perl is a bit backwards about this, IMO. We should be using the newest version and features unless we specify otherwise, IMO, but that's not how it works, ATM. If 5.10 isn't available to you and your students just use print in future examples. I will be excluding the shebang line from future examples too.

Now let's expand hello world a bit. Have them Make it print

Hello, programming!
Here we go!

First thing is to make use of concatenation

use feature 'say';

say "Hello, programming"
    . "Here we go";  

This example is inspired by Programming: Principles and Practice Using C++ ( This is an excellent book and should be the textbook for BOTH C++ classes at Baker. I recommend all novice to intermediate programmers have a copy whether you like C++, or not, this book is excellent ). Please note that the . operator is in the front. This draws a programmers attention to it and notes that it's a continuation. See Perl Best Practices for more details on this.

Your output will be.

Hello, programming Here we go

That's not what we want! obviously say appends a newline to the end but doesn't help with long strings.

use feature 'say';

say "Hello, programming\n"                                                                            
    . "Here we go";  

Much Better.

Simple Game

Now let's up the ante a bit. Again not there first programming class, the following example is rewritten in Perl from Head First Programming: A Learner's Guide to Programming Using the Python Language which arguably should be the textbook for Intro to Programming. This is the first example that book gives in chapter 1 (but it gives it in python).
note: you can use < STDIN > but blogger screws it up because of the brackets. This code works exactly the same.

So now you can discuss: scalars, if then else statements,and readline. There are also two additional pragma's strict and warnings that you should tell your students to always use unless they have a 'good' reason to disable them (these have been there since the beginning).

So usually on the first night of class you're lucky to get past hello world. I think this is all doable. In 1 small program I just took you all the way to Chapter 6 in Perl by Example (3rd Edition) the chapter on control structures. Remember, this is not your students first class... you don't need to hold their hand through all this. Race through it so they can improve on it as the term goes on. Instead of spending 4 weeks (assuming a chapter a week) on if then statements you can now spend 10

Homework

Start a blog using whatever site you prefer (preexisting blogs acceptable) and add it to Iron Man. Do a first post that talks a little about you (blog can be anonymous). Include a copy of the simple game and what you think would be better about it. Also mention anything you'd like to do with, or know about, Perl.

Teaching Perl - Week 1 - ( Part1 )

Make sure you've read Part 0 first.

As I said before Baker provides a Linux server for the perl class. However, none of the students have had a unix class. So I advise touching on it and giving them a link to these tutorials if they want to use it, and a helping hand. The book also talks about ActiveState but this isn't the recommended way to do perl on windows anymore. Avoiding crux will prevent you from wasting a week or more on unix which is not what the class is about (my instructor failed horribly at this, and at knowing unix (or perl for that matter) well enough to cover it).

Strawberry

Strawberry Perl is the recommend open source binaries for windows these days and should work just like the unix ones. You should cover not only normal installation; but installing to their network drives (use portable edition), so they can have strawberry on any baker computer on campus; and to their flash drives (use portable edition), so they can take it with them to any computer running windows.

CPAN

Next you should mention CPAN. CPAN is the #1 resource in perl, if you need to do something, most of the time someone else has done it for you and it's on CPAN. Don't forget to mention search. and kobesearch.cpan.org.

local::lib

After talking about CPAN use it. Help your students install a local::lib esp if you're using the schools linux server which won't allow installs of global CPAN modules.

Task::Kensho

You'll want to recommend that students check out Task::Kensho which a CPAN module that lists all the best modules for common tasks, they may not be the only module for that task but they are the recommended ones. You could have them install them with cpanp -i Task::Kensho at this point or just recommend they look them over to see if they might find them useful. These are all probably ahead of where your students are at, but it never hurts to point useful libraries out early. Just tell them they can get back to them later. Note: local::lib is on Task::Kensho.

P.S. Week 1 will be multipart I have to leave now but I want to hit publish. coming next is how to teach them to do their first program the right way so they don't have to learn it twice.

Teaching Perl - Part 0 - Preface

This is my 2 cents on how perl should be getting taught at my school. My school's curriculum sucks in general across the board. But assuming I can't change what classes are taught, when and what prerequisites this is how I would teach perl given the current computer science curriculum.

Since people reading this are probably not familiar with the Fail that is Baker College (I would not choose Baker if I had to start again but credits don't transfer easily, so it was easier to continue on with it). Baker has 10 week courses, and with tech courses they are seemingly exclusively scheduled 6-10pm once per week. So this factors into how I'll lay this out.

Specifically for the perl class the curriculum provides the schools crux server which is running CentOS linux or something... (I'm not sure which distro it is atm). It does not have the prerequisite of Linux 101 though, and it's not in there curriculum. I don't believe it has any prerequisites other than intro to programming. It is a 300 level course, however. This means that most/all students in the course have probably taken a few C++, Visual Basic, and Java classes (there are about 6 of these classes at the 200 level and I imagine most have done at least 2 of the 6).

The book required at Baker is currently Perl by Example (4th Edition). I'd suggest telling students it's recommended but not required (If you won't get in trouble) in any event you and students shouldn't need to use this book at all, because a great book Beginning Perl is free online. Also the non book resources for perl are practically limitless and you should be encouraging the use of these. Some of them include

• learn.perl.org
• perldoc
• kobesearch.cpan.org or
• search.cpan.org
• Iron Man or
• SayPerl (AKA Iron Man with all posts translated)
• Perl Channel on Freenode if they need to ask for help

You should have a copy of Perl Best Practices. I've been told that you should read this with a grain of salt, that it's only like 50% accurate these days... but I think it's still a good place to start, and you can pick and choose what to use from it. You may also want a copy of Intermediate Perl For more advanced discussion on references and Object Oriented perl.

You may also want a good resource as a Linux CLI reference. This series of tutorials was recommended to me.

I will continue to post on the subject matter that I would cover each week, in this 10 part series (programmers count from zero ;) ). I may also discuss Perl by Example (3rd Edition) because that was the required book when I took the class, even though 4ed was out.

Update: Week 1 (part1)