CPTG286K Programming - Perl

CPTG286K Programming - Perl Chapter 1: A Stroll Through Perl Instructor: Denny Lin

Course Objectives • Learn to program in Perl • Use effective documentation techniques • Use clear programming style

Lecture 1 Outline • Hello world program • Storing keyboard input into a scalar variable • If-then-else string comparison • Storing and accessing arrays • Else if blocks • Storing and accessing hashes

Important UNIX commands • Use the pico editor to create PERL scripts: $ pico ex1.pl • Make sure PERL scripts have execute permissions $ chmod +x ex1.pl • Run PERL scripts by typing its full name $ ex1.pl

Hello, world! # This is the standard Hello, world! program # Call the PERL compiler using -w (warning) switch #!/usr/bin/perl -w print (“Hello, world!\n”);

Storing input from the keyboard • Get keyboard input with the <STDIN> construct • Store input in a scalar variable called $name Ex1.pl: #!/usr/bin/perl -w print “What is your name?”; # produce prompt $name = <STDIN>; # read keyboard chomp ($name); # rid trailing newline print “Hello, $name!\n”; # print output

If-then-else string comparison • Statement blocks appear within curly brackets { } • Use the eq operator to compare equality of two strings, and ne to determine inequality • Use clear indentation style • DO NOT put curly brackets in comment: if ($name eq “Randal”) { # this end of block is never read }

If-then-else comparison example Ex2.pl: #!/usr/bin/perl -w print “What is your name?”; # produce prompt $name = <STDIN>; # read keyboard chomp ($name); # rid trailing newline if ($name eq “Randal”) { print “Hello, Randal! How good of you to be here!\n”; } else { print “Hello, $name!\n”; } # ordinary greeting NOTE. The following DOES NOT work (why?): { print “Hello, $name!\n”; # ordinary greeting }

Arrays in PERL • Each element of an array stores scalar variables • Array variable names start with an @ during assignment. For example: @words = (“camel”, “llama”, “alpaca”); • Use the qw() operator to quote words in an array. For example: @words = qw(camel llama alpaca);

Accessing PERL arrays • Elements of an array are accessed as scalar variables: • $words[0] is camel • setting $i to 2, $words[$i] is alpaca

If-then-elsif-else block • Note that an else if block is spelled elsif: if ($words[$i] eq $guess) # is guess correct? { $correct = “yes”; # guess is correct } elsif ($i < 2) # more words to look at? { $i = $i + 1; # increment $i } else { print “Wrong, try again. What is the secret word?”; $guess = <STDIN>; # read keyboard chomp ($guess); # rid trailing newline $i = 0; }

Storing tables in a Hash • Hashes hold scalar values referenced by a key • Hash variables start with a % during assignment. For example: %words = qw( fred camel barney llama betty alpaca wilma alpaca );

Accessing table data in a Hash • Hash data elements are accessed as scalar variables, and addressed with curly brackets: • $words{“betty”} is alpaca • setting $person to betty, $words{$person} is alpaca

Lecture 2 Outline • String operations • Pattern match operator • Substitution operator • Translation operator • What Is Truth? • Subroutines • Filehandles

String Operations • Pattern match operator • Use the =~ operator to match strings • Use slashes to make sure white/spaces are significant • Use ^ to specify a “start with” pattern • Use \b to denote word boundary • Use /i to ignore case

Pattern Matching Example Ex3.pl #!/usr/bin/perl #Turn off warning: notice no –w switch […rest of program deleted …] chomp ($name); # rid trailing newline If ($name =~ /^randal\b/i) # Match names starting # with randal, use word # boundary, and ignore case { print “Hello, Randal! How good of you to be here!\n”; } […rest of program deleted …]

String Operations (cont’d) • Substitution operator • Use the s operator to perform substitutions delimited by slashes • Regular expressions specify from and to what the operator will substitute. From and to entries are separated by a slash

Substitution example • To substitute a string with another that doesn’t contain non-word characters • \W specifies all non-word characters • .* specifies characters to the end of string • A blank “to” entry substitutes all “from” entries • The following finds the first non-word character in $name, and substitutes them with blanks to the end of string $name =~ s/\W.*//;

String Operations (cont’d) • Translation operator • Use the tr operator to perform translations delimited by slashes • Regular expressions specify from and to what the operator will translate. From and to entries are separated by a slash

Translation example • To translate a list of uppercase characters into lowercase: • Specify the A-Z list in the “from” entry • Specify the a-z list in the “to” entry • The following turns any uppercase characters in $name into lowercase characters $name =~ tr/A-Z/a-z/;

What Is Truth? • In PERL: • Any string is true except for “” and “0” • Any number is true except 0 • Any reference is true • Any undefined value is false From “Programming Perl” 3rd Edition Page 29-30

Subroutines • Defined using sub subroutine_name { } • The my() operator defines private parameters stored in the @_ local array • Subroutines return values using the return statement

Subroutine example sub good_word { my($somename, $someguess) = @_; # name of parameters $somename =~ s/\W.*//; # remove everything # after first word $somename =~ tr/A-Z/a-z/; # lowercase everything if ($someone eq “randal”) { return 1; } # return true elsif (($words{$someone} || “groucho”) eq $someguess { return 1; } # return true else { return 0; } # return false }

Filehandles • Create user-defined filehandles to access files • Use the open() function to assign a filehandle to a file • Access file contents by assigning the scalar variable a filehandle value • Close the file with the close() operator

