Transcript Plunging Into Perl

Plunging Into Perl
While Avoiding the Deep End
(mostly)
Some Perl nomenclature
PERL – Practical Extraction and Report
Language
Some Perl nomenclature
PERL – Practical Extraction and Report
Language
PERL – Pathologically Eclectic Rubbish
Lister (if you’re so inclined)
Some Perl nomenclature
PERL – Practical Extraction and Report
Language
PERL – Pathologically Eclectic Rubbish
Lister (if you’re so inclined)
TMTOWTDI – There’s More Than One Way
To Do It
Some Perl attributes
it’s a scripted language, not compiled faster, easier development
runs plenty fast for most things
Some Perl attributes
it’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
$name
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]
etc.
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 $var = something;
my @array = ();
A variable declared like this is
visible throughout your program.
Best Practices!
Variable scope, and good practices
use strict;
my $var;
my $var = something;
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
String manipulation & other stuff
a better substring example
String manipulation & other stuff
index function, find the location of a string in a
string
String manipulation & other stuff
The split function. Here we split string $l into
pieces at every space character.
Less common usage: take only 1st 2 pieces.
String manipulation & other stuff
find “db ratio” anywhere in $l
Actually, the 2nd statement should be:
$l =~ s/^ +//;
The ^ means start looking at the start of
the line.
String manipulation & other stuff
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.
String manipulation & other stuff
An often convenient way of populating an array.
String manipulation & other stuff
Given
$stuff = “this is me”;
These are not equivalent:
“print $stuff”
‘print $stuff’
`print $stuff`
String manipulation & other stuff
Given
$stuff = “this is me”;
These are not equivalent:
“print $stuff” is “print this is me”
‘print $stuff’
`print $stuff`
String manipulation & other stuff
Given
$stuff = “this is me”;
These are not equivalent:
“print $stuff” is “print this is me”
‘print $stuff’ is ‘print $stuff’
`print $stuff`
String manipulation & other stuff
Given
$stuff = “this is me”;
`print $stuff` would have the
operating system try to execute the
command <print this is me>
String manipulation & other stuff
This 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 whatever outputs to the screen
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 whatever outputs to the screen
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
program dissection – ratiocheck.pl
In line 3 above, a file is slurped,
i.e., the entire file is read into an array
via the <> mechanism.
program dissection – ratiocheck.pl
This is a more typical use of the split
function. Here, $item is separated into
two pieces at the “|” character.
program dissection – ratiocheck.pl
We want to check every database in
alphabetical order. We are then calling the
checkit subroutine for each database.
program dissection – ratiocheck.pl
The system function executes its string as
an O.S. command. Here we are mailing a
file to two different people.
program dissection – ratiocheck.pl
This subroutine takes 1 argument.
program dissection – ratiocheck.pl
Remember our generic templates? Here
they are used as a format string for the
sprintf function.
$generic_path = "/m1/voyager/%s/data/";
program dissection – ratiocheck.pl
The –s test returns a file’s size.
(There are several dozen different –x file tests.)
program dissection – ratiocheck.pl
Compute the files’ size ratio with
sufficient decimal places.
program dissection – ratiocheck.pl
\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.
program dissection – ratiocheck.pl
Here we have a hash reference…
we are checking if the ratio is greater than
the threshold for the current database.
program dissection – ratiocheck.pl
This is a busy printf statement…
the alert text gets a string, a character,
and a string embedded in it.
program dissection – ratiocheck.pl
The first argument is a string, which is
the output of the sprintf statement, which
outputs the threshold value for this
database.
program dissection – ratiocheck.pl
The second argument is a character. We
print the “%” character, whose ASCII
value is 37.
program dissection – ratiocheck.pl
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 stuff
What 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
Get the data coming from your query.
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
Get the data coming from your query.
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 $sth_patron.
Finally, you can close your database
connection.
CPAN
Comprehensive Perl Archive Network
http://cpan.org
You name it and somebody has probably
written a Perl module for it, and you’ll find it
here.
There are also good Perl links here; look for
the Perl Bookmarks link.
CPAN
Installing modules
You need to be root for systemwide installation
on Unix systems.
On Windows machines, you’ll need to be
administrator.
You can install them “just for yourself” with a bit
of tweaking, and without needing root access.
If you’re not a techie, you’ll probably want to
find someone who is, to install modules.
Installing modules is beyond the scope of this
presentation.
Perl on your PC
You can get Perl for your PC from ActiveState.
They typically have two versions available; I
recommend the newer one. Get the MSI version.
Installation is easy and painless, but it may take
some time to complete.
A lot of modules are included with this
distribution; many additional modules are
available. Module installation is made easy via
the Perl Package Manager (PPM). Modules not
found this way will require manual installation,
details of which are beyond the scope of this
presentation.
Date and Time in Perl, basic
### "create" today's date
my ($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.
Modifiers are optional; others are
available.
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.
Modifiers are optional; others are
available.
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
Learning Perl
Perl in a Nutshell
I use
these
two
a
lot
Highly recommended once
you’re experienced.
Programming Perl
Perl Cookbook
Perl Best Practices
Advanced Perl Programming
These are all O’Reilly books.
Resources
CPAN
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.pl
some program code for math with dates
snippet.grep
various regular expressions I’ve found useful
Plunging Into Perl.ppt
this presentation
Thanks for listening.
Questions?
[email protected]
269.387.3885
Picture © 2005 by Roy Zimmer