Perl Basics (20090307 classroom)

IRC Log

-!- nirik changed the topic of #fedora-classroom to: Fedora IRC Classroom - Some Perl basics with your teacher Doran Barton ( fozzmoo ) - See Classroom for class schedule today and more info.		15:00
@nirik	take it away fozzmoo...	15:00
fozzmoo	For those who will following along, here are some slides: http://www.xmission.com/~fozz/perl_basics.pdf	15:00
fozzmoo	I'm Doran Barton, programmer, sysadmin, RHCE, geek, and Perl junkie.	15:01
fozzmoo	My e-mail address is on slide #1.	15:01
fozzmoo	How many people are actively here for this session as opposed to idling... idly?	15:01
Ritmo2k	Active	15:02
* nirik is watching along.		15:02
mharris	3.14 people :)	15:02
* SMParrish here		15:02
fozzmoo	heh.	15:02
baconfork	present	15:02
fozzmoo	Just wanted to make sure I'm not presenting to a bunch of cardboard cutouts. :)	15:02
mharris	LOL	15:02
fozzmoo	Let's move on to slide #2	15:02
heftig	online	15:02
* delhage is a bit of a cardboard cutout, but here		15:03
fozzmoo	This is just a summary of some learning resources I threw together.	15:03
fozzmoo	The O'Reilly books are, without a doubt, the best printed material you can get on Perl.	15:03
fozzmoo	That doesn't mean other publishers don't have good stuff... but, honestly, I wouldn't know. All the brains in the community seem to write or work for ORA	15:03
fozzmoo	In addition to perlmonks.org as a site to use as a resource, I would also recommend perl.com (operated, again, by O'Reilly and Associates)	15:04
fozzmoo	And then, there may be a local Perl users group near you - check pm.org	15:04
fozzmoo	Any questions before we move to slide #3?	15:04
fozzmoo	okay then.	15:05
fozzmoo	Perl. What is it?	15:05
fozzmoo	It's an interpreted scripting language.	15:05
fozzmoo	But, like other modern languages, the definition is beginning to blur.	15:06
fozzmoo	The actual mechanics of the Perl interpreter is similar to that of Java.	15:06
fozzmoo	And, it will be even more so with Perl6.	15:06
fozzmoo	Because of the interpreter nature, and because of a large and active open source community, the Perl interpreter is available on many platforms.	15:06
fozzmoo	Linux, of course, is probably where most of the activity is at, followed by other Unix variants and then Windows.	15:07
fozzmoo	The creator of the Perl language, Larry Wall, has a deep background in linguistics.	15:07
fozzmoo	He proudly proclaims that Perl is a natural language.	15:08
fozzmoo	I won't go into that here, but you can read about it if you're interested in what that means.	15:08
fozzmoo	One thing it does mean is that Perl *can* be easy to follow and read.	15:08
fozzmoo	Unfortunately, many programs test that hypothesis. :)	15:08
fozzmoo	programmers, I mean.	15:08
fozzmoo	It is free and Free.	15:09
fozzmoo	And, as of Perl5, it is object oriented and is becoming more and more so as we approach Perl6.	15:09
fozzmoo	The, mantra, if you will, of the Perl community is TMTOWDI	15:09
fozzmoo	"There is more than one way to do it."	15:09
fozzmoo	This is evidenced by the many, many different Perl solutions to a given problem.	15:10
fozzmoo	Perl is a very flexible language and it seems there's always a more efficient way to do things.	15:10
fozzmoo	The trick is balancing efficiency with readability. :)	15:10
fozzmoo	Going back to slide 2 for a moment...	15:10
fozzmoo	Damian Conway has written an excellent book called "Perl Best Practices"	15:11
fozzmoo	If you're going to do some serious development using Perl, you *must* get this book.	15:11
fozzmoo	Don't learn how to program in Perl by reading random code snippets you find online.	15:11
fozzmoo	Any questions before I go to my soapbox slide (#4)?	15:11
SSlater	I have heard that perl is good for manipulating data?	15:12
