AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perldata.txt

Text File | 1999-09-09 | 23.8 KB | 578 lines

NAME
       perldata - Perl data types

DESCRIPTION
       Variable names

       Perl has three data structures: scalars, arrays of
       scalars, and associative arrays of scalars, known as
       "hashes".  Normal arrays are indexed by number, starting
       with 0.  (Negative subscripts count from the end.)  Hash
       arrays are indexed by string.

       Scalar values are always named with '$', even when
       referring to a scalar that is part of an array.  It works
       like the English word "the".  Thus we have:

           $days               # the simple scalar value "days"
           $days[28]           # the 29th element of array @days
           $days{'Feb'}        # the 'Feb' value from hash %days
           $#days              # the last index of array @days

       but entire arrays or array slices are denoted by '@',
       which works much like the word "these" or "those":

           @days               # ($days[0], $days[1],... $days[n])
           @days[3,4,5]        # same as @days[3..5]
           @days{'a','c'}      # same as ($days{'a'},$days{'c'})

       and entire hashes are denoted by '%':

           %days               # (key1, val1, key2, val2 ...)

       In addition, subroutines are named with an initial '&',
       though this is optional when it's otherwise unambiguous
       (just as "do" is often redundant in English).  Symbol
       table entries can be named with an initial '*', but you
       don't really care about that yet.

       Every variable type has its own namespace.  You can,
       without fear of conflict, use the same name for a scalar
       variable, an array, or a hash (or, for that matter, a
       filehandle, a subroutine name, or a label).  This means
       that $foo and @foo are two different variables.  It also
       means that $foo[1] is a part of @foo, not a part of $foo.
       This may seem a bit weird, but that's okay, because it is
       weird.

       Since variable and array references always start with '$',
       '@', or '%', the "reserved" words aren't in fact reserved
       with respect to variable names.  (They ARE reserved with
       respect to labels and filehandles, however, which don't
       have an initial special character.  You can't have a
       filehandle named "log", for instance.  Hint: you could say
       open(LOG,'logfile') rather than open(log,'logfile').
       Using uppercase filehandles also improves readability and
       protects you from conflict with future reserved words.)
       Case IS significant--"FOO", "Foo" and "foo" are all
       different names.  Names that start with a letter or
       underscore may also contain digits and underscores.

       It is possible to replace such an alphanumeric name with
       an expression that returns a reference to an object of
       that type.  For a description of this, see the perlref
       manpage.

       Names that start with a digit may only contain more
       digits.  Names which do not start with a letter,
       underscore,  or digit are limited to one character, e.g.
       $% or $$.  (Most of these one character names have a
       predefined significance to Perl.  For instance, $$ is the
       current process id.)

       Context

       The interpretation of operations and values in Perl
       sometimes depends on the requirements of the context
       around the operation or value.  There are two major
       contexts: scalar and list.  Certain operations return list
       values in contexts wanting a list, and scalar values
       otherwise.  (If this is true of an operation it will be
       mentioned in the documentation for that operation.)  In
       other words, Perl overloads certain operations based on
       whether the expected return value is singular or plural.
       (Some words in English work this way, like "fish" and
       "sheep".)

       In a reciprocal fashion, an operation provides either a
       scalar or a list context to each of its arguments.  For
       example, if you say

           int( <STDIN> )

       the integer operation provides a scalar context for the
       <STDIN> operator, which responds by reading one line from
       STDIN and passing it back to the integer operation, which
       will then find the integer value of that line and return
       that.  If, on the other hand, you say

           sort( <STDIN> )

       then the sort operation provides a list context for
       <STDIN>, which will proceed to read every line available
       up to the end of file, and pass that list of lines back to
       the sort routine, which will then sort those lines and
       return them as a list to whatever the context of the sort
       was.

       Assignment is a little bit special in that it uses its
       left argument to determine the context for the right
       argument.  Assignment to a scalar evaluates the righthand
       side in a scalar context, while assignment to an array or
       array slice evaluates the righthand side in a list
       context.  Assignment to a list also evaluates the
       righthand side in a list context.

       User defined subroutines may choose to care whether they
       are being called in a scalar or list context, but most
       subroutines do not need to care, because scalars are
       automatically interpolated into lists.  See the wantarray
       entry in the perlfunc manpage.

       Scalar values

       All data in Perl is a scalar or an array of scalars or a
       hash of scalars.  Scalar variables may contain various
       kinds of singular data, such as numbers, strings, and
       references.  In general, conversion from one form to
       another is transparent.  (A scalar may not contain
       multiple values, but may contain a reference to an array
       or hash containing multiple values.)  Because of the
       automatic conversion of scalars, operations and functions
       that return scalars don't need to care (and, in fact,
       can't care) whether the context is looking for a string or
       a number.

