This is the second set of slightly updated slides from a Perl programming course that I held some years ago. I want to share it with everyone looking for intransitive Perl-knowledge. A table of content for all presentations can be found at i-can.eu. The source code for the examples and the presentations in ODP format are on https://github.com/kberov/PerlProgrammingCourse

1. Perl Programming
Course
Scalar data types
Krasimir Berov
I-can.eu

2. Contents
1. Perl variables/data types
2. Numeric operators
3. String operators
4. Scalar functions (summary)
5. Undefined quantities
6. Type-casting in Perl?
7. References

3. Perl variable/data types
Perl has three main variable types:
– scalars
– arrays
– hashes
... Plus two additional:
– typeglobs
– filehandles
See: perlintro, perldata

4. Variable names
● Values are usually referred to by name, or through a
named reference.
● The first character tells you to what sort/type of data
structure it refers ($,@,%).
● The rest is the name of the particular value to which it
refers.
● Usually this name is a single identifier – a string
beginning with a letter or underscore, and containing
letters, underscores, and digits.
● It may be also a chain of identifiers, separated by ::
(see perlmod/Packages, perlref).

5. Perl Scalars
● A scalar can contain a single string (of any size, limited
only by the available memory), number, or a reference
to something (see perlref).
● There are also a number of "magic" scalars with
names that look like punctuation or line noise ($_, $/,
$] ...).
my $animal = "camel";
my $answer = 22;
print "Me: Hello $animal! How old are you?n";
print "$animal: $answer.$/";
print '-'x 20, $/;
print 'Named reference: ',${animal},$/;
$Other::animal = 'llama';
print "From package 'Other': $Other::animaln";
print 'Perl version: ',$], $/;

6. Perl Arrays
● Arrays are ordered lists of scalars indexed by
number, starting with 0.
my @animals = ("camel", "llama", "пиле");
my @numbers = (23, 42, 69);
my @mixed = ("camel", 42, 1.23);
print @animals .$/;#what the...;) scalar context
print "@animals" . $/;#interpolated array
print "@animals @numbers" . $/;#interpolated arrays
print @animals, @numbers, $/;#list context

7. Perl Hashes
● Hashes are unordered collections of scalar
values indexed by their associated string key.
my %fruit_colors = (
apple => "red",
banana => "yellow",
);
print
map { "$_ => $fruit_colors{$_}n" }
sort
keys %fruit_colors;
print "%fruit_colorsn"; #hashes can NOT be interpolated

8. Back to Scalars?!..

9. Scalar values
● All data in Perl is a scalar, an array of scalars, or a
hash of scalars.
● A scalar may contain one single value which is:
– String
– Number
– Reference (more later)
– Filehandle (another time (I/O) )
● Conversion from one form to another is
transparent
● Scalar values are by default undefined.

10. What are scalars (revisited)
● A scalar is a single string, number, or a reference
to something.
● A scalar value is interpreted as TRUE in the
Boolean sense if it is not the null string or the
number 0 (or its string equivalent, "0").
#try this on the commandline
>perl -e 'print "truen" if "0" '
>perl -e 'print "falsen" unless "0" '
>...

11. Scalar functions or operators?
● Many of the built-in functions in Perl are referred
often to as named operators
● There are several categories depending on the
usage, type of manipulated or produced data, etc.
● For example:
– Functions for SCALARs or strings
– Numeric functions
– Regular expressions and pattern matching...
● One function/operator may fall in one or more categories
depending on the context.

12. Assignment operator
● The most common operation on a scalar (or
array, or hash) variable is assignment.
● This way we give a value to a variable.
● This way we give a name to a literal value.
my $name = 'Larry Wall';
print 'My name is ', $name, $/;

13. Numeric operators
● Named operators (functions) that act on numbers
and produce numbers
– abs, hex, oct

14. Numeric operators
● abs VALUE
abs
Returns the absolute value of its argument. If VALUE
is omitted, uses $_.
● See perlfunc/abs
my $answer = -22;
print abs $answer;
print abs "$answer";

15. Numeric operators
● hex EXPR
hex
Interprets EXPR as a hex string and returns the
decimal value. (To convert strings that might start with
either 0, 0x, or 0b, see oct.) If EXPR is omitted, uses
$_.
● To present something as hex, look into printf, sprintf, or
unpack.
● See perlfunc/hex
print hex '0xBf';
print hex 'bF';

16. Numeric operators
● oct EXPR
oct
Interprets EXPR as an octal string and returns the
corresponding value.
– If EXPR starts off with 0x, interprets it as a hex string.
– If EXPR starts off with 0b, it is interpreted as a binary
string.
– Leading whitespace is ignored in all three cases.
– If EXPR is omitted, uses $_.
– To go the other way (produce a number in octal), use
sprintf() or printf()
print( oct 0b10, $/);
print( oct '0xBf', $/);
print( oct '07', $/);
print( oct '0777', $/);

17. String operators
● Named operators (functions) for SCALARs or
strings
– length, chop and chomp, uc/lc

18. String operators
● length EXPR
length
Returns the length in characters of the value of EXPR.
– If EXPR is omitted, returns length of $_.
– Cannot be used on an array or hash to find out how
many elements these have.
– For that, use scalar @array and scalar keys %hash
respectively.
– if the EXPR is in Unicode, you will get the number of
characters, not the number of bytes
use utf8;
print( length 'kniga' , $/);
use bytes;
print( length 'книга', $/);
no bytes;
print( length 'книга', $/);

