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   1  =head1 NAME
   3  perldata - Perl data types
   5  =head1 DESCRIPTION
   7  =head2 Variable names
   8  X<variable, name> X<variable name> X<data type> X<type>
  10  Perl has three built-in data types: scalars, arrays of scalars, and
  11  associative arrays of scalars, known as "hashes".  A scalar is a 
  12  single string (of any size, limited only by the available memory),
  13  number, or a reference to something (which will be discussed
  14  in L<perlref>).  Normal arrays are ordered lists of scalars indexed
  15  by number, starting with 0.  Hashes are unordered collections of scalar 
  16  values indexed by their associated string key.
  18  Values are usually referred to by name, or through a named reference.
  19  The first character of the name tells you to what sort of data
  20  structure it refers.  The rest of the name tells you the particular
  21  value to which it refers.  Usually this name is a single I<identifier>,
  22  that is, a string beginning with a letter or underscore, and
  23  containing letters, underscores, and digits.  In some cases, it may
  24  be a chain of identifiers, separated by C<::> (or by the slightly
  25  archaic C<'>); all but the last are interpreted as names of packages,
  26  to locate the namespace in which to look up the final identifier
  27  (see L<perlmod/Packages> for details).  It's possible to substitute
  28  for a simple identifier, an expression that produces a reference
  29  to the value at runtime.   This is described in more detail below
  30  and in L<perlref>.
  31  X<identifier>
  33  Perl also has its own built-in variables whose names don't follow
  34  these rules.  They have strange names so they don't accidentally
  35  collide with one of your normal variables.  Strings that match
  36  parenthesized parts of a regular expression are saved under names
  37  containing only digits after the C<$> (see L<perlop> and L<perlre>).
  38  In addition, several special variables that provide windows into
  39  the inner working of Perl have names containing punctuation characters
  40  and control characters.  These are documented in L<perlvar>.
  41  X<variable, built-in>
  43  Scalar values are always named with '$', even when referring to a
  44  scalar that is part of an array or a hash.  The '$' symbol works
  45  semantically like the English word "the" in that it indicates a
  46  single value is expected.
  47  X<scalar>
  49      $days        # the simple scalar value "days"
  50      $days[28]        # the 29th element of array @days
  51      $days{'Feb'}    # the 'Feb' value from hash %days
  52      $#days        # the last index of array @days
  54  Entire arrays (and slices of arrays and hashes) are denoted by '@',
  55  which works much like the word "these" or "those" does in English,
  56  in that it indicates multiple values are expected.
  57  X<array>
  59      @days        # ($days[0], $days[1],... $days[n])
  60      @days[3,4,5]    # same as ($days[3],$days[4],$days[5])
  61      @days{'a','c'}    # same as ($days{'a'},$days{'c'})
  63  Entire hashes are denoted by '%':
  64  X<hash>
  66      %days        # (key1, val1, key2, val2 ...)
  68  In addition, subroutines are named with an initial '&', though this
  69  is optional when unambiguous, just as the word "do" is often redundant
  70  in English.  Symbol table entries can be named with an initial '*',
  71  but you don't really care about that yet (if ever :-).
  73  Every variable type has its own namespace, as do several
  74  non-variable identifiers.  This means that you can, without fear
  75  of conflict, use the same name for a scalar variable, an array, or
  76  a hash--or, for that matter, for a filehandle, a directory handle, a
  77  subroutine name, a format name, or a label.  This means that $foo
  78  and @foo are two different variables.  It also means that C<$foo[1]>
  79  is a part of @foo, not a part of $foo.  This may seem a bit weird,
  80  but that's okay, because it is weird.
  81  X<namespace>
  83  Because variable references always start with '$', '@', or '%', the
  84  "reserved" words aren't in fact reserved with respect to variable
  85  names.  They I<are> reserved with respect to labels and filehandles,
  86  however, which don't have an initial special character.  You can't
  87  have a filehandle named "log", for instance.  Hint: you could say
  88  C<open(LOG,'logfile')> rather than C<open(log,'logfile')>.  Using
  89  uppercase filehandles also improves readability and protects you
  90  from conflict with future reserved words.  Case I<is> significant--"FOO",
  91  "Foo", and "foo" are all different names.  Names that start with a
  92  letter or underscore may also contain digits and underscores.
  93  X<identifier, case sensitivity>
  94  X<case>
  96  It is possible to replace such an alphanumeric name with an expression
  97  that returns a reference to the appropriate type.  For a description
  98  of this, see L<perlref>.
