Preliminaries

• Evaluation
• Homeworks, etc.

Remarks on coding and debugging

• Halting problem "repeat forever"
• Proofing programs correct

Debugging

• Compile time errors vs. Run time errors
  • Compile time errors : syntax errors and inconsistent use of typed data elements:
  • Run-time errors : data dependency, exceeding capacities, invalid operations after changes of state
• Tracing execution
• Error messages and how to interpret them

History and background of PERL

• C shell, C, sed, awk, grep, and Unix hacking
• System administrators dream

Our interest in PERL

• String handling and text manipulation
• Source of "canned" code for virtually everything.

Sources of PERL info

• Online PERL Manual
• A bunch of PERL resources : FAQ, list of other sites
• PERL - Camel Book examples.

PERL syntax and semantics

Getting started : "console I/O"

#!/usr/bin/perl
print "Enter Hexadecimal Number: ";     # Ask for a number.
$answer = ;                         # Input the number.
print hex($answer),"\n";                # Print out new number.

Statements

        if EXPR
        unless EXPR
        while EXPR
        until EXPR

Flow control in PERL

        do {
                $_ = ;
                ...
        } until $_ eq ".\n";

Let's read some more PERL!

   # a simpleminded Pascal comment stripper
   # (warning: assumes no { or } in strings)
   line: while () {
           while (s|({.*}.*){.*}|$1 |) {}
           s|{.*}| |;
           if (s|{.*| |) {
                   $front = $_;
                   while () {
                           if (/}/) {      # end of comment?
                                   s|^|$front{|;
                                   redo line;
                           }
                   }
           }
           print;
   }

Labels in PERL

     line: while () {
             next line if /^#/;      # discard comments
             ...
     }

Variables

• Filehandles
• $ : Scalar variables
• @ : "normal" array
• % : associative arrays
• * :

Data conversion

Building arrays from strings

               while (<>) {
                       chop;   # avoid \n on last field
                       @array = split(/:/);
                       ...
               }

Both of these do the same thing...

               do 'stat.pl'; 
               eval \`cat stat.pl\`;

          s/\n// ;
         chop ;

String functions

chop
eval
crypt
index
rindex
length
q
substr

Arrays and list functions

delete
each
grep
keys
join
pop
push
reverse
shift
sort

A couple examples

    @foo = grep(!/^#/, @bar);    # weed out comments

    $_ = join(':',
                $login,$passwd,$uid,$gid,$gcos,$home,$shell);

Pattern matching

m/PATTERN/gio
/PATTERN/gio
?PATTERN?
s/PATTERN/REPLACEMENT/gieo

Some examples:

           s/\bgreen\b/mauve/g;                # don't change wintergreen
           ($foo = $bar) =~ s/bar/foo/;
           $_ = 'abc123xyz';
           s/\d+/$&*2/e;               # yields 'abc246xyz'
           s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
           s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

study
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds

           $ARGV[1] =~ y/A-Z/a-z/;     # canonicalize to lower case
           $cnt = tr/*/*/;             # count the stars in $_
           $cnt = tr/0-9//;            # count the digits in $_
           tr/a-zA-Z//s;       # bookkeeper -> bokeper
           ($HOST = $host) =~ tr/a-z/A-Z/;
           y/a-zA-Z/ /cs;      # change non-alphas to single space
           tr/\200-\377/\0-\177/;# delete 8th bit

Format strings

# a report on the /etc/passwd file
format STDOUT_TOP =
                        Passwd File
Name                Login    Office   Uid   Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name,              $login,  $office,$uid,$gid, $home
.

$_ The default input and pattern-searching space.

               while (<>) {... # only equivalent in while!
               while ($_ = <>) {...

               /^Subject:/
               $_ =~ /^Subject:/

               y/a-z/A-Z/
               $_ =~ y/a-z/A-Z/

               chop
               chop($_)

               $foo{$a,$b,$c}
               $foo{join($;, $a, $b, $c)}

Special Variables

$ARGV

contains the name of the current file when reading from <>.

@ARGV

The array ARGV contains the command line arguments

$_

The default input and pattern-searching space.

$!

If used in a numeric context, yields the current value of errno

$#

output format for printed numbers.

$%

current page number

$&

string matched by the last successful pattern match (not counting any matches hidden within a

$'

string following whatever was matched by the last successful pattern match

$*

Set to 1 to do multiline matching within a string, 0 to tell perl that it can assume that strings contain a

$+

last bracket matched by the last search pattern.

$,

output field separator for the print operator.

$-

number of lines left on the page of the currently selected output channel.

$.

current input line number of the last filehandle that was read.

$/

input record separator, newline by default.

$0

Contains the name of the file containing the perl script being executed.

$:

current set of characters after which a string may be broken

$;

subscript separator for multi-dimensional array emulation.

$

$=

current page length

$@

perl syntax error message from the last eval command. $ARGVcontains the name of the current file when reading from <>.

$^L

What formats output to perform a formfeed.

$~

name of the current report format for the currently selected output channel. Default is name of the

$^

name of the current top-of-page format

$""This is like $, except that it applies to array values interpolated into a double-quoted string

$^T

time at which the script began running, in seconds since the epoch. $^W

current value of the warning switch.

$[

index of the first element in an array, and of the first character in a substring. Default is 0, but you

$\

output record separator for the print operator. Ordinarily the print operator simply prints out the

$\`

string preceding whatever was matched by the last successful pattern match