ABC of Perl programming

Perl ABC Part I

David Young
yangboh@cn.ibm.com
Jan. 2011

Perl ABC Data Structure

Data Structure
Scalar
List
Hash
Reference
Filehandle
Function


Scalar
A number
A string
A reference
List
A list is ordered scalar data.
Hash
Associative arrays


List examples
A list is ordered scalar data.
#+begin_src perl
@a = ("fred","barney","betty","wilma"); # ugh!
@a = qw(fred barney betty wilma);       # better!
@a = qw(
    fred
    barney
    betty
    wilma
);          # same thing
#+end_src


Hash examples
"associative arrays"

#+begin_src perl #+begin_src perl
%words = ( %words = qw(
    fred   => "camel",     fred   camel
    barney => "llama",     barney llama
    betty  => "alpaca",     betty  alpaca
    wilma  => "alpaca",     wilma  alpaca
); );
#+end_src #+end_src


Hash
continue …
#+begin_src perl
@words = qw(
    fred       camel
    barney     llama
    betty      alpaca
    wilma      alpaca
);
%words = @words;
#+end_src


Special Things – Nested List
There is NOT anythig like list of lists

#+begin_src perl
@a = qw(fred barney betty wilma);
@b = ("tom", @a, "jerry"); # be careful

# what you actually get is
@b = qw(tom fred barney betty wilma jerry);
#+end_src


Special Things – Nested List
But … there is nested list in the real world
What you really mean is
#+begin_src perl
@b = ("tom", [ @a ], "jerry");
@b = ("tom", @a, "jerry");
#+end_src
#+begin_src perl
$c = [ @a ];  $c = @a;
@b = ("tom", $c, "jerry");
#+end_src


Special Things – Nested Hash
There is nested hash in the real world
#+begin_src perl
$words_nest{ mash } = {
    captain  => "pierce",
%words_nest = (
    major    => "burns",
    fred    => "camel",
    barney  => "llama",     corporal => "radar",
};
    betty   => "alpaca",
    wilma   => "alpaca",
    jetsons => {
        husband   => "george",
        wife      => "jane",
        "his boy" => "elroy",  # Key quotes needed.
    },
);

#+end_src

Perl ABC Data Access

Data Access
Access Scalar Data
Access List Data
Access Hash Data


Scalar
$fred   = "pay"; $fredday = "wrong!";
$barney = "It's $fredday";
                  # not payday, but "It's wrong!"
$barney = "It's ${fred}day";
                  # now, $barney gets "It's payday"
$barney2 = "It's $fred"."day";
                  # another way to do it
$barney3 = "It's " . $fred . "day";
                  # and another way


List -- access individully
#+begin_src perl
@b = ("tom", [ @a ], "jerry");
#+end_src
#+begin_src perl
$c = @a;                       # $c = 4;
$c = $b[0];                    # $c = tom

#+end_src


List -- slicing access
#+begin_src perl
@b = ("tom", [ @a ], "jerry");
#+end_src
#+begin_src perl
@c = @a;                    # list copy
($aa, $ab, $ac @c) = @a;    # @c = qw(wilma);
@c = @a[1,2,3];
           # @c = qw(barney betty wilma);
@c = @a[1..3];              # same thing
@a[1,2] = @a[2,1];          # switch value
#+end_src


List – access as a whole
foreach
map
grep


foreach
#+begin_src perl
@a = (3,5,7,9);
foreach $one (@a) {
$one *= 3;
}

# @a is now (9,15,21,27)
Notice how altering $one in fact altered each element of @a.
This is a feature, not a bug.


map
@a = (3,5,7,9);
@b = map { $_ * 3 } @a;            # @b is now (9,15,21,27)
@c = map { $_ > 5 } @a;            # @c is now (,1,1)

grep
@a = (3,5,7,9);
@c = grep { $_ > 5 } @a;           # @c is now (7,9)
@c = grep { $_ > 5 ? $_ : ()} @a;  # equivalent as map


map and equivalent foreach
@a = (3,5,7,9);
@b = map { $_ * 3 } @a; # @b is now (9,15,21,27)

# equivalent foreach
foreach my $a (@a) {
push @b, $a * 3; # did not return values
}


sub time3 {
map and equivalent foreach
   my $num = shift;
@a = (3,5,7,9);    return $num * 3
}
@b = map { $_ * 3 } @a;
$func = sub {
   my $num = shift;
   return $num * 3
}

# equivalents sub my_map {
@b = map &time3($_) @a; my ($func, $data) = @_;
@b = map &$func($_) @a; foreach $a (@$data) {
@b = my_map &time3, @a;     push @b, &$func($a);
@b = my_map $func, @a; }
return @b;
}


Hash -- access individully
#+begin_src perl
%words = (
    fred   => "camel",
    barney => "llama",
    betty  => "alpaca",
    wilma  => "alpaca",
);
#+end_src

#+begin_src perl

$c = $words{"fred"};   # $c = camel
$d = "barney";
$e = $words{$d};       # $e = llama

#+end_src


Hash -- access as a whole
#+begin_src perl #+begin_src perl
%words = (    @key_list = keys(%words);
  fred   => "camel",    @value_list = values(%words);
  barney => "llama", #+end_src
  betty  => "alpaca",
  wilma  => "alpaca", #+begin_src perl
); foreach $key (keys(%words){
#+end_src      print $words{$key}, "n";
}
#+end_src

#+begin_src perl
foreach $value (values(%words){
     print $value, "n";
}
#+end_src


List – access nested elements
#+begin_src perl
@b = ("tom", [ @a ], "jerry");
@b = ("tom", @a, "jerry");
#+end_src

#+begin_src perl
$a = $b[1];              # $a = [ @a ]
$c = $b[1]>[1];         # $c = barney
$c = @b[1][1];           # same thing
$c = @$a>[1];           # same thing
$c = ${$a}[1];           # same thing
#+end_src


Hash – access nested elements
%h_nest = (
    fred    => "camel",
    barney  => "llama",
    betty   => "alpaca",
    wilma   => "alpaca",
    jetsons => {
        husband   => "george",
        wife      => "jane",
        "his boy" => "elroy",
    },
);

$c = $h_nest{"jetsons"}{"wife"};  # $c = jane
$j = "jetsons";  $w = "wife";
$c = $h_nest{$j}{$w};             # same thing

$jet = $h_nest("jetsons"};        # $jet has a hash
$d = $jet{"husband"};             # $d = george


Reference
# Create some variables
$a      = "mama mia";
@array  = (10, 20);
%hash   = ("laurel" => "hardy", "nick" =>  "nora");

# Now create references to them
$r_a     = $a;      # $ra now "refers" to (points to) $a
$r_array = @array;
$r_hash  = %hash;


Access Reference Data
# Now create references to them
$r_a     = $a;      # $ra now "refers" to (points to) $a
$r_array = @array;
$r_hash  = %hash;
# Now access the referenced data
$r_a;         # the address
$$r_a;        # the $a;       "mama mia";
$r_array      # the address
@$r_array     # the array     (10, 20);
@$r_array[1]  # the element   20;

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