 100  Names that start with a digit may contain only more digits.  Names
 101  that do not start with a letter, underscore, digit or a caret (i.e.
 102  a control character) are limited to one character, e.g.,  C<$%> or
 103  C<$$>.  (Most of these one character names have a predefined
 104  significance to Perl.  For instance, C<$$> is the current process
 105  id.)
 107  =head2 Context
 108  X<context> X<scalar context> X<list context>
 110  The interpretation of operations and values in Perl sometimes depends
 111  on the requirements of the context around the operation or value.
 112  There are two major contexts: list and scalar.  Certain operations
 113  return list values in contexts wanting a list, and scalar values
 114  otherwise.  If this is true of an operation it will be mentioned in
 115  the documentation for that operation.  In other words, Perl overloads
 116  certain operations based on whether the expected return value is
 117  singular or plural.  Some words in English work this way, like "fish"
 118  and "sheep".
 120  In a reciprocal fashion, an operation provides either a scalar or a
 121  list context to each of its arguments.  For example, if you say
 123      int( <STDIN> )
 125  the integer operation provides scalar context for the <>
 126  operator, which responds by reading one line from STDIN and passing it
 127  back to the integer operation, which will then find the integer value
 128  of that line and return that.  If, on the other hand, you say
 130      sort( <STDIN> )
 132  then the sort operation provides list context for <>, which
 133  will proceed to read every line available up to the end of file, and
 134  pass that list of lines back to the sort routine, which will then
 135  sort those lines and return them as a list to whatever the context
 136  of the sort was.
 138  Assignment is a little bit special in that it uses its left argument
 139  to determine the context for the right argument.  Assignment to a
 140  scalar evaluates the right-hand side in scalar context, while
 141  assignment to an array or hash evaluates the righthand side in list
 142  context.  Assignment to a list (or slice, which is just a list
 143  anyway) also evaluates the righthand side in list context.
 145  When you use the C<use warnings> pragma or Perl's B<-w> command-line 
 146  option, you may see warnings
 147  about useless uses of constants or functions in "void context".
 148  Void context just means the value has been discarded, such as a
 149  statement containing only C<"fred";> or C<getpwuid(0);>.  It still
 150  counts as scalar context for functions that care whether or not
 151  they're being called in list context.
 153  User-defined subroutines may choose to care whether they are being
 154  called in a void, scalar, or list context.  Most subroutines do not
 155  need to bother, though.  That's because both scalars and lists are
 156  automatically interpolated into lists.  See L<perlfunc/wantarray>
 157  for how you would dynamically discern your function's calling
 158  context.
 160  =head2 Scalar values
 161  X<scalar> X<number> X<string> X<reference>
 163  All data in Perl is a scalar, an array of scalars, or a hash of
 164  scalars.  A scalar may contain one single value in any of three
 165  different flavors: a number, a string, or a reference.  In general,
 166  conversion from one form to another is transparent.  Although a
 167  scalar may not directly hold multiple values, it may contain a
 168  reference to an array or hash which in turn contains multiple values.
 170  Scalars aren't necessarily one thing or another.  There's no place
 171  to declare a scalar variable to be of type "string", type "number",
 172  type "reference", or anything else.  Because of the automatic
 173  conversion of scalars, operations that return scalars don't need
 174  to care (and in fact, cannot care) whether their caller is looking
 175  for a string, a number, or a reference.  Perl is a contextually
 176  polymorphic language whose scalars can be strings, numbers, or
 177  references (which includes objects).  Although strings and numbers
 178  are considered pretty much the same thing for nearly all purposes,
 179  references are strongly-typed, uncastable pointers with builtin
 180  reference-counting and destructor invocation.
 182  A scalar value is interpreted as TRUE in the Boolean sense if it is not
 183  the null string or the number 0 (or its string equivalent, "0").  The
 184  Boolean context is just a special kind of scalar context where no 
 185  conversion to a string or a number is ever performed.
 186  X<boolean> X<bool> X<true> X<false> X<truth>
 188  There are actually two varieties of null strings (sometimes referred
 189  to as "empty" strings), a defined one and an undefined one.  The
 190  defined version is just a string of length zero, such as C<"">.
 191  The undefined version is the value that indicates that there is
 192  no real value for something, such as when there was an error, or
 193  at end of file, or when you refer to an uninitialized variable or
 194  element of an array or hash.  Although in early versions of Perl,
 195  an undefined scalar could become defined when first used in a
 196  place expecting a defined value, this no longer happens except for
 197  rare cases of autovivification as explained in L<perlref>.  You can
 198  use the defined() operator to determine whether a scalar value is
 199  defined (this has no meaning on arrays or hashes), and the undef()
 200  operator to produce an undefined value.
