Intro to Perl - Day 2

Goals

• manipulate <STDIN> in an @array context
• manipulate command line arguments
• look at the major control structures
• manipulate $scalars, @arrays, and %hashes and their elements
• open, manipulate, lock, and close filehandles
• open, read, and close directory handles

Review

POST

1. What should be on the first line of your script?
2. Is there a useful switch that shows you verbose warnings?
3. How is an @array different from a %hash?
4. Extra Credit: How do permissions on scripts differ from permissions on binary executables?

manipulate <STDIN> in an @array context

We can do a similar thing by reading the input into an @array context: @input = <STDIN>;
The method of getting the actual input into the array differs a bit, as this context expects several lines terminated by an EOF marker [CTRL-D]. You can do this manually or pipe the text to your script.

manipulate command line arguments

$0 would contain "myscript.pl"
@ARGV would contain the entire array of passed args
$ARGV[0] would contain "first"
$ARGV[1] would contain "second"
$ARGV[2] would contain "third"
$#ARGV would contain the subscript of the last element

• verify that ($#ARG + 1) >= (minimum args)
• or... check for the existence of $ARGV[$min - 1]

You can also use an interesting function of the @ARGV array to specify files as sources for STDIN: pass them on the command line and then use bizarre-looking diamond operator <> in a loop. See p 73 in your text for more info.

Control structures

if/unless

if (true_expression) {
	do your thing;
}

elsif (some_other_expression) {
	another thing;
}

else {
	final option;
}

while/until

while (true_expression) {
	do this;
}

for

for (starting_value; evaluation; re-initialize value) {
	do this;
}

*foreach*

foreach $temp (@array/list) {
	do this with $temp;
}

Misc

• there is no "case" structure
• "next;" to crash out of a particular iteration
• "last;" to crash out of a loop/block
• "redo;" to start the current loop/block again
• You can label blocks if you like:

  
  EACHLINE: while (<MYFILE>) {
  	do this;
  	do that;
  	if (something specific happens) {
  		last EACHLINE; #silly here, but could be useful in nests
  	}
  }
  

manipulate $scalars, @arrays, and %hashes.

$scalars

• chop() and chomp()
• forcing uppercase, lowercase character or word
• substr(var, offset [, length])
• split(/[delimiter]/, $scalar)
• join('[delimiter]', [list or @array]) (use pack and unpack for fixed-length binary fields)
• substitution: $var =~ s/[regex]/[replacement]/gi
• matching: $var =~ /[regex]/

@arrays

• sort()
• rev() also works on scalars
• push(@array, list) onto end of stack, pop(@array) from end of stack
• shift(@array) from front of stack, unshift(@array, list) from front of stack
• splice() for more complex stack operations
• array slices @array[element..element]

%hashes

• keys(%hash) returns keys as an @array
• values(%hash) returns values (might not be unique)
• each(%hash), returns a two-element hash with each iteration
  while (($key,$value) = each (%hash)) from pp 160 of the camel book

open, manipulate, lock, and close filehandles

• slurp the whole file up, close the filehandle and work with the whole thing in memory (small files, frequent accesses, unstable system)
• hold the filehandle open while you iterate through it (larger files, less frequent "hits", stable system)

open the file handle

open(FILEHANDLE, [mode]file);
open(DATA, "</home/jason/clients.txt"); 	#open for input
open(DATA, "/home/jason/clients.txt"); 		#same!
open(DATA, ">/home/jason/clients.txt"); 	#open for output 
open(DATA, ">>/home/jason/clients.txt"); 	#open for append
open(DATA, "+>/home/jason/clients.txt"); 	#read and write; seek();

An idiomatic example

#define mnemonics for lock 
$FLOCK_SHARED=1;
$FLOCK_EXCLUSIVE=2;
$FLOCK_NOFAIL=4; #don't wait, even if you can't 
$FLOCK_RELEASE=8;

#set a $scalar with the filename in it for convenience
$filename="/home/jason/clients.txt";

#open the file or die trying
#note use of the $! special var that indicates OS error
open(DATA, ">>$filename") or
	die ("could not open $filename: $!"); 

#get an exclusive lock
flock(DATA, $FLOCK_EXCLUSIVE);

#do your business here
# YOUR CODE HERE
# like print DATA ("This is some new information.\n");


#close the filehandle
#automagically releases the advisory lock
close(DATA);

open, read, and close directory handles

• opendir(DATADIR, "$somedir") or die $!;
• @allfiles=readdir(DATADIR);
  • from the camel book. Avoid the "." and ".." files thusly: @allfiles=grep (!/^\.\.?$/, readdir(DATADIR) );
• closedir(DATADIR);

open, close named pipes

$mailprog="/usr/lib/sendmail -t";

open(MAIL, "|$mailprog") or die $!;

#this is a "here" document
print MAIL <<end; 
To: $some_fellow
From: $my_email
Subject: Check this out!

$message
end
#the marker needs to be against the left margin, even if this is an indented loop

close(MAIL); #the mail goes out!

Homework

• report formatting 116-128
• special printing formats
• subroutines
• scope