       Scalars aren't necessarily one thing or another.  There's
       no place to declare a scalar variable to be of type
       "string", or of type "number", or type "filehandle", or
       anything else.  Perl is a contextually polymorphic
       language whose scalars can be strings, numbers, or
       references (which includes objects).  While strings and
       numbers are considered pretty much same thing for nearly
       all purposes, references are strongly-typed uncastable
       pointers with built-in reference-counting and destructor
       invocation.

       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").  The Boolean context is just a
       special kind of scalar context.

       There are actually two varieties of null scalars: defined
       and undefined.  Undefined null scalars are returned when
       there is no real value for something, such as when there
       was an error, or at end of file, or when you refer to an
       uninitialized variable or element of an array.  An
       undefined null scalar may become defined the first time
       you use it as if it were defined, but prior to that you
       can use the defined() operator to determine whether the
       value is defined or not.

       To find out whether a given string is a valid non-zero
       number, it's usually enough to test it against both
       numeric 0 and also lexical "0" (although this will cause
       -w noises).  That's because strings that aren't numbers
       count as 0, just as the do in awk:

           if ($str == 0 && $str ne "0")  {
               warn "That doesn't look like a number";
           }

       That's usually preferable because otherwise you won't
       treat IEEE notations like NaN or Infinity properly.  At
       other times you might prefer to use a regular expression
       to check whether data is numeric.  See the perlre manpage
       for details on regular expressions.

           warn "has nondigits"        if     /\D/;
           warn "not a whole number"   unless /^\d+$/;
           warn "not an integer"       unless /^[+-]?\d+$/
           warn "not a decimal number" unless /^[+-]?\d+\.?\d*$/
           warn "not a C float"
               unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

       The length of an array is a scalar value.  You may find
       the length of array @days by evaluating $#days, as in csh.
       (Actually, it's not the length of the array, it's the
       subscript of the last element, since there is (ordinarily)
       a 0th element.)  Assigning to $#days changes the length of
       the array.  Shortening an array by this method destroys
       intervening values.  Lengthening an array that was
       previously shortened NO LONGER recovers the values that
       were in those elements.  (It used to in Perl 4, but we had
       to break this make to make sure destructors were called
       when expected.)  You can also gain some measure of
       efficiency by preextending an array that is going to get
       big.  (You can also extend an array by assigning to an
       element that is off the end of the array.)  You can
       truncate an array down to nothing by assigning the null
       list () to it.  The following are equivalent:

           @whatever = ();
           $#whatever = $[ - 1;

       If you evaluate a named array in a scalar context, it
       returns the length of the array.  (Note that this is not
       true of lists, which return the last value, like the C
       comma operator.)  The following is always true:

           scalar(@whatever) == $#whatever - $[ + 1;

       Version 5 of Perl changed the semantics of $[: files that
       don't set the value of $[ no longer need to worry about
       whether another file changed its value.  (In other words,
       use of $[ is deprecated.)  So in general you can just
       assume that
           scalar(@whatever) == $#whatever + 1;

       Some programmers choose to use an explicit conversion so
       nothing's left to doubt:

           $element_count = scalar(@whatever);

       If you evaluate a hash in a scalar context, it returns a
       value which is true if and only if the hash contains any
       key/value pairs.  (If there are any key/value pairs, the
       value returned is a string consisting of the number of
       used buckets and the number of allocated buckets,
       separated by a slash.  This is pretty much only useful to
       find out whether Perl's (compiled in) hashing algorithm is
       performing poorly on your data set.  For example, you
       stick 10,000 things in a hash, but evaluating %HASH in
       scalar context reveals "1/16", which means only one out of
       sixteen buckets has been touched, and presumably contains
       all 10,000 of your items.  This isn't supposed to happen.)

       Scalar value constructors

       Numeric literals are specified in any of the customary
       floating point or integer formats:

           12345
           12345.67
           .23E-10
           0xffff              # hex
           0377                # octal
           4_294_967_296       # underline for legibility

       String literals are usually delimited by either single or
       double quotes.  They work much like shell quotes:  double-
       quoted string literals are subject to backslash and
       variable substitution; single-quoted strings are not
       (except for "\'" and "\\").  The usual Unix backslash
       rules apply for making characters such as newline, tab,
       etc., as well as some more exotic forms.  See the qq entry
       in the perlop manpage for a list.

       You can also embed newlines directly in your strings, i.e.
       they can end on a different line than they begin.  This is
       nice, but if you forget your trailing quote, the error
       will not be reported until Perl finds another line
       containing the quote character, which may be much further
       on in the script.  Variable substitution inside strings is
       limited to scalar variables, arrays, and array slices.
       (In other words, identifiers beginning with $ or @,
       followed by an optional bracketed expression as a
       subscript.)  The following code segment prints out "The
       price is $100."