fozzmoo	SSlater: It is. Perl's original forte was processing text data.	15:13
SSlater	Great.	15:13
fozzmoo	That made it ideal, later on, for doing things with HTML, XML, etc.	15:13
fozzmoo	Another strength Perl brings to the table with this is it's regex support which has kind of defined regular expression syntax for the rest of the open source world.	15:14
* fozzmoo slaps himself for not phrasing that better.		15:14
fozzmoo	Anyway, yeah. Perl rocks for text processing.	15:14
fozzmoo	Perl was first unleashed on the world in the mid 1980s.	15:15
fozzmoo	So, it's pretty old.	15:15
fozzmoo	A lot has changed since then, of course.	15:15
fozzmoo	The syntax borrowed heavily from C, Bourne shell, and awk.	15:15
fozzmoo	The original PHP interpreter was written in Perl.	15:16
fozzmoo	this possibly explains why PHP's syntax is, in many ways, similar to Perl.	15:16
fozzmoo	In my opinion, Perl is technologically on a much higher plane than PHP.	15:16
fozzmoo	A lot more ReallySmartPeople(tm) have worked on Perl and have made it a very sophisticated language.	15:17
fozzmoo	This is somewhat of an achiles heel, though.	15:17
fozzmoo	I've written about this on my blog <http://fozzolog.fozzilinymoo.org/tech/> - Perl is nowhere near as "accessible" for web developers as PHP is.	15:18
fozzmoo	But PHP apps have a limit to their scalability.	15:18
fozzmoo	Perl apps, not so much.	15:18
fozzmoo	Python is another language you may consider.	15:19
fozzmoo	I'm not well versed in Python, but I would say it also has very smart people involved.	15:19
fozzmoo	The advantage Perl has over Python is the huge breadth of modules available online for re-use in your apps.	15:19
fozzmoo	But Python's moving fast.	15:20
fozzmoo	I only know one guy who has done a lot of Perl development and Python development.	15:20
fozzmoo	Specifically, he's doing django work.	15:20
fozzmoo	He likes it, but he misses the flexibility of Perl where you can swap modules in and out easily where django is more restrictive about what packages you can use for what. It's not all or nothing, but it's not easy to mix and match.	15:21
fozzmoo	Java is... well...	15:21
fozzmoo	Have you ever seen a Java one-liner?	15:21
fozzmoo	I rest my case.	15:21
heftig	no :P	15:21
delhage	:)	15:22
fozzmoo	Any questions before we zip to #5?	15:22
fozzmoo	BTW, if you're late arriving: http://www.xmission.com/~fozz/perl_basics.pdf	15:23
fozzmoo	Perl was the first language I played with that had a standard embedded documentation system.	15:23
fozzmoo	It's called POD - Plain Old Documentation.	15:24
fozzmoo	If you're on a Linux system, you probably have some Perl libs installed...	15:24
fozzmoo	Go find one of these like LWP.pm and open it in an editor	15:24
wonderer|chatzil	how do I test this the fastest way?	15:25
fozzmoo	locate LWP.pm	15:25
wonderer|chatzil	ok	15:25
fozzmoo	Then, edit the file...	15:25
fozzmoo	You should see the inline documentation right away.	15:25
fozzmoo	You can read this at the command line with the 'perldoc' command.	15:26
fozzmoo	i.e. 'perldoc LWP'	15:26
fozzmoo	This explains what you can do with the LWP module.	15:26
fozzmoo	For a table of contents, so to speak, you can type 'perldoc perl'	15:26
fozzmoo	this gives you all the base POD volumes you can read.	15:26
fozzmoo	If you're really cheap and you don't want to go buy a book...	15:26
fozzmoo	you can learn pretty much everything from these online PODs.	15:27
fozzmoo	The wit and charm of the main devs is littered throughout, just like in the books. :)	15:27
fozzmoo	The example on slide #5 (perldoc -f printf) is how you bring up documentation on a specific built-in function.	15:28
fozzmoo	Those are stored in the perlfunc POD.	15:28
fozzmoo	All this documentation is available online as well, via many sites. perl.com is one.	15:28