 201  X<defined> X<undefined> X<undef> X<null> X<string, null>
 203  To find out whether a given string is a valid non-zero number, it's
 204  sometimes enough to test it against both numeric 0 and also lexical
 205  "0" (although this will cause noises if warnings are on).  That's 
 206  because strings that aren't numbers count as 0, just as they do in B<awk>:
 208      if ($str == 0 && $str ne "0")  {
 209      warn "That doesn't look like a number";
 210      }
 212  That method may be best because otherwise you won't treat IEEE
 213  notations like C<NaN> or C<Infinity> properly.  At other times, you
 214  might prefer to determine whether string data can be used numerically
 215  by calling the POSIX::strtod() function or by inspecting your string
 216  with a regular expression (as documented in L<perlre>).
 218      warn "has nondigits"    if     /\D/;
 219      warn "not a natural number" unless /^\d+$/;             # rejects -3
 220      warn "not an integer"       unless /^-?\d+$/;           # rejects +3
 221      warn "not an integer"       unless /^[+-]?\d+$/;
 222      warn "not a decimal number" unless /^-?\d+\.?\d*$/;     # rejects .2
 223      warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
 224      warn "not a C float"
 225      unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
 227  The length of an array is a scalar value.  You may find the length
 228  of array @days by evaluating C<$#days>, as in B<csh>.  However, this
 229  isn't the length of the array; it's the subscript of the last element,
 230  which is a different value since there is ordinarily a 0th element.
 231  Assigning to C<$#days> actually changes the length of the array.
 232  Shortening an array this way destroys intervening values.  Lengthening
 233  an array that was previously shortened does not recover values
 234  that were in those elements.  (It used to do so in Perl 4, but we
 235  had to break this to make sure destructors were called when expected.)
 236  X<$#> X<array, length>
 238  You can also gain some minuscule measure of efficiency by pre-extending
 239  an array that is going to get big.  You can also extend an array
 240  by assigning to an element that is off the end of the array.  You
 241  can truncate an array down to nothing by assigning the null list
 242  () to it.  The following are equivalent:
 244      @whatever = ();
 245      $#whatever = -1;
 247  If you evaluate an array in scalar context, it returns the length
 248  of the array.  (Note that this is not true of lists, which return
 249  the last value, like the C comma operator, nor of built-in functions,
 250  which return whatever they feel like returning.)  The following is
 251  always true:
 252  X<array, length>
 254      scalar(@whatever) == $#whatever - $[ + 1;
 256  Version 5 of Perl changed the semantics of C<$[>: files that don't set
 257  the value of C<$[> no longer need to worry about whether another
 258  file changed its value.  (In other words, use of C<$[> is deprecated.)
 259  So in general you can assume that
 260  X<$[>
 262      scalar(@whatever) == $#whatever + 1;
 264  Some programmers choose to use an explicit conversion so as to 
 265  leave nothing to doubt:
 267      $element_count = scalar(@whatever);
 269  If you evaluate a hash in scalar context, it returns false if the
 270  hash is empty.  If there are any key/value pairs, it returns true;
 271  more precisely, the value returned is a string consisting of the
 272  number of used buckets and the number of allocated buckets, separated
 273  by a slash.  This is pretty much useful only to find out whether
 274  Perl's internal hashing algorithm is performing poorly on your data
 275  set.  For example, you stick 10,000 things in a hash, but evaluating
 276  %HASH in scalar context reveals C<"1/16">, which means only one out
 277  of sixteen buckets has been touched, and presumably contains all
 278  10,000 of your items.  This isn't supposed to happen.  If a tied hash
 279  is evaluated in scalar context, a fatal error will result, since this
 280  bucket usage information is currently not available for tied hashes.
