Perl Notes



Below are the commands for



    - Perl program setup

    - Variables

    - Arrays

    - DBM

    - String search and manipulation

    - Other



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PERL PROGRAM SETUP



Perl progams read in arguments in a similar way to C



$numArgs = $#ARGV + 1;

print "thanks, you gave me $numArgs command-line arguments.\n";



foreach $argnum (0 .. $#ARGV) {

   print "$ARGV[$argnum]\n";

}

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VARIABLES



my $myvar;

my @mylist;

my %myhash = ();



$myhash{$key} = $value;

for $key (keys %myhash) {;}

print "size of hash:  " . keys( %hash ) . ".\n";

print "Value EXISTS, but may be undefined.\n" if exists  $hash{ $key };

my %hash_copy = %hash;  # copy a hash



----------------------------------------------------

ARRAYS





----------------------------------------------------

DBM





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STRING SEARCH AND MANIPULATION



crypt(STRING1, STRING2) -- Encrypts STRING1

index(STRING, SUBSTRING, POSITION) -- Returns the position

   of the first occurrence of SUBSTRING in STRING at or

   after POSITION.

lc(STRING) -- Returns a string with every letter of STRING

   in lowercase. For instance, lc("ABCD") returns "abcd".

length(STRING) -- Returns the length of STRING.

split(PATTERN, STRING, LIMIT) -- Breaks up a string based 

   on some delimiter. In an array context, it returns a 

   list of the things that were found. In a scalar 

   context, it returns the number of things found.



if ($string =~ m/regex/) {

  print 'match';

} else {

  print 'no match';

}



Perl has a host of special variables that get filled after 

every m// or s/// regex match. $1, $2, $3, etc. hold the 

backreferences. $+ holds the last (highest-numbered) 

backreference. $& (dollar ampersand) holds the entire regex 

match.



@- is an array of match-start indices into the string. $-[0] 

holds the start of the entire regex match, $-[1] the start 

of the first backreference, etc. Likewise, @+ holds 

match-end indices (ends, not lengths).



$' (dollar followed by an apostrophe or single quote) holds 

the part of the string after (to the right of) the regex 

match. $` (dollar backtick) holds the part of the string 

before (to the left of) the regex match. Using these 

variables is not recommended in scripts when performance 

matters, as it causes Perl to slow down all regex matches in 

your entire script.



All these variables are read-only, and persist until the 

next regex match is attempted.



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HASH and MYSQL



    my $answers = 'a,b,c,d,e';



    my $sql = "select max_time, $answers from questions " .

              'where question_number=?';

    my $hash_ref = sql_fetch_hashref( $sql, $q );



    my @answers = split ',', $answers;



    my $max_time = $hash_ref->{max_time} || '60';



    my $hash_ref_ans;

    for my $letter ( @answers ) {

        $hash_ref_ans->{ $letter } = $hash_ref->{ $letter }

            if defined $hash_ref->{ $letter };

    }