fozzmoo	For installed CPAN modules, you can read docs on cpan.org if you'd like	15:29
fozzmoo	Now, a word about CPAN (Comprehensive Perl Archive Network)	15:29
fozzmoo	it seems every other scripting language known to man has CPAN-envy.	15:29
fozzmoo	PHP has PEAR. Python has... what it has.	15:30
fozzmoo	Java hasn't figured out what it has...	15:30
fozzmoo	CPAN is a distributed repository of modules you can install and use in your apps.	15:30
fozzmoo	The Perl community has built a very nice automated testing framework and has established high standards for what constitutes a modules accepted for inclusion in CPAN.	15:31
fozzmoo	it's not a matter of just uploading something.	15:31
fozzmoo	It has been well documented and has to abide by certain conventions.	15:31
fozzmoo	So, let's move to #6	15:31
fozzmoo	I'd like to wrap up in about 10-15 minutes for Q&A. :)	15:32
fozzmoo	We've got 19 slides left.	15:32
fozzmoo	Slide 6... the requisite hello world demo.	15:32
fozzmoo	First line is the "magic" line, just like you'd expect with shell scripting or python scripting.	15:32
fozzmoo	This tells your shell what interpreter to use.	15:32
fozzmoo	So, if your Perl interpreter isn't at /usr/bin/perl, replace that accordingly.	15:33
fozzmoo	If you're using Fedora, RHEL, CentOS, etc. it probably is in /usr/bin	15:33
* fozzmoo beeps and moves to Slide #7		15:33
fozzmoo	So, you save your script in a file. Call it what you want, but typically they're named with a .pl extension.	15:34
fozzmoo	To run it, you can simply give the filename to the interpreter as a command line parameter.	15:34
fozzmoo	i.e. perl filename.pl	15:34
fozzmoo	If you want it to be a standalone executable, then you do what you would do with a shell script or other script. Make sure you have the necessary magic line at the top of the file, assign execute permissions to the file, and then you can run it from the command line by itself (granted it's in your path, or you use ./cruel.pl)	15:35
fozzmoo	Speak up if you are wondering anything... I'm moving on.	15:36
* fozzmoo beeps and moves to Slide #8		15:36
fozzmoo	Rules: Case sensitive, semicolons at the end of most statements.	15:36
fozzmoo	Comments begin with #	15:36
fozzmoo	(like in shell scripts)	15:36
fozzmoo	Indentation isn't part of the syntax like Python, but it is highly recommended. :)	15:37
* fozzmoo beeps and moves to Slide #9		15:37
fozzmoo	Now we get into some of the meat of Perl.	15:37
fozzmoo	Unlike C or Java, Perl is loosely typed.	15:37
fozzmoo	That will change somewhat with Perl6, but for now, Perl variables are broken down into three types: scalars, lists, and hashes.	15:38
fozzmoo	Scalars contain any singular data which can be a number, a character, a string.	15:38
fozzmoo	Lists are ordered collections of scalars.	15:38
fozzmoo	Hashes are indexed collections of scalars.	15:39
heftig	so lists have no holes?	15:39
fozzmoo	holes?	15:39
fozzmoo	Could you ellaborate a bit, heftig?	15:39
heftig	well, empty items	15:40
fozzmoo	No. They do not.	15:40
heftig	okay	15:40
fozzmoo	So, the way you access data in a variable is using context.	15:40
fozzmoo	The $ is "scalar context"	15:40
fozzmoo	So, $x is a variable that contains scalar data.	15:41
fozzmoo	The @ is "list context"	15:41
fozzmoo	@x is a list and $x[2] is the scalar at the 3rd element in @x	15:41
fozzmoo	(starting at zero)	15:41
fozzmoo	Finally, % is "hash context"	15:42
fozzmoo	%x is a hash and $x{name} is the value stored in %x referenced by the key "name"	15:42
fozzmoo	I think PHP calls this a dictionary.	15:42
* fozzmoo beeps and moves to Slide #10		15:43
* fozzmoo beeps and moves to Slide #11		15:43
fozzmoo	Couple notes about lists...	15:43
fozzmoo	the 'scalar' function, given a list variable, will return the number of elements in the list.	15:44