 281  X<hash, scalar context> X<hash, bucket> X<bucket>
 283  You can preallocate space for a hash by assigning to the keys() function.
 284  This rounds up the allocated buckets to the next power of two:
 286      keys(%users) = 1000;        # allocate 1024 buckets
 288  =head2 Scalar value constructors
 289  X<scalar, literal> X<scalar, constant>
 291  Numeric literals are specified in any of the following floating point or
 292  integer formats:
 294      12345
 295      12345.67
 296      .23E-10             # a very small number
 297      3.14_15_92          # a very important number
 298      4_294_967_296       # underscore for legibility
 299      0xff                # hex
 300      0xdead_beef         # more hex   
 301      0377                # octal (only numbers, begins with 0)
 302      0b011011            # binary
 304  You are allowed to use underscores (underbars) in numeric literals
 305  between digits for legibility.  You could, for example, group binary
 306  digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
 307  or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
 308  X<number, literal>
 310  String literals are usually delimited by either single or double
 311  quotes.  They work much like quotes in the standard Unix shells:
 312  double-quoted string literals are subject to backslash and variable
 313  substitution; single-quoted strings are not (except for C<\'> and
 314  C<\\>).  The usual C-style backslash rules apply for making
 315  characters such as newline, tab, etc., as well as some more exotic
 316  forms.  See L<perlop/"Quote and Quote-like Operators"> for a list.
 317  X<string, literal>
 319  Hexadecimal, octal, or binary, representations in string literals
 320  (e.g. '0xff') are not automatically converted to their integer
 321  representation.  The hex() and oct() functions make these conversions
 322  for you.  See L<perlfunc/hex> and L<perlfunc/oct> for more details.
 324  You can also embed newlines directly in your strings, i.e., they can end
 325  on a different line than they begin.  This is nice, but if you forget
 326  your trailing quote, the error will not be reported until Perl finds
 327  another line containing the quote character, which may be much further
 328  on in the script.  Variable substitution inside strings is limited to
 329  scalar variables, arrays, and array or hash slices.  (In other words,
 330  names beginning with $ or @, followed by an optional bracketed
 331  expression as a subscript.)  The following code segment prints out "The
 332  price is $Z<>100."
 333  X<interpolation>
 335      $Price = '$100';    # not interpolated
 336      print "The price is $Price.\n";    # interpolated
 338  There is no double interpolation in Perl, so the C<$100> is left as is.
 340  By default floating point numbers substituted inside strings use the
 341  dot (".")  as the decimal separator.  If C<use locale> is in effect,
 342  and POSIX::setlocale() has been called, the character used for the
 343  decimal separator is affected by the LC_NUMERIC locale.
 344  See L<perllocale> and L<POSIX>.
 346  As in some shells, you can enclose the variable name in braces to
 347  disambiguate it from following alphanumerics (and underscores).
 348  You must also do
 349  this when interpolating a variable into a string to separate the
 350  variable name from a following double-colon or an apostrophe, since
 351  these would be otherwise treated as a package separator:
 352  X<interpolation>
 354      $who = "Larry";
 355      print PASSWD "$who}::0:0:Superuser:/:/bin/perl\n";
 356      print "We use $who}speak when $who}'s here.\n";
 358  Without the braces, Perl would have looked for a $whospeak, a
 359  C<$who::0>, and a C<$who's> variable.  The last two would be the
 360  $0 and the $s variables in the (presumably) non-existent package
 361  C<who>.
 363  In fact, an identifier within such curlies is forced to be a string,
 364  as is any simple identifier within a hash subscript.  Neither need
 365  quoting.  Our earlier example, C<$days{'Feb'}> can be written as
 366  C<$days{Feb}> and the quotes will be assumed automatically.  But
 367  anything more complicated in the subscript will be interpreted as an
 368  expression.  This means for example that C<$version{2.0}++> is
 369  equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
 371  =head3 Version Strings
 372  X<version string> X<vstring> X<v-string>
 374  B<Note:> Version Strings (v-strings) have been deprecated.  They will
 375  be removed in some future release after Perl 5.8.1.  The marginal
 376  benefits of v-strings were greatly outweighed by the potential for
 377  Surprise and Confusion.
 379  A literal of the form C<v1.20.300.4000> is parsed as a string composed
 380  of characters with the specified ordinals.  This form, known as
 381  v-strings, provides an alternative, more readable way to construct
 382  strings, rather than use the somewhat less readable interpolation form
 383  C<"\x{1}\x{14}\x{12c}\x{fa0}">.  This is useful for representing
 384  Unicode strings, and for comparing version "numbers" using the string
 385  comparison operators, C<cmp>, C<gt>, C<lt> etc.  If there are two or
 386  more dots in the literal, the leading C<v> may be omitted.
 388      print v9786;              # prints SMILEY, "\x{263a}"
 389      print v102.111.111;       # prints "foo"
 390      print 102.111.111;        # same
 392  Such literals are accepted by both C<require> and C<use> for
 393  doing a version check.  Note that using the v-strings for IPv4
 394  addresses is not portable unless you also use the
 395  inet_aton()/inet_ntoa() routines of the Socket package.
