plunging into perl while avoiding the deep end (mostly)

Plunging Into Perl While Avoiding the Deep End

(mostly)

Some Perl nomenclaturePERL – Practical Extraction and Report

Language

PERL – Pathologically Eclectic Rubbish Lister (if you’re so inclined)

Language

PERL – Pathologically Eclectic Rubbish Lister (if you’re so inclined)

TMTOWTDI – There’s More Than One Way To Do It

Some Perl attributesit’s a scripted language, not compiled - faster, easier development runs plenty fast for most things

Loose variable typing - both good and bad, but mostly good

Your first program

#!/usr/local/bin/perl

print "Hello, World\n";

“Protecting” your program (Unix)

By default, your program is not executable.

chmod 744 your_program

You can execute it as owner of the file, anyone else can only read it.

Variables

can be text or number: a character,a whole page of text,or any kind of number

context determines type

can go “both” ways

Variables, array of

@employee

Array of $employee variables

$employee[0]

$employee[1]

Variables, hash of

$lib{‘thisone’} = “2 days”;

$lib{‘thatone’} = “5 days”;

Thus can use

$grace_period = $lib{$libname}

when $libname is thatone,

$grace_period is 5 days

Variables, list of

($var1, $var2, $var3) =function_that_does_something;

This function returns a list of elements.

A list is always inside parentheses ().

Variables, assigning a value to

$var = value or expression

$array[n] = something;

@array = (); # empty array

%hash = (); # empty hash

Can be done almost anywhere, anytime.

Variable scope, and good practices

use strict;

Requires that you declare all variables like this:

my $var;

my $var = something;

my @array = ();

Also makes Perl check your code.

Best Practices!

Variable scope, and good practices

use strict;

my $var;

my @array = ();

A variable declared like this is visible throughout your program.

Best Practices!

Variable scope, and good practicesuse strict;

my $var;

my @array = ();

A “my” declaration within code grouped within { and } is visible only in that section of code; it does not exist elsewhere.

Best Practices!

Scope: where in a program a variable exists.

File input and output (I/O)

Using command line arguments

Usage: program.pl infile outfile $ARGV[0] $ARGV[1]

String manipulation & other stuff

substring function

a better substring example

index function, find the location of a string in a string

The split function. Here we split string $l into pieces at every space character.

Less common usage: take only 1st 2 pieces.

Actually, the 2nd statement should be: $l =~ s/^ +//;

The ^ means start looking at the start of the line.

find “db ratio” anywhere in $l

Instead of using $inline[n], $inline[n+1], etc., to refer to elements of array @inline, here we can refer to @inline’s elements via $l in this example. Often makes for clearer and simpler code.

An often convenient way of populating an array.

String manipulation & other stuffGiven

$stuff = “this is me”;

These are not equivalent:

“print $stuff”

‘print $stuff’

`print $stuff`

“print $stuff” is “print this is me”

‘print $stuff’

`print $stuff`

“print $stuff” is “print this is me”

‘print $stuff’ is ‘print $stuff’

`print $stuff`

String manipulation & other stuffGiven $stuff = “this is me”;

`print $stuff` would have the operating system try to execute the command <print this is me>

String manipulation & other stuffThis form should be used as$something = `O.S. command`

Example: $listing = ‘ls *.pl`;

The output of this ls command is placed, as possibly a large string, into the variable $listing. This syntax allows powerful processing capabilities within a program.

printf, sprintf

printf(“%s lines here”, $counter)

if $counter is 42, we get

42 lines here

for the output

printf, sprintf

printf(“%c lines here”, $counter)

if $counter is 42, we get

* lines here

for the output, since 42 is the ASCII value for “*”, and we’re printing a character

printf, sprintf

Some additional string formatting…

%s – output length is length($var)

%10s – output length is absolutely 10 (right justified)

%10.20s – output length is min 10, max 20

%-10.10s – output length is absolutely 10 (left justified)

Any padding is with space characters.

printf, sprintf

Some additional number formatting…

%d – output length is length($var)

%10d – output length is absolutely 10 (leading space padded)

%-10d – left justified, absolutely 10 (trailing space padded)

%-10.10d – right justified, absolutely 10 (leading zero padded)

printf, sprintf

Still more number formatting…

%f – output length is length($var)

%10.10f – guarantees 10 positions to the right of the decimal (zero padded)

printf, sprintf

printf whatever outputs to the screen

printf, sprintf

printf file whatever outputs to that file

Ex: printf file (“this is %s fun\n”, $much);

(print functions just like the above, as to output destination.)

printf, sprintf

printf file whatever outputs to that file

Ex: printf file (“this is %s fun\n”, $much);

(print functions just like the above, as to output destination.)

sprintf is just like any printf, except that its output always goes to a string variable.

Ex: $var = sprintf(“this is %s fun\n”, $much);

ratiocheck.pl, what it does

When the ratio of sizes of certain files related to a database exceeds a threshold, it’s probably time to do an index regen on that database.

ratiocheck.pl, what it does

When the ratio of sizes of certain files related to a database exceeds a threshold, it’s probably time to do an index regen on that database.

This program computes these ratios for several databases, each with its own threshold, and flags those that are candidates for index regeneration.

program dissection – ratiocheck.pl

set up some variables

two of these are templates for printing

In line 3 above, a file is slurped, i.e., the entire file is read into an array via the <> mechanism.

This is a more typical use of the split function. Here, $item is separated into two pieces at the “|” character.

We want to check every database in alphabetical order. We are then calling the checkit subroutine for each database.