baconfork	?	15:44
fozzmoo	Is that weird?	15:44
fozzmoo	So, if you have @x	15:44
fozzmoo	You can call $num = scalar @x;	15:45
fozzmoo	And $num will contain the number of elements in @x	15:45
delhage	?	15:45
Ritmo2k	I thought $var returned the size of @var if it exists?	15:45
fozzmoo	Another way of looking at it is that the 'scalar' function forces Perl to look at the list in scalar context.	15:45
delhage	Are command line arguments passed in as a list?	15:46
baconfork	how does push/pop relate to shift/unshift?	15:46
fozzmoo	Ritmo2k: No, $var and @var are two different variables (but $var[0] is a specific element in @var)	15:46
fozzmoo	delhage: It depends on which end of the list you're working with.	15:46
delhage	ok....	15:47
fozzmoo	delhage: See 'perldoc -f push'	15:47
fozzmoo	push "pushes values onto the end of the list"	15:47
fozzmoo	Whereas unshift pushes values onto the beginning.	15:47
fozzmoo	It's handy because you can treat your list as a queue or a stack right out of the chute.	15:48
delhage	fozzmoo: I think that was re baconfork's question?	15:48
fozzmoo	delhage: You're right. Sorry.	15:48
Ritmo2k	Yea, that was wrong, but when do you use 'scalar' and not just $size = @array ?	15:48
fozzmoo	baconfork: That was for YOU!	15:48
baconfork	I got now	15:48
fozzmoo	Ritmo2k: For readability. :)	15:48
Ritmo2k	Ahh	15:48
delhage	I got really confused for a second ;)	15:48
fozzmoo	delhage: To answer YOUR question...	15:48
fozzmoo	delhage: Yes, command line parameters are passed in as a list.	15:48
fozzmoo	I'll talk about that in a sec.	15:48
delhage	ok	15:49
* fozzmoo beeps and moves to slide #12		15:49
fozzmoo	Hashes are the weirdest thing for people learning Perl.	15:49
fozzmoo	usually.	15:49
fozzmoo	One way to imagine them is that they're lists but instead of using integers to reference data, you can use strings.	15:49
fozzmoo	As such, they're not ordered.	15:49
fozzmoo	They can be treated as a set.	15:50
fozzmoo	On slide #12, there's an example of how we would populate a list.	15:50
fozzmoo	%student = (name1 => "value1", name2 => "value2", ...);	15:50
fozzmoo	And then, to access a specific value, we use the name, or "key" as a reference:	15:51
fozzmoo	$student{'student_no'}	15:51
fozzmoo	(and we do so in scalar context)	15:51
* fozzmoo beeps and moves to slide #13		15:51
Ritmo2k	So hashes are _only_ pairs: Keys/Values, but the values can be anything?	15:51
fozzmoo	Ritmo2k: yes. The values are just scalars.	15:52
fozzmoo	As are the keys	15:52
fozzmoo	And, if you treat a hash in list context, it's just a list of scalars where the odd scalars are keys and the even scalars are values.	15:52
fozzmoo	(Sorry- that's probably weird to comprehend.)	15:53
Ritmo2k	Nah, but a hash of hashes wouldnt look good that way, right?	15:53
fozzmoo	You can do hashes of hashes, but what you need to do is use what's called a 'reference'	15:53
fozzmoo	and I wasn't planning on talking about that this time.	15:53
fozzmoo	But a reference is akin to a "pointer" in C.	15:53
fozzmoo	it lets you treat a hash as a scalar.	15:54
fozzmoo	Once you have a way of accessing a whole hash via a scalar, you can store it as a value in a hash.	15:54
fozzmoo	Instant hash of hashes.	15:54
heftig	similar to php's "variable variables"?	15:54
fozzmoo	$my_hash{key} = \%otherhash;	15:54
fozzmoo	heftig: I'm not that familiar, but probably.	15:54
fozzmoo	Okay. I'm just going to keep going until 23:00 and I'll take question as I go. :)	15:55
fozzmoo	The 'keys' function returns the keys (of the key-value pairs) of a hash.	15:56
fozzmoo	it returns them as a list.	15:56
fozzmoo	but you're not guaranteed any kind of order to them.	15:56
fozzmoo	So: @keys = keys %hash;	15:56