 397  Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
 398  are not v-strings before the C<< => >> operator (which is usually used
 399  to separate a hash key from a hash value), instead they are interpreted
 400  as literal strings ('v65').  They were v-strings from Perl 5.6.0 to
 401  Perl 5.8.0, but that caused more confusion and breakage than good.
 402  Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
 403  be v-strings always.
 405  =head3 Special Literals
 406  X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
 407  X<end> X<data> X<^D> X<^Z>
 409  The special literals __FILE__, __LINE__, and __PACKAGE__
 410  represent the current filename, line number, and package name at that
 411  point in your program.  They may be used only as separate tokens; they
 412  will not be interpolated into strings.  If there is no current package
 413  (due to an empty C<package;> directive), __PACKAGE__ is the undefined
 414  value.
 415  X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
 417  The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
 418  may be used to indicate the logical end of the script before the actual
 419  end of file.  Any following text is ignored.
 421  Text after __DATA__ but may be read via the filehandle C<PACKNAME::DATA>,
 422  where C<PACKNAME> is the package that was current when the __DATA__
 423  token was encountered.  The filehandle is left open pointing to the
 424  contents after __DATA__.  It is the program's responsibility to
 425  C<close DATA> when it is done reading from it.  For compatibility with
 426  older scripts written before __DATA__ was introduced, __END__ behaves
 427  like __DATA__ in the top level script (but not in files loaded with
 428  C<require> or C<do>) and leaves the remaining contents of the
 429  file accessible via C<main::DATA>.
 431  See L<SelfLoader> for more description of __DATA__, and
 432  an example of its use.  Note that you cannot read from the DATA
 433  filehandle in a BEGIN block: the BEGIN block is executed as soon
 434  as it is seen (during compilation), at which point the corresponding
 435  __DATA__ (or __END__) token has not yet been seen.
 437  =head3 Barewords
 438  X<bareword>
 440  A word that has no other interpretation in the grammar will
 441  be treated as if it were a quoted string.  These are known as
 442  "barewords".  As with filehandles and labels, a bareword that consists
 443  entirely of lowercase letters risks conflict with future reserved
 444  words, and if you use the C<use warnings> pragma or the B<-w> switch, 
 445  Perl will warn you about any
 446  such words.  Some people may wish to outlaw barewords entirely.  If you
 447  say
 449      use strict 'subs';
 451  then any bareword that would NOT be interpreted as a subroutine call
 452  produces a compile-time error instead.  The restriction lasts to the
 453  end of the enclosing block.  An inner block may countermand this
 454  by saying C<no strict 'subs'>.
 456  =head3 Array Joining Delimiter
 457  X<array, interpolation> X<interpolation, array> X<$">
 459  Arrays and slices are interpolated into double-quoted strings
 460  by joining the elements with the delimiter specified in the C<$">
 461  variable (C<$LIST_SEPARATOR> if "use English;" is specified), 
 462  space by default.  The following are equivalent:
 464      $temp = join($", @ARGV);
 465      system "echo $temp";
 467      system "echo @ARGV";
 469  Within search patterns (which also undergo double-quotish substitution)
 470  there is an unfortunate ambiguity:  Is C</$foo[bar]/> to be interpreted as
 471  C</$foo}[bar]/> (where C<[bar]> is a character class for the regular
 472  expression) or as C</$foo[bar]}/> (where C<[bar]> is the subscript to array
 473  @foo)?  If @foo doesn't otherwise exist, then it's obviously a
 474  character class.  If @foo exists, Perl takes a good guess about C<[bar]>,
 475  and is almost always right.  If it does guess wrong, or if you're just
 476  plain paranoid, you can force the correct interpretation with curly
 477  braces as above.
 479  If you're looking for the information on how to use here-documents,
 480  which used to be here, that's been moved to
 481  L<perlop/Quote and Quote-like Operators>.
 483  =head2 List value constructors
 484  X<list>
 486  List values are denoted by separating individual values by commas
 487  (and enclosing the list in parentheses where precedence requires it):
 489      (LIST)
 491  In a context not requiring a list value, the value of what appears
 492  to be a list literal is simply the value of the final element, as
 493  with the C comma operator.  For example,
 495      @foo = ('cc', '-E', $bar);
 497  assigns the entire list value to array @foo, but
 499      $foo = ('cc', '-E', $bar);
 501  assigns the value of variable $bar to the scalar variable $foo.