The system function executes its string as an O.S. command. Here we are mailing a file to two different people.

This subroutine takes 1 argument.

Remember our generic templates? Here they are used as a format string for the sprintf function.

$generic_path = "/m1/voyager/%s/data/";

The –s test returns a file’s size.(There are several dozen different –x file tests.)

Compute the files’ size ratio with sufficient decimal places.

\n means new line, loosely equivalent to a CR, or carriage return.

Since we want to print the “%” character, we have to escape it with the “\” backslash.

Here we have a hash reference… we are checking if the ratio is greater than the threshold for the current database.

This is a busy printf statement…

the alert text gets a string, a character, and a string embedded in it.

The first argument is a string, which is the output of the sprintf statement, which outputs the threshold value for this database.

The second argument is a character. We print the “%” character, whose ASCII value is 37.

The third argument is a string. In this case, the string consists of 35 asterisks.

A string followed by “xN” will occur N times.

ratiocheck.pl, output

Here’s what the output looks like:

DBI stuffWhat is it and why might I want it?

DBI is the DataBase Interface module for Perl. You will also need the specific DBD (DataBase Driver) module for Oracle.

This enables Perl to perform queries against your Voyager database.

Both of these should already be on your Voyager box.

DBI stuff, how to

You need four things to connect to Voyager:

machine name your.machine.here.edu

username your_username

password your_password

SID VGER (or LIBR)

DBI stuff, how to

$dbh is the handle for the database

$sth is the handle for the query

Create a query…then execute it.

NOTE: SQL from Access will most likely NOT work here!

DBI stuff, how to

Get the data coming from your query.

DBI stuff, how to

You’ll need a Perl variable for each column returned in the query.

Commonly a list of variables is used; you could also use an array.

DBI stuff, how to

You’ll need a Perl variable for each column returned in the query.

Commonly a list of variables is used; you could also use an array.

Typically, you get your data in a while loop, but you could have

$var = $sth->fetchrow_array;

when you know you’re getting a single value.

DBI stuff, how to

When you’re done with a query, you should finish it. This becomes important when you have multiple queries in succession.

You can have multiple queries open at the same time. In that case, make the statement handles unique…$sth2, or $sth_patron.

Finally, you can close your database connection.

Comprehensive Perl Archive Network

http://cpan.org

You name it and somebody has probably written a Perl module for it, and you’ll find it here.

There are also good Perl links here; look for the Perl Bookmarks link.

Installing modules

You need to be root for systemwide installation on Unix systems.

On Windows machines, you’ll need to be administrator.

You can install them “just for yourself” with a bit of tweaking, and without needing root access.

If you’re not a techie, you’ll probably want to find someone who is, to install modules.

Installing modules is beyond the scope of this presentation.

Perl on your PC

You can get Perl for your PC from ActiveState.

They typically have two versions available; I recommend the newer one. Get the MSI version.

Installation is easy and painless, but it may take some time to complete.

A lot of modules are included with this distribution; many additional modules are available. Module installation is made easy via the Perl Package Manager (PPM). Modules not found this way will require manual installation, details of which are beyond the scope of this presentation.

Date and Time in Perl, basic

### "create" today's date

my ($sec, $min, $hour, $day, $month, $year, $wday, $yday, $isdst) = localtime;

This gets the date and time information from the system.

Date and Time in Perl, basic

### "create" today's date

my ($sec, $min, $hour, $day, $month, $year, $wday, $yday, $isdst) = localtime;

my $today = sprintf ("%4.4d.%2.2d.%2.2d", $year+1900, $month+1, $day);

This puts today’s date in “Voyager” format, 2006.04.26

Date and Time in Perl

The program, datemath.pl, is part of your handout. The screenshot below shows its output.

Regular expressions, matching

m/PATTERN/gi

If the m for matching is not there, it is assumed.

The g modifier means to find globally, all occurrences.

The i modifier means matching case insensitive.

Modifiers are optional; others are available.

Regular expressions, substituting

s/PATTERN/REPLACEWITH/gi

The s says that substitution is the intent.

The g modifier means to substitute globally, all occurrences.

The i modifier means matching case insensitive.

Regular expressions, translating

tr/SEARCHFOR/REPLACEWITH/cd

The tr says that translation is the intent.

The c modifier means translate whatever is not in SEARCHFOR.

The d modifier means to delete found but unreplaced characters.

Regular expressions

Look in the Perl book (see Resources) for an explanation on how to use regular expressions. You can look around elsewhere, at Perl sites, and in other books, for more information and examples.

Looking at explained examples can be very helpful in learning how to use regular expressions.

(I’ve enclosed some I’ve found useful; see Resources.)

Regular expressions

Very powerful mechanism.

Often hard to understand at first glance.

Can be rather obtuse and frustrating!

If one way doesn’t work, keep at it. Most likely there is a way that works!

Resources

Advanced Perl Programming

Learning Perl

Perl in a Nutshell

Programming Perl

Perl CookbookPerl Best Practices

I use these

two a lot

Highly recommended once you’re experienced.

These are all O’Reilly books.

ResourcesCPAN

http://cpan.org

Active State Perl

http://activestate.com/Products/Download/Download.plex?id=ActivePerl

The files listed below are available at

http://homepages.wmich.edu/~zimmer/files/eugm2006

datemath.plsome program code for math with dates

snippet.grepvarious regular expressions I’ve found useful

Plunging Into Perl.pptthis presentation

Thanks for listening.

Questions?

roy.zimmer@wmich.edu

269.387.3885

plunging into perl while avoiding the deep end (mostly)

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stuff index function

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