IRIX Base Documentation 1998 November/IRIX 6.5.2 Base Documentation November 1998.img/usr/share/catman/u_man/cat1/perldata.z/perldata

Text File | 1998-10-30 | 31KB | 727 lines

PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



NNNNAAAAMMMMEEEE
     perldata - Perl data types

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     VVVVaaaarrrriiiiaaaabbbblllleeee nnnnaaaammmmeeeessss

     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.

     Values are usually referred to by name (or through a named reference).
     The first character of the name tells you to what sort of data structure
     it refers.  The rest of the name tells you the particular value to which
     it refers.  Most often, it consists of a single _i_d_e_n_t_i_f_i_e_r, that is, a
     string beginning with a letter or underscore, and containing letters,
     underscores, and digits.  In some cases, it may be a chain of
     identifiers, separated by :: (or by ', but that's deprecated); all but
     the last are interpreted as names of packages, to locate the namespace in
     which to look up the final identifier (see the Packages entry in the
     _p_e_r_l_m_o_d manpage for details).  It's possible to substitute for a simple
     identifier an expression which produces a reference to the value at
     runtime; this is described in more detail below, and in the _p_e_r_l_r_e_f
     manpage.

     There are also special variables whose names don't follow these rules, so
     that they don't accidentally collide with one of your normal variables.
     Strings which match parenthesized parts of a regular expression are saved
     under names containing only digits after the $ (see the _p_e_r_l_o_p manpage
     and the _p_e_r_l_r_e manpage).  In addition, several special variables which
     provide windows into the inner working of Perl have names containing
     punctuation characters (see the _p_e_r_l_v_a_r manpage).

     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 '%':





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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



         %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.

     Because 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 _I_S 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 _p_e_r_l_r_e_f manpage.

     Names that start with a digit may contain only 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.)

     CCCCoooonnnntttteeeexxxxtttt

     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,



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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     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 _p_e_r_l_f_u_n_c manpage.

     SSSSccccaaaallllaaaarrrr vvvvaaaalllluuuueeeessss

     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
     the same thing for nearly all purposes, references are strongly-typed
     uncastable pointers with builtin 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 _d_e_f_i_n_e_d() operator to



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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     determine whether the value is defined or not.

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

         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
     _p_e_r_l_r_e 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 ccccsssshhhh.  (Actually, it's not the
     length of the array, it's the subscript of the last element, because
     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
     _N_O _L_O_N_G_E_R recovers the values that were in those elements.  (It used to
     in Perl 4, but we had to break this to make sure destructors were called
     when expected.)  You can also gain some measure of efficiency by pre-
     extending 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 assume that





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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



         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 useful only 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.)

     SSSSccccaaaallllaaaarrrr vvvvaaaalllluuuueeee ccccoooonnnnssssttttrrrruuuuccccttttoooorrrrssss

     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 section on _Q_u_o_t_e _a_n_d _Q_u_o_t_e_l_i_k_e _O_p_e_r_a_t_o_r_s in the _p_e_r_l_o_p
     manpage for a list.

     Octal or hex representations in string literals (e.g. '0xffff') are not
     automatically converted to their integer representation.  The _h_e_x() and
     _o_c_t() functions make these conversions for you.  See the hex entry in the
     _p_e_r_l_f_u_n_c manpage and the oct entry in the _p_e_r_l_f_u_n_c manpage for more
     details.

     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, names beginning
     with $ or @, followed by an optional bracketed expression as a
     subscript.)  The following code segment prints out "The price is $100."



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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



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

     As in some shells, you can put curly brackets around the name 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, because single quote is a valid (though deprecated) character
     in a variable name (see the Packages entry in the _p_e_r_l_m_o_d manpage).

     Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
     represent the current filename, line number, and package name at that
     point in your program.  They may be used only as separate tokens; they
     will not be interpolated into strings.  If there is no current package
     (due to a package; directive), __PACKAGE__ is the undefined value.

     The tokens __END__ and __DATA__ 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 a DATA filehandle: main::DATA for __END__,
     or PACKNAME::DATA (where PACKNAME is the current package) for __DATA__.
     The two control characters ^D and ^Z are synonyms for __END__ (or
     __DATA__ in a module).  See the _S_e_l_f_L_o_a_d_e_r manpage for more description
     of __DATA__, and an example of its use.  Note that you cannot read from
     the DATA filehandle in a BEGIN block: the BEGIN block is executed as soon
     as it is seen (during compilation), at which point the corresponding
     __DATA__ (or __END__) token has not yet been seen.

     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 ----wwww
     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'.




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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     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 empty
     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





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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



             myfunc(<<"THIS", 23, <<'THAT');
         Here's a line
         or two.
         THIS
         and here's 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;


     LLLLiiiisssstttt vvvvaaaalllluuuueeee ccccoooonnnnssssttttrrrruuuuccccttttoooorrrrssss

     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 the value 3 to $foo:

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

     You may have an optional comma before the closing parenthesis of a 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



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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     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
     _N_O_T interpolate, see the _p_e_r_l_r_e_f 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.  For example:

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

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

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

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

     You may assign to undef in a list.  This is useful for throwing away some
     of the return values of a function:

         ($dev, $ino, undef, undef, $uid, $gid) = stat($file);

     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, because most list functions return a null list when finished,
     which when assigned produces a 0, which is interpreted as FALSE.




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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     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 _l_o_c_a_l() or _m_y().

     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 arranges for its left-hand operand to be interpreted
     as a string, if it's a bareword which would be a legal identifier.  This
     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
        );




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PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))                                                        PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))



     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 _p_e_r_l_f_u_n_c
     manpage for examples of how to arrange for an output ordering.

     TTTTyyyyppppeeeegggglllloooobbbbssss aaaannnndddd FFFFiiiilllleeeehhhhaaaannnnddddlllleeeessss

     Perl uses an internal type called a _t_y_p_e_g_l_o_b 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.  It also used to be the preferred way to pass
     filehandles into a function, but now that we have the *foo{THING}
     notation it isn't often needed for that, either.  It is still needed to
     pass new filehandles into functions (*HANDLE{IO} only works if HANDLE has
     already been used).

     If you need to use a typeglob to save away a filehandle, do it this way:

         $fh = *STDOUT;

     or perhaps as a real reference, like this:

         $fh = \*STDOUT;

     This is also a 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');

     Another way to create local filehandles is with IO::Handle and its ilk,
     see the bottom of the open() entry in the _p_e_r_l_f_u_n_c manpage.

     See the _p_e_r_l_r_e_f manpage, the _p_e_r_l_s_u_b manpage, and the section on _S_y_m_b_o_l
     _T_a_b_l_e_s in the _p_e_r_l_m_o_d manpage for more discussion on typeglobs.
















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