 502  Note that the value of an actual array in scalar context is the
 503  length of the array; the following assigns the value 3 to $foo:
 505      @foo = ('cc', '-E', $bar);
 506      $foo = @foo;                # $foo gets 3
 508  You may have an optional comma before the closing parenthesis of a
 509  list literal, so that you can say:
 511      @foo = (
 512          1,
 513          2,
 514          3,
 515      );
 517  To use a here-document to assign an array, one line per element,
 518  you might use an approach like this:
 520      @sauces = <<End_Lines =~ m/(\S.*\S)/g;
 521          normal tomato
 522          spicy tomato
 523          green chile
 524          pesto
 525          white wine
 526      End_Lines
 528  LISTs do automatic interpolation of sublists.  That is, when a LIST is
 529  evaluated, each element of the list is evaluated in list context, and
 530  the resulting list value is interpolated into LIST just as if each
 531  individual element were a member of LIST.  Thus arrays and hashes lose their
 532  identity in a LIST--the list
 534      (@foo,@bar,&SomeSub,%glarch)
 536  contains all the elements of @foo followed by all the elements of @bar,
 537  followed by all the elements returned by the subroutine named SomeSub 
 538  called in list context, followed by the key/value pairs of %glarch.
 539  To make a list reference that does I<NOT> interpolate, see L<perlref>.
 541  The null list is represented by ().  Interpolating it in a list
 542  has no effect.  Thus ((),(),()) is equivalent to ().  Similarly,
 543  interpolating an array with no elements is the same as if no
 544  array had been interpolated at that point.
 546  This interpolation combines with the facts that the opening
 547  and closing parentheses are optional (except when necessary for
 548  precedence) and lists may end with an optional comma to mean that
 549  multiple commas within lists are legal syntax. The list C<1,,3> is a
 550  concatenation of two lists, C<1,> and C<3>, the first of which ends
 551  with that optional comma.  C<1,,3> is C<(1,),(3)> is C<1,3> (And
 552  similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.)  Not that
 553  we'd advise you to use this obfuscation.
 555  A list value may also be subscripted like a normal array.  You must
 556  put the list in parentheses to avoid ambiguity.  For example:
 558      # Stat returns list value.
 559      $time = (stat($file))[8];
 561      # SYNTAX ERROR HERE.
 562      $time = stat($file)[8];  # OOPS, FORGOT PARENTHESES
 564      # Find a hex digit.
 565      $hexdigit = ('a','b','c','d','e','f')[$digit-10];
 567      # A "reverse comma operator".
 568      return (pop(@foo),pop(@foo))[0];
 570  Lists may be assigned to only when each element of the list
 571  is itself legal to assign to:
 573      ($a, $b, $c) = (1, 2, 3);
 575      ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
 577  An exception to this is that you may assign to C<undef> in a list.
 578  This is useful for throwing away some of the return values of a
 579  function:
 581      ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
 583  List assignment in scalar context returns the number of elements
 584  produced by the expression on the right side of the assignment:
 586      $x = (($foo,$bar) = (3,2,1));       # set $x to 3, not 2
 587      $x = (($foo,$bar) = f());           # set $x to f()'s return count
 589  This is handy when you want to do a list assignment in a Boolean
 590  context, because most list functions return a null list when finished,
 591  which when assigned produces a 0, which is interpreted as FALSE.
 593  It's also the source of a useful idiom for executing a function or
 594  performing an operation in list context and then counting the number of
 595  return values, by assigning to an empty list and then using that
 596  assignment in scalar context. For example, this code:
 598      $count = () = $string =~ /\d+/g;
 600  will place into $count the number of digit groups found in $string.
 601  This happens because the pattern match is in list context (since it
 602  is being assigned to the empty list), and will therefore return a list
 603  of all matching parts of the string. The list assignment in scalar
 604  context will translate that into the number of elements (here, the
 605  number of times the pattern matched) and assign that to $count. Note
 606  that simply using
 608      $count = $string =~ /\d+/g;
 610  would not have worked, since a pattern match in scalar context will
 611  only return true or false, rather than a count of matches.
 613  The final element of a list assignment may be an array or a hash:
 615      ($a, $b, @rest) = split;
 616      my($a, $b, %rest) = @_;
 618  You can actually put an array or hash anywhere in the list, but the first one
 619  in the list will soak up all the values, and anything after it will become
 620  undefined.  This may be useful in a my() or local().