fozzmoo	The example of #13 shows how we can use a foreach loop to iterate over that list.	15:56
fozzmoo	A little preview of a future slide.	15:56
* fozzmoo beeps and moves to Slide #14		15:57
fozzmoo	Default variables are another weird thing in Perl.	15:57
fozzmoo	A little bewildering for people who are unaccustomed.	15:57
fozzmoo	Generally, I would advise not using default variables because they present a readability issue.	15:57
fozzmoo	but you often need to.	15:57
* fengshaun wonders where the slides are		15:57
fozzmoo	http://www.xmission.com/~fozz/perl_basics.pdf	15:58
fengshaun	thanks!	15:58
fozzmoo	fengshaun: Just wrapping up, but there you go.	15:58
fengshaun	yup	15:58
fozzmoo	delhage asked earlier about command line arguments.	15:58
fozzmoo	They are passed in via the default list: @_	15:58
fozzmoo	So, if you look at the default list at the top of your script, you'll be looking at parameters passed in from the command line.	15:59
fozzmoo	If you had a filename as a parameter, you could say: $filename = shift @_;	15:59
fozzmoo	Now, you don't have to give @_ to the shift function. if you just say shift; then it will assume you want it to do the shift operation on the default list.	15:59
* fozzmoo beeps and moves to #15		16:00
fozzmoo	15-17 talk about quoting rules.	16:00
fozzmoo	Fairly similar to shell, actually.	16:00
* nirik notes we are at the end of the timeslot... but you can go over if you like as this is the last class today. ;)		16:00
fozzmoo	interpolation inside double quotes.	16:00
delhage	is that like shift in shell scripting?	16:00
delhage	right	16:00
* delhage has a bit of a lag		16:00
fozzmoo	delhage: yeah, in this specific case, it is.	16:00
delhage	ok	16:01
fozzmoo	single quotes - no interpolation. So if you put variables inside the quotes, you'll get the literal string back without the variable value.	16:01
fozzmoo	Example is shown on #17	16:01
* fozzmoo beeps and slides to #18		16:01
fozzmoo	Back quotes are command substitution.	16:01
fozzmoo	Just like in shell.	16:02
fozzmoo	Nice way to grab the output of a command and assign to a scalar.	16:02
fozzmoo	Finally, some control statements.	16:02
* fozzmoo beeps and moves to slide #19		16:02
fozzmoo	if-else-elsif	16:02
fozzmoo	Prior to Perl 5.10, there was nothing like a switch or case statement in Perl...	16:03
fozzmoo	so nested if-elsif structures were really the only way to do what you'd normally do with a case/switch structure.	16:03
delhage	oh	16:03
fozzmoo	But Perl 5.10 has a given-when structure.	16:03
* fozzmoo beeps and moves to #20		16:03
delhage	when was 5.10 released?	16:04
fozzmoo	In the last year or so.	16:04
delhage	ok	16:04
fozzmoo	It's in Fedora 8 and forward, I believe...	16:04
Ritmo2k	Heh, perl monks sugegst slide 21 is the wrong way to do this: http://www.perlmonks.org/?node_id=46769	16:04
fozzmoo	but RHEL5 and variants are still on Perl 5.8	16:05
delhage	yeah, looks wrong	16:05
fozzmoo	you mean slide #22?	16:05
Ritmo2k	err yeah	16:06
delhage	oh	16:06
fozzmoo	The for loop in Perl is really no different than foreach()\	16:06
fozzmoo	they just iterate over lists.	16:06
fozzmoo	The C-style for(init; condition; increment) is supported.	16:07
fozzmoo	But it's not really the "Perl way"	16:07
fozzmoo	Ritmo2k: thanks for pointing out how useful PerlMonks.org can be!	16:07
Ritmo2k	lol	16:07
delhage	did you skip #21?	16:07
fozzmoo	Kinda did, didn't we?	16:08
fozzmoo	#21 just talks about comparison operators.	16:08
fozzmoo	I think it's pretty self-explanatory.	16:08
fozzmoo	One thing I didn't include is file tests...	16:08
fozzmoo	similar to what shells provide.	16:08
delhage	so it is totally the opposite of the test function in shell scripting	16:08
delhage	?	16:08
fozzmoo	So you can do if(-f $file)	16:09
delhage	where == is equality between strings	16:09