19. String operators
● chop VARIABLE
chop( LIST )
chop
Chops off the last character of a string and returns the character
chopped.
– If VARIABLE is omitted, chops $_.
– If VARIABLE is a hash, it chops the hash's values, but
not its keys.
– If you chop a list, each element is chopped. Only the
value of the last chop is returned.
#!/usr/bin/perl -C
#binmode(STDOUT, ':encoding(cp866)');#on win32
use utf8;
binmode(STDOUT, ':utf8');
my ($bob_latin, $bob_cyr) = ('bob', 'боб');
print( chop($bob_latin) , $/, chop($bob_cyr) , $/);

20. String operators
● chomp VARIABLE
chomp( LIST )
chomp
Safer version of chop.
– Removes any trailing string that corresponds to the
current value of $/.
– Returns the total number of characters removed
from all its arguments.
#binmode(STDOUT, ':encoding(cp866)');#on win32
use utf8;
binmode(STDOUT, ':utf8');
my ($bob_latin, $bob_cyr) = ("bobn", "боб$/");
print( $bob_latin, $bob_cyr, $/ );
print( chomp($bob_latin,$bob_cyr) , $/ );
print( $bob_latin, $bob_cyr, $/ );

21. String operators
● lc EXPR
lc
Returns a lowercased version of EXPR. If EXPR is
omitted, uses $_.
● uc EXPR
uc
Returns an uppercased version of EXPR. If EXPR is
omitted, uses $_.
#binmode(STDOUT, ':encoding(cp866)');#on win32
use utf8;
binmode(STDOUT, ':utf8');
my ($lcstr, $ucstr) = ("BOBn", "боб$/");
print( lc $lcstr, uc($ucstr), $/ );

22. Un/defined quantities
● undef => nothing, empty, void
● defined => something not undef :)

23. Un/defined quantities
● undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue.
– Use only on a scalar value, an array (using @), a hash (using
%), a subroutine (using &), or a typeglob (using *)...
– Always returns the undefined value.
#use strict; use warnings; use diagnostics;
my $name;
print $name ,$/;
$name ="Larry";
print $name ,$/;
undef $name ;
print $name ,$/;

24. Un/defined quantities
● defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other
than the undefined value undef.
– If EXPR is not present, $_ will be checked.
– Allows you to distinguish undef from other values.
– A simple Boolean test will not distinguish among undef,
zero, the empty string, and "0", which are all equally
my false.
$data;
print $data if defined($data);
$data = 0;
print defined($data);
print $data if defined($data);
undef $data;
print defined($data);
$_ = 2;
print defined;

25. Scalar functions (summary)
● Functions for SCALARs or strings
– chomp, chop, chr, crypt, hex, index, lc, lcfirst,
length, oct, ord, pack, q//, qq//, reverse, rindex,
sprintf, substr, tr///, uc, ucfirst, y///
● Numeric functions
– abs, atan2, cos, exp, hex, int, log, oct, rand, sin,
sqrt, srand
● Miscellaneous functions
● defined, dump, eval, formline, local, my, our, reset,
scalar, undef, wantarray
● See perlfunc

26. Type casting operations :X :(
● C Operators Missing From Perl:
– Type-casting operator
– ...
● From perlglossary:
– type casting: Converting data from one type to
another. C permits this. Perl does not need it. Nor
want it.
● I hope you have in mind references :)...

27. References
● A scallar can also contain a reference.
● A reference is just a piece of data pointing to another piece of data
(anonimous or named).
● In Perl, a reference is always a scalar, although the data it refers to
may not be
● Languages like C and C++ have a feature that's similar to references,
called pointers.
● Pointers leave interpretation of what's there for the programmer
● References only store memory locations for specific, clearly defined
data structures – maybe not predefined, but defined nevertheless.
● References allow you to leave the arrangement of computer memory to
the computer itself.

28. References
● ref EXPR
ref
Returns a non-empty string if EXPR is a reference, the
empty string otherwise.
– If EXPR is not specified, $_ will be used.
– The value returned depends on the type of thing the
reference is a reference to.
– Builtin types include:
SCALAR, ARRAY, HASH, CODE, REF, GLOB, LVALUE,
FORMAT, IO, Regexp
– If the referenced object has been blessed into a
package, then that package name is returned instead.
– You can think of ref as a typeof operator.

29. References
● Example
use Data::Dumper;
my %hash = (me =>'you' );
my @array = ('we',%hash,['them']);
my $scalar = @array;
print ref $scalar, $/;
print $scalar,$/;
print Dumper($scalar);
● References will be discussed another time

30. Scalar data types
Questions?

31. Exercises
1. Write a program that converts a given hex digit to decimal
and displays it on the screen.
2. Write a program which displays the absolute of a negative
number.
3. Write a program which removes the last letter from a string
(no matter what the string is) and displays the letter on the
screen.
4. Write a program which converts a number to its
corresponding character and displays the letter on the
screen.
5. Write a program which prints the string 'Здрасти' 3 times on
3 separate lines using only one print statement.