 622  A hash can be initialized using a literal list holding pairs of
 623  items to be interpreted as a key and a value:
 625      # same as map assignment above
 626      %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
 628  While literal lists and named arrays are often interchangeable, that's
 629  not the case for hashes.  Just because you can subscript a list value like
 630  a normal array does not mean that you can subscript a list value as a
 631  hash.  Likewise, hashes included as parts of other lists (including
 632  parameters lists and return lists from functions) always flatten out into
 633  key/value pairs.  That's why it's good to use references sometimes.
 635  It is often more readable to use the C<< => >> operator between key/value
 636  pairs.  The C<< => >> operator is mostly just a more visually distinctive
 637  synonym for a comma, but it also arranges for its left-hand operand to be
 638  interpreted as a string -- if it's a bareword that would be a legal simple
 639  identifier (C<< => >> doesn't quote compound identifiers, that contain
 640  double colons). This makes it nice for initializing hashes:
 642      %map = (
 643                   red   => 0x00f,
 644                   blue  => 0x0f0,
 645                   green => 0xf00,
 646     );
 648  or for initializing hash references to be used as records:
 650      $rec = {
 651                  witch => 'Mable the Merciless',
 652                  cat   => 'Fluffy the Ferocious',
 653                  date  => '10/31/1776',
 654      };
 656  or for using call-by-named-parameter to complicated functions:
 658     $field = $query->radio_group(
 659                 name      => 'group_name',
 660                 values    => ['eenie','meenie','minie'],
 661                 default   => 'meenie',
 662                 linebreak => 'true',
 663                 labels    => \%labels
 664     );
 666  Note that just because a hash is initialized in that order doesn't
 667  mean that it comes out in that order.  See L<perlfunc/sort> for examples
 668  of how to arrange for an output ordering.
 670  =head2 Subscripts
 672  An array is subscripted by specifying a dollar sign (C<$>), then the
 673  name of the array (without the leading C<@>), then the subscript inside
 674  square brackets.  For example:
 676      @myarray = (5, 50, 500, 5000);
 677      print "Element Number 2 is", $myarray[2], "\n";
 679  The array indices start with 0. A negative subscript retrieves its 
 680  value from the end.  In our example, C<$myarray[-1]> would have been 
 681  5000, and C<$myarray[-2]> would have been 500.
 683  Hash subscripts are similar, only instead of square brackets curly brackets
 684  are used. For example:
 686      %scientists = 
 687      (
 688          "Newton" => "Isaac",
 689          "Einstein" => "Albert",
 690          "Darwin" => "Charles",
 691          "Feynman" => "Richard",
 692      );
 694      print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
 696  =head2 Slices
 697  X<slice> X<array, slice> X<hash, slice>
 699  A common way to access an array or a hash is one scalar element at a
 700  time.  You can also subscript a list to get a single element from it.
 702      $whoami = $ENV{"USER"};             # one element from the hash
 703      $parent = $ISA[0];                  # one element from the array
 704      $dir    = (getpwnam("daemon"))[7];  # likewise, but with list
 706  A slice accesses several elements of a list, an array, or a hash
 707  simultaneously using a list of subscripts.  It's more convenient
 708  than writing out the individual elements as a list of separate
 709  scalar values.
 711      ($him, $her)   = @folks[0,-1];              # array slice
 712      @them          = @folks[0 .. 3];            # array slice
 713      ($who, $home)  = @ENV{"USER", "HOME"};      # hash slice
 714      ($uid, $dir)   = (getpwnam("daemon"))[2,7]; # list slice
 716  Since you can assign to a list of variables, you can also assign to
 717  an array or hash slice.
 719      @days[3..5]    = qw/Wed Thu Fri/;
 720      @colors{'red','blue','green'} 
 721                     = (0xff0000, 0x0000ff, 0x00ff00);
 722      @folks[0, -1]  = @folks[-1, 0];
 724  The previous assignments are exactly equivalent to
 726      ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
 727      ($colors{'red'}, $colors{'blue'}, $colors{'green'})
 728                     = (0xff0000, 0x0000ff, 0x00ff00);
 729      ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
 731  Since changing a slice changes the original array or hash that it's
 732  slicing, a C<foreach> construct will alter some--or even all--of the
 733  values of the array or hash.