fozzmoo	delhage: yeah. it is different.	16:09
delhage	and -eq between numbers	16:09
fozzmoo	It's more like... Fortran. :)	16:09
delhage	damn...	16:09
delhage	that's confusing	16:09
fozzmoo	Shell deviated from C. :)	16:09
delhage	well, fortran doesn't have -eq ;)	16:10
delhage	or does it?	16:10
* delhage hasn't written fortran in 15 years		16:10
fozzmoo	It's been too long. I was going to say VMS DCL, but nobody would know what I was talking about.	16:10
fozzmoo	I	16:10
fozzmoo	I'll end there...	16:10
delhage	ok, the slide is correct then	16:11
@nirik	thanks for the class fozzmoo !	16:11
heftig	thanks for the class, fozzmoo	16:11
fozzmoo	and let everyone read on to the built-in functions on slide #25	16:11
fengshaun	thanks fozzmoo!	16:11
Ritmo2k	Cheers!	16:11
fozzmoo	you're welcome!	16:11
fozzmoo	I'd love to come back and talk about Perl regex.	16:11
Ritmo2k	I would so be in on that!	16:11
SSlater	Given I know nothing of Perl bar the last hour, can Perl be used to interrogate a USB RS232 device to aquire data?	16:11
fozzmoo	if there's interest, put it in the classroom wiki.	16:11
-!- nirik changed the topic of #fedora-classroom to: Fedora IRC Classroom - Next class at 18:00 UTC 2009-03-08 - See Classroom for class schedule and more info.		16:11
heftig	i still still confusing pcre with other syntaxes	16:11
fengshaun	that just made me to add Perl to my language inventory	16:11
delhage	thanks fozzmoo, but I have one more question if that's OK?	16:11
fozzmoo	SSlater: Sure. Check CPAN. cpan.org. :)	16:11
fozzmoo	delhage: Sure.	16:11
* Ritmo2k polsihing off oreilly Sed&Awk and RegEx books on desk		16:12
SSlater	Thanks fozzmoo, great class.	16:12
* herlo points out that this is the last class so feel free to linger and discuss		16:12
fengshaun	fozzmoo: yea, please come back for regex!	16:12
delhage	fozzmoo: I'm not really sure how to pose the question though... ;)	16:12
fozzmoo	delhage: Think about it and e-mail me later. :)	16:12
delhage	fozzmoo: about CPAN and package based distros like fedora	16:12
fozzmoo	SSlater: yw	16:12
fozzmoo	delhage: Ahhh.	16:12
fozzmoo	Yeah. There are a couple programs that will build RPMs for you.	16:13
fozzmoo	cpan2rpm	16:13
fozzmoo	and cpanspec	16:13
delhage	fozzmoo: you probably see where I am going?	16:13
fozzmoo	And another one called cpanflute.	16:13
fozzmoo	I prefer to build all my CPAN packages as RPMs.	16:13
fozzmoo	It can be a pain, but it's worth it.	16:13
fozzmoo	Fortunately, for Fedora, most of the good packages are in standard repos.	16:13
delhage	I know from experience that it can be a real hell to use a mix of rpms and modules installed via the CPAN shell	16:13
heftig	php introduced me to regexps, so i'm most comfortable with the perl-compatible syntax. any others still confuse me :)	16:13
Ritmo2k	fozzmoo, when using cpan2rpm, how do you handle the sometimes seemingly endless recursive loop of deps when building out stuff?	16:13
fozzmoo	Ritmo2k: perseverance	16:14
delhage	fozzmoo: thanks	16:14
Ritmo2k	LOL	16:14
Ritmo2k	is there no frontend to recursively check for deps?	16:14
fozzmoo	That's another thing I've blogged about - it's a barrier to entry for Perl.	16:14
fozzmoo	Ritmo2k: Not that I know of, but I would hope someone resolves that.	16:14
fozzmoo	Some people have told me they've hacked stuff together to do it...	16:15
fozzmoo	but they weren't comfortable releasing it to the world.	16:15
fozzmoo	I wish they would. :)	16:15
fozzmoo	Thanks all for attending. Good questions.	16:15
delhage	thank you	16:15
fengshaun	thanks fozzmoo	16:16
fozzmoo	yw	16:16
fozzmoo	yvvvvw	16:16
Ritmo2k	foxx, Ill be keeping an eye out for the regex class:)	16:16
Ritmo2k	fozz	16:16
fengshaun	me too!	16:17

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