NAMEperldata - Perl data types
DESCRIPTION
Variable namesPerl has three built-in data types: scalars, arrays of scalars, and associative arrays of scalars, known as ``hashes''. Normal arrays are ordered lists of scalars indexed by number, starting with 0 and with negative subscripts counting from the end. Hashes are unordered collections of scalar values indexed by their associated string key. 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. Usually this name is a single identifier,
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 Perl also has its own built-in variables whose names don't follow
these rules. They have strange names so they don't accidentally
collide with one of your normal variables. Strings that match
parenthesized parts of a regular expression are saved under names
containing only digits after the Scalar values are always named with '$', even when referring to a scalar that is part of an array or a hash. The '$' symbol works semantically like the English word ``the'' in that it indicates a single value is expected. $days # the simple scalar value "days" $days[28] # the 29th element of array @days $days{'Feb'} # the 'Feb' value from hash %days $#days # the last index of array @days Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much like the word ``these'' or ``those'' does in English, in that it indicates multiple values are expected. @days # ($days[0], $days[1],... $days[n]) @days[3,4,5] # same as ($days[3],$days[4],$days[5]) @days{'a','c'} # same as ($days{'a'},$days{'c'}) Entire hashes are denoted by '%': %days # (key1, val1, key2, val2 ...) In addition, subroutines are named with an initial '&', though this is optional when unambiguous, just as the word ``do'' is often redundant in English. Symbol table entries can be named with an initial '*', but you don't really care about that yet (if ever :-). Every variable type has its own namespace, as do several
non-variable identifiers. This means that you can, without fear
of conflict, use the same name for a scalar variable, an array, or
a hash--or, for that matter, for a filehandle, a directory handle, a
subroutine name, a format name, or a label. This means that $foo
and @foo are two different variables. It also means that Because variable references always start with '$', '@', 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
It is possible to replace such an alphanumeric name with an expression that returns a reference to the appropriate type. For a description of this, see the perlref manpage. Names that start with a digit may contain only more digits. Names
that do not start with a letter, underscore, or digit are limited to
one character, e.g.,
ContextThe 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: list and scalar. 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 scalar context for the <> 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 list context for <>, 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 right-hand side in scalar context, while assignment to an array or hash evaluates the righthand side in list context. Assignment to a list (or slice, which is just a list anyway) also evaluates the righthand side in list context. When you use the User-defined subroutines may choose to care whether they are being called in a void, scalar, or list context. Most subroutines do not need to bother, though. That's because both scalars and lists are automatically interpolated into lists. See wantarray in the perlfunc manpage for how you would dynamically discern your function's calling context.
Scalar valuesAll data in Perl is a scalar, an array of scalars, or a hash of scalars. A scalar may contain one single value in any of three different flavors: a number, a string, or a reference. In general, conversion from one form to another is transparent. Although a scalar may not directly hold multiple values, it may contain a reference to an array or hash which in turn contains multiple values. Scalars aren't necessarily one thing or another. There's no place to declare a scalar variable to be of type ``string'', type ``number'', type ``reference'', or anything else. Because of the automatic conversion of scalars, operations that return scalars don't need to care (and in fact, cannot care) whether their caller is looking for a string, a number, or a reference. Perl is a contextually polymorphic language whose scalars can be strings, numbers, or references (which includes objects). Although 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 where no conversion to a string or a number is ever performed. There are actually two varieties of null strings (sometimes referred
to as ``empty'' strings), a defined one and an undefined one. The
defined version is just a string of length zero, such as To find out whether a given string is a valid non-zero number, it's sometimes 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 they do in awk: if ($str == 0 && $str ne "0") { warn "That doesn't look like a number"; } That method may be best because otherwise you won't treat IEEE
notations like warn "has nondigits" if /\D/; warn "not a natural number" unless /^\d+$/; # rejects -3 warn "not an integer" unless /^-?\d+$/; # rejects +3 warn "not an integer" unless /^[+-]