Intermediate Perl - Day 1

Goals

• review variable scope
• review subroutine argument passing
• learn argument prototyping
• learn to use symbolic and hard references
• build anonymous structures
• learn to dereference
• use the perl debugger

review: variable scope

• global (actually in main::)
• my (preferred): scope is limited to the block or function
• local (deprecated): like my, only called functions can also see the variable. Use my except in those rare instances where it is too restrictive (mainly in formats).

review: subroutine passing

• passing by value vs. reference
• @arrays are greedy and will slurp up everything, given the chance:
  • pass the array at the end
  • limit the array with slice notation
  • pass a reference (see below)
  • use prototyping (see below)
• %hashes are flattened into 2D when passing; only one hash is easily passed (again, use a reference)
• incoming args found in @_ (the default array)

Practicum: subroutine passing

prototyping

sub Example {
	#do nifty stuff in here;
}

#	this sub requires two scalars and allows
#	for an optional array
#	note that all could be stored in one big list
sub Example ($$;@){
	#do nifty stuff in here;
}

• The above example would allow the scalars to be passed in an array (like @someArgs). To enforce that separate datatypes get passed, use a backslash in front of the marker: $$;\@
• the prototyping only works when you call the subroutines without the &.
• the prototype must precede the call, either by code or by require in the code before the sub is called.

references

writing better code

• use strict;
• use warnings;

perl debugger

• leverages behavior of gdb, etc
• call it with the -d switch
• waits before each statement
• h help
• p [expression] prints the value of [expression]
• V [package] [vars] prints out variables
• T stack backtrace: tells what a subroutine was called from
• s step into subroutines: next statement, including statements in subroutines
• n next statement: as above only does not enter subroutines
• [enter] repeats last step/next
• L List breakpoints.
• t toggle trace mode; turns on codeline printing
• b 10 break on line 10
• b $counter > 5 break when $counter is greater than 5
• b SUB foo break on invocation of &foo
• d 10 delete breakpoint on 10
• D delete all breakpoints
• W delete all watches
• W [var] set a watch on [var]

• man perldebug
• perl -Dr regular expression dissection

Homework

• namespace (packages)
• read comp.lang.perl

Remember, your login is based on your machine's hostname, not on any other number.
~/[initials] refers to the subdirectory under your homedir, named after your initials. Everything except for .dotfiles will be stored in your ~/[initials] directory.