           $Price = '$100';    # not interpreted
           print "The price is $Price.\n";     # interpreted

       As in some shells, you can put curly brackets around the
       identifier to delimit it from following alphanumerics.  In
       fact, an identifier within such curlies is forced to be a
       string, as is any single identifier within a hash
       subscript.  Our earlier example,

           $days{'Feb'}

       can be written as

           $days{Feb}

       and the quotes will be assumed automatically.  But
       anything more complicated in the subscript will be
       interpreted as an expression.

       Note that a single-quoted string must be separated from a
       preceding word by a space, since single quote is a valid
       (though deprecated) character in an identifier (see the
       Packages entry in the perlmod manpage).

       Two special literals are __LINE__ and __FILE__, which
       represent the current line number and filename at that
       point in your program.  They may only be used as separate
       tokens; they will not be interpolated into strings.  In
       addition, the token __END__ may be used to indicate the
       logical end of the script before the actual end of file.
       Any following text is ignored, but may be read via the
       DATA filehandle.  (The DATA filehandle may read data only
       from the main script, but not from any required file or
       evaluated string.)  The two control characters ^D and ^Z
       are synonyms for __END__ (or __DATA__ in a module; see the
       SelfLoader manpage for details on __DATA__).

       A word that has no other interpretation in the grammar
       will be treated as if it were a quoted string.  These are
       known as "barewords".  As with filehandles and labels, a
       bareword that consists entirely of lowercase letters risks
       conflict with future reserved words, and if you use the -w
       switch, Perl will warn you about any such words.  Some
       people may wish to outlaw barewords entirely.  If you say

           use strict 'subs';

       then any bareword that would NOT be interpreted as a
       subroutine call produces a compile-time error instead.
       The restriction lasts to the end of the enclosing block.
       An inner block may countermand this by saying no strict
       'subs'.

       Array variables are interpolated into double-quoted
       strings by joining all the elements of the array with the
       delimiter specified in the $" variable ($LIST_SEPARATOR in
       English), space by default.  The following are equivalent:

           $temp = join($",@ARGV);
           system "echo $temp";

           system "echo @ARGV";

       Within search patterns (which also undergo double-quotish
       substitution) there is a bad ambiguity:  Is /$foo[bar]/ to
       be interpreted as /${foo}[bar]/ (where [bar] is a
       character class for the regular expression) or as
       /${foo[bar]}/ (where [bar] is the subscript to array
       @foo)?  If @foo doesn't otherwise exist, then it's
       obviously a character class.  If @foo exists, Perl takes a
       good guess about [bar], and is almost always right.  If it
       does guess wrong, or if you're just plain paranoid, you
       can force the correct interpretation with curly brackets
       as above.

       A line-oriented form of quoting is based on the shell
       "here-doc" syntax.  Following a << you specify a string to
       terminate the quoted material, and all lines following the
       current line down to the terminating string are the value
       of the item.  The terminating string may be either an
       identifier (a word), or some quoted text.  If quoted, the
       type of quotes you use determines the treatment of the
       text, just as in regular quoting.  An unquoted identifier
       works like double quotes.  There must be no space between
       the << and the identifier.  (If you put a space it will be
       treated as a null identifier, which is valid, and matches
       the first blank line.)  The terminating string must appear
       by itself (unquoted and with no surrounding whitespace) on
       the terminating line.

               print <<EOF;
           The price is $Price.
           EOF

               print <<"EOF";  # same as above
           The price is $Price.
           EOF

               print <<`EOC`;  # execute commands
           echo hi there
           echo lo there
           EOC

               print <<"foo", <<"bar"; # you can stack them
           I said foo.
           foo
           I said bar.
           bar
               myfunc(<<"THIS", 23, <<'THAT');
           Here's a line
           or two.
           THIS
           and here another.
           THAT

       Just don't forget that you have to put a semicolon on the
       end to finish the statement, as Perl doesn't know you're
       not going to try to do this:

               print <<ABC
           179231
           ABC
               + 20;

       List value constructors

       List values are denoted by separating individual values by
       commas (and enclosing the list in parentheses where
       precedence requires it):

           (LIST)

       In a context not requiring a list value, the value of the
       list literal is the value of the final element, as with
       the C comma operator.  For example,

           @foo = ('cc', '-E', $bar);

       assigns the entire list value to array foo, but

           $foo = ('cc', '-E', $bar);

       assigns the value of variable bar to variable foo.  Note
       that the value of an actual array in a scalar context is
       the length of the array; the following assigns to $foo the
       value 3:

           @foo = ('cc', '-E', $bar);
           $foo = @foo;                # $foo gets 3

       You may have an optional comma before the closing
       parenthesis of an list literal, so that you can say:

           @foo = (
               1,
               2,
               3,
           );

       LISTs do automatic interpolation of sublists.  That is,
       when a LIST is evaluated, each element of the list is
       evaluated in a list context, and the resulting list value
       is interpolated into LIST just as if each individual
       element were a member of LIST.  Thus arrays lose their
       identity in a LIST--the list

           (@foo,@bar,&SomeSub)

       contains all the elements of @foo followed by all the
       elements of @bar, followed by all the elements returned by
       the subroutine named SomeSub when it's called in a list
       context.  To make a list reference that does NOT
       interpolate, see the perlref manpage.

       The null list is represented by ().  Interpolating it in a
       list has no effect.  Thus ((),(),()) is equivalent to ().
       Similarly, interpolating an array with no elements is the
       same as if no array had been interpolated at that point.

       A list value may also be subscripted like a normal array.
       You must put the list in parentheses to avoid ambiguity.
       Examples:

           # Stat returns list value.
           $time = (stat($file))[8];

           # SYNTAX ERROR HERE.
           $time = stat($file)[8];  # OOPS, FORGOT PARENS

           # Find a hex digit.
           $hexdigit = ('a','b','c','d','e','f')[$digit-10];

           # A "reverse comma operator".
           return (pop(@foo),pop(@foo))[0];

       Lists may be assigned to if and only if each element of
       the list is legal to assign to:

           ($a, $b, $c) = (1, 2, 3);

           ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);

       Array assignment in a scalar context returns the number of
       elements produced by the expression on the right side of
       the assignment:

           $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
           $x = (($foo,$bar) = f());           # set $x to f()'s return count

       This is very handy when you want to do a list assignment
       in a Boolean context, since most list functions return a
       null list when finished, which when assigned produces a 0,
       which is interpreted as FALSE.

       The final element may be an array or a hash:
           ($a, $b, @rest) = split;
           local($a, $b, %rest) = @_;

       You can actually put an array or hash anywhere in the
       list, but the first one in the list will soak up all the
       values, and anything after it will get a null value.  This
       may be useful in a local() or my().

       A hash literal contains pairs of values to be interpreted
       as a key and a value:

           # same as map assignment above
           %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

       While literal lists and named arrays are usually
       interchangeable, that's not the case for hashes.  Just
       because you can subscript a list value like a normal array
       does not mean that you can subscript a list value as a
       hash.  Likewise, hashes included as parts of other lists
       (including parameters lists and return lists from
       functions) always flatten out into key/value pairs.
       That's why it's good to use references sometimes.

       It is often more readable to use the => operator between
       key/value pairs.  The => operator is mostly just a more
       visually distinctive synonym for a comma, but it also
       quotes its left-hand operand, which makes it nice for
       initializing hashes:

           %map = (
                        red   => 0x00f,
                        blue  => 0x0f0,
                        green => 0xf00,
          );

       or for initializing hash references to be used as records:

           $rec = {
                       witch => 'Mable the Merciless',
                       cat   => 'Fluffy the Ferocious',
                       date  => '10/31/1776',
           };

       or for using call-by-named-parameter to complicated
       functions:

          $field = $query->radio_group(
                      name      => 'group_name',
                      values    => ['eenie','meenie','minie'],
                      default   => 'meenie',
                      linebreak => 'true',
                      labels    => \%labels
          );

       Note that just because a hash is initialized in that order
       doesn't mean that it comes out in that order.  See the
       sort entry in the perlfunc manpage for examples of how to
       arrange for an output ordering.

       Typeglobs and FileHandles

       Perl uses an internal type called a typeglob to hold an
       entire symbol table entry.  The type prefix of a typeglob
       is a *, because it represents all types.  This used to be
       the preferred way to pass arrays and hashes by reference
       into a function, but now that we have real references,
       this is seldom needed.

       One place where you still use typeglobs (or references
       thereto) is for passing or storing filehandles.  If you
       want to save away a filehandle, do it this way:

           $fh = *STDOUT;

       or perhaps as a real reference, like this:

           $fh = \*STDOUT;

       This is also the way to create a local filehandle.  For
       example:

           sub newopen {
               my $path = shift;
               local *FH;  # not my!
               open (FH, $path) || return undef;
               return \*FH;
           }
           $fh = newopen('/etc/passwd');

       See the perlref manpage, the perlsub manpage, and the
       section on Symbols Tables in the perlmod manpage for more
       discussion on typeglobs.  See the open entry in the
       perlfunc manpage for other ways of generating filehandles.