Filehandle example sub init_words { open(WORDSLIST, “wordslist”); while ($name = <WORDSLIST>) # read name { chomp ($name); # rid trailing newline $word = <WORDSLIST>; # read word chomp ($word); # rid trailing newline $words{$name} = $word; # Put into hash table } close (WORDSLIST); # close file }

Lecture 3 Outline • Checking age of files • Sending e-mail warnings • Reading the next file • Formatting output • Renaming files • Saving hash tables into a database • Retrieving information from a database

Checking the age of files • Assign a filehandle to a file for examination • Use the –M file test operator to check the age of the file • Compare the filehandle to the number of days

File age checking example sub init_words { open (WORDSLIST, “wordslist”) || # open file or die “Can’t open wordlist: $!”; # exit & print error if (-M WORDSLIST >= 7.0) # is age of file >= 7 days? { # print error and exit die “Sorry, the wordslist is older than seven days.”; } while ($name = <WORDSLIST>) # read name { chomp ($name); # rid trailing newline $word = <WORDSLIST>; # read word chomp ($word); # rid trailing newline $words{$name} = $word; # put into hash table } close (WORDSLIST) || # close file or die “Couldn’t close wordlist: $!”; # print error and exit }

Sending e-mail warnings • Use the open() function to create a filehandle to the MAIL process • Pipe your e-mail address into the MAIL process • Write message into the MAIL process • Close filehandle to the MAIL process

Example sending e-mail warning sub good_word { my($somename, $someguess) = @_; #name of parameters […rest of program deleted…] else { # mail joedoe@lasierra.edu open MAIL, “|mail joedoe\@lasierra.edu”; # write text of e-mail print MAIL “Warning: $someone guessed $someguess\n”; close MAIL; # close MAIL filehandle return 0; # return value is false } }

Reading the next file • The glob function returns the next filename that matches a search pattern • Put the glob function in a while loop to list all files in a directory

Example of reading next files sub init_words { while ( defined ($filename = glob(“*.secret”)) ) { # find next file until undef open (WORDSLIST, $filename) || # open file or die “Can’t open wordlist: $!”; # print error, exit if (-M WORDSLIST < 7.0) # complement test { while ($name = <WORDSLIST>) { # read until undef chomp $name; # rid trailing newline $word = <WORDSLIST>; # get word chomp $word; # rid trailing newline $words{name} = $word; # put into table } } close (WORDSLIST) || die “Couldn’t close wordlist: $!”; } }

Formatting output • The format statement is used to layout reports by formatting output variables • A single write; command is used execute a report • Formats definitions contain a format name, and a template definition • Template definitions contain fieldlines and fieldholders

Format definition example format STDOUT = @<<<<<<<<<<<<<< @<<<<<<<< @<<<<<<<<<<<< $filename, $name, $word . format STDOUT_TOP = Page @<< $% Filename Name Word =============== ========= ============= .

Sample Output Page 1 Filename Name Word =============== ========= ============= barney.secret christina rabbit barney.secret joey fish barney.secret jennifer jellyfish fred.secret fred camel fred.secret barney llama fred.secret betty alpaca fred.secret wilma alpaca

Renaming files • Use the rename function to alter the name of files • When used in conjunction with a check for the age of next files, older files can be automatically renamed

Example renaming expired files sub init_words { while ( defined($filename = glob("*.secret")) ) { open(WORDSLIST, $filename) || die "Can't open wordlist: $!"; if (-M WORDSLIST < 7.0) { […rest of program deleted…] } else { rename ($filename,"$filename.old") || die "can't rename $filename to $filename.old: $!"; } close (WORDSLIST) || die "Couldn't close wordlist: $!"; } }

Saving hash tables in a database • The dbmopen() statement can create a hash table that stores information on a database file • Information is stored into the hash table by assigning values for a key • The dbmclose() statement disconnects the hash from the database file

Saving hash table data example […the following goes at the end of the main section of Ex3.pl…] # log all successful accesses # last_good hash contains successful accesses dbmopen (%last_good,”lastdb”,0666); $last_good{$name} = time; dbmclose(%last_good);

Retrieving info from database • Use the dbmopen() statement to get hash table data from database file • Use a foreach loop to process each entry (key) from the database file. • Use the sort and keys functions to produce a sorted list • Store and calculate result from hash into scalar variable • Execute report using a write; statement

Example retrieving database info Ex4.pl #!/usr/bin/perl dbmopen (%last_good,"lastdb",0666); # open lastdb using # 0666 file permission foreach $name (sort keys %last_good) # process each entry { $when = $last_good{$name}; # assign hash data to # scalar variable which # contains data in seconds $hours = (time - $when) / 3600; # compute hours ago write; # execute report } format STDOUT = User @<<<<<<<<<<<<<: last correct guess was @<<< hours ago. $name, $hours .

Sample Output User Betty : last correct guess was 0.01 hours ago. User Denny : last correct guess was 0.09 hours ago. User Fred : last correct guess was 0.09 hours ago. User barney : last correct guess was 0.01 hours ago. User christina : last correct guess was 0.00 hours ago. User jennifer : last correct guess was 0.00 hours ago. User joey : last correct guess was 0.00 hours ago. User wilma : last correct guess was 0.00 hours ago.

CPTG286K Programming - Perl

CPTG286K Programming - Perl

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