Fundamental of Perl programming

1. Perl ABC Part I
David Young
yangboh@cn.ibm.com
Jan. 2011

2. Perl ABC Data Structure
Data Structure
Scalar
List
Hash
Reference
Filehandle
Function

3. Perl ABC Data Structure
Scalar
A number
A string
A reference
List
A list is ordered scalar data.
Hash
Associative arrays

4. Perl ABC Data Structure
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

5. Perl ABC Data Structure
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

6. Perl ABC Data Structure
Hash
continue …
#+begin_src perl
@words = qw(
fred camel
barney llama
betty alpaca
wilma alpaca
);
%words = @words;
#+end_src

7. Perl ABC Data Structure
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

8. Perl ABC Data Structure
Special Things – Nested List
But … there is nested list in the real world
What you really mean is
#+begin_src perl
@a = qw(fred barney betty wilma);
@b = ("tom", [ @a ], "jerry");
@b = ("tom", @a, "jerry");
#+end_src
#+begin_src perl
$c = [ @a ]; $c = @a;
@b = ("tom", $c, "jerry");
#+end_src

9. Perl ABC Data Structure
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

10. Perl ABC Data Access
Data Access
Access Scalar Data
Access List Data
Access Hash Data

11. Perl ABC Data Access
Data Access
Access Scalar Data
Access List Data
Access Hash Data

12. Perl ABC Data Access
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

13. Perl ABC Data Access
Data Access
Access Scalar Data
Access List Data
Access Hash Data

14. Perl ABC Data Access
List -- access individully
#+begin_src perl
@a = qw(fred barney betty wilma);
@b = ("tom", [ @a ], "jerry");
#+end_src
#+begin_src perl
$c = @a; # $c = 4;
$c = $b[0]; # $c = tom
#+end_src

15. Perl ABC Data Access
List -- slicing access
#+begin_src perl
@a = qw(fred barney betty wilma);
@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

16. Perl ABC Data Access
List – access as a whole
foreach
map
grep

17. Perl ABC Data Access
List – access as a whole
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.

18. Perl ABC Data Access
List – access as a whole
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

19. Perl ABC Data Access
List – access as a whole
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
}

20. Perl ABC Data Access
List – access as a whole
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;
}

21. Perl ABC Data Access
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

22. Perl ABC Data Access
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

23. Perl ABC Data Access
List – access nested elements
#+begin_src perl
@a = qw(fred barney betty wilma);
@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

24. Perl ABC Data Access
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

25. Perl ABC Data Access
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;

26. Perl ABC Data Access
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;

27. Perl ABC Data Access
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;