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Man Page for PERLDATA

     perldata - Perl data structures

     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.)


     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

     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

     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.

     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.

     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

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

     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:

         0xffff              # hex
         0377                # octal
         4_294_967_296       # underline for legibility

     String literals are 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

     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.0;     # interpreted

     As in some shells, you can put curly brackets around the
     identifier to delimit it from following alphanumerics.  Also
     note that a single-quoted string must be separated from a
     preceding word by a space, since single quote is a valid
     (though discouraged) 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

     A word that doesn't have any 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, 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--see the Merry Christmas example below.)  The
     terminating string must appear by itself (unquoted and with
     no surrounding whitespace) on the terminating line.

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

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

             print << x 10;  # Legal but discouraged.  Use  <<"".
         Merry Christmas!

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

             print <<"foo", <<"bar"; # you can stack them
         I said foo.
         I said bar.

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

     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
             + 20;

     List value constructors

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


     In a context not requiring an 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

         @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 = (

     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


     contains all the elements of @foo followed by all the
     elements of @bar, followed by all the elements returned by
     the subroutine named SomeSub.  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.

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

         # 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);

     The final element may be an array or a hash:

         ($a, $b, @rest) = split;
         local($a, $b, %rest) = @_;

     You can actually put an array anywhere in the list, but the
     first array 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);

     It is often more readable to use the => operator between
     key/value pairs (the => operator is actually nothing more
     than a more visually distinctive synonym for a comma):

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

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

         $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2

     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.



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