Scalar data types
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This is the second set of slightly updated slides from a Perl programming course that I held some years ago. ...

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

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Scalar data types Scalar data types Presentation Transcript

  • Perl Programming Course Scalar data typesKrasimir BerovI-can.eu
  • Contents1. Perl variables/data types2. Numeric operators3. String operators4. Scalar functions (summary)5. Undefined quantities6. Type-casting in Perl?7. References
  • Perl variable/data typesPerl has three main variable types: – scalars – arrays – hashes... Plus two additional: – typeglobs – filehandlesSee: perlintro, perldata
  • 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 ($,@,%).● The rest is the name of the particular value to which it refers.● Usually this name is a single identifier – a string beginning with a letter or underscore, and containing letters, underscores, and digits.● It may be also a chain of identifiers, separated by :: (see perlmod/Packages, perlref).
  • Perl Scalars● A scalar can contain a single string (of any size, limited only by the available memory), number, or a reference to something (see perlref).● There are also a number of "magic" scalars with names that look like punctuation or line noise ($_, $/, $] ...). 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: $Other::animaln"; print Perl version: ,$], $/;
  • Perl Arrays● Arrays are ordered lists of scalars indexed by number, starting with 0. my @animals = ("camel", "llama", "пиле"); my @numbers = (23, 42, 69); my @mixed = ("camel", 42, 1.23); print @animals .$/;#what the...;) scalar context print "@animals" . $/;#interpolated array print "@animals @numbers" . $/;#interpolated arrays print @animals, @numbers, $/;#list context
  • Perl Hashes● Hashes are unordered collections of scalar values indexed by their associated string key. my %fruit_colors = ( apple => "red", banana => "yellow", ); print map { "$_ => $fruit_colors{$_}n" } sort keys %fruit_colors; print "%fruit_colorsn"; #hashes can NOT be interpolated
  • Back to Scalars?!..
  • 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.
  • 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" >...
  • 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.
  • 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, $/;
  • Numeric operators● Named operators (functions) that act on numbers and produce numbers – abs, hex, oct
  • 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";
  • 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;
  • 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, $/);
  • String operators● Named operators (functions) for SCALARs or strings – length, chop and chomp, uc/lc
  • 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, $/);
  • 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 $_. – If VARIABLE is a hash, it chops the hashs values, but not its keys. – If you chop a list, each element is chopped. Only the value of the last chop is returned. #!/usr/bin/perl -C #binmode(STDOUT, :encoding(cp866));#on win32 use utf8; binmode(STDOUT, :utf8); my ($bob_latin, $bob_cyr) = (bob, боб); print( chop($bob_latin) , $/, chop($bob_cyr) , $/);
  • 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, $/ );
  • 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), $/ );
  • Un/defined quantities● undef => nothing, empty, void● defined => something not undef :)
  • 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 $name ,$/;
  • 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;
  • 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
  • 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 :)...
  • 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 thats similar to references, called pointers.● Pointers leave interpretation of whats 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.
  • 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.
  • 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
  • Scalar data typesQuestions?
  • Exercises1. 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.