 735      foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } 
 737      foreach (@hash{qw[key1 key2]}) {
 738          s/^\s+//;           # trim leading whitespace
 739          s/\s+$//;           # trim trailing whitespace
 740          s/(\w+)/\u\L$1/g;   # "titlecase" words
 741      }
 743  A slice of an empty list is still an empty list.  Thus:
 745      @a = ()[1,0];           # @a has no elements
 746      @b = (@a)[0,1];         # @b has no elements
 747      @c = (0,1)[2,3];        # @c has no elements
 749  But:
 751      @a = (1)[1,0];          # @a has two elements
 752      @b = (1,undef)[1,0,2];  # @b has three elements
 754  This makes it easy to write loops that terminate when a null list
 755  is returned:
 757      while ( ($home, $user) = (getpwent)[7,0]) {
 758          printf "%-8s %s\n", $user, $home;
 759      }
 761  As noted earlier in this document, the scalar sense of list assignment
 762  is the number of elements on the right-hand side of the assignment.
 763  The null list contains no elements, so when the password file is
 764  exhausted, the result is 0, not 2.
 766  If you're confused about why you use an '@' there on a hash slice
 767  instead of a '%', think of it like this.  The type of bracket (square
 768  or curly) governs whether it's an array or a hash being looked at.
 769  On the other hand, the leading symbol ('$' or '@') on the array or
 770  hash indicates whether you are getting back a singular value (a
 771  scalar) or a plural one (a list).
 773  =head2 Typeglobs and Filehandles
 774  X<typeglob> X<filehandle> X<*>
 776  Perl uses an internal type called a I<typeglob> to hold an entire
 777  symbol table entry.  The type prefix of a typeglob is a C<*>, because
 778  it represents all types.  This used to be the preferred way to
 779  pass arrays and hashes by reference into a function, but now that
 780  we have real references, this is seldom needed.  
 782  The main use of typeglobs in modern Perl is create symbol table aliases.
 783  This assignment:
 785      *this = *that;
 787  makes $this an alias for $that, @this an alias for @that, %this an alias
 788  for %that, &this an alias for &that, etc.  Much safer is to use a reference.
 789  This:
 791      local *Here::blue = \$There::green;
 793  temporarily makes $Here::blue an alias for $There::green, but doesn't
 794  make @Here::blue an alias for @There::green, or %Here::blue an alias for
 795  %There::green, etc.  See L<perlmod/"Symbol Tables"> for more examples
 796  of this.  Strange though this may seem, this is the basis for the whole
 797  module import/export system.
 799  Another use for typeglobs is to pass filehandles into a function or
 800  to create new filehandles.  If you need to use a typeglob to save away
 801  a filehandle, do it this way:
 803      $fh = *STDOUT;
 805  or perhaps as a real reference, like this:
 807      $fh = \*STDOUT;
 809  See L<perlsub> for examples of using these as indirect filehandles
 810  in functions.
 812  Typeglobs are also a way to create a local filehandle using the local()
 813  operator.  These last until their block is exited, but may be passed back.
 814  For example:
 816      sub newopen {
 817          my $path = shift;
 818          local  *FH;  # not my!
 819          open   (FH, $path)          or  return undef;
 820          return *FH;
 821      }
 822      $fh = newopen('/etc/passwd');
 824  Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
 825  for filehandle manipulations, although they're still needed to pass brand
 826  new file and directory handles into or out of functions. That's because
 827  C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
 828  In other words, C<*FH> must be used to create new symbol table entries;
 829  C<*foo{THING}> cannot.  When in doubt, use C<*FH>.
 831  All functions that are capable of creating filehandles (open(),
 832  opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
 833  automatically create an anonymous filehandle if the handle passed to
 834  them is an uninitialized scalar variable. This allows the constructs
 835  such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
 836  create filehandles that will conveniently be closed automatically when
 837  the scope ends, provided there are no other references to them. This
 838  largely eliminates the need for typeglobs when opening filehandles
 839  that must be passed around, as in the following example:
 841      sub myopen {
 842          open my $fh, "@_"
 843               or die "Can't open '@_': $!";
 844          return $fh;
 845      }
 847      {
 848          my $f = myopen("</etc/motd");
 849          print <$f>;
 850          # $f implicitly closed here
 851      }
 853  Note that if an initialized scalar variable is used instead the
 854  result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
 855  to C<open( *{'zzz'}, ...)>.
 856  C<use strict 'refs'> forbids such practice.
 858  Another way to create anonymous filehandles is with the Symbol
 859  module or with the IO::Handle module and its ilk.  These modules
 860  have the advantage of not hiding different types of the same name
 861  during the local().  See the bottom of L<perlfunc/open()> for an
 862  example.
 864  =head1 SEE ALSO
 866  See L<perlvar> for a description of Perl's built-in variables and
 867  a discussion of legal variable names.  See L<perlref>, L<perlsub>,
 868  and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
 869  the C<*foo{THING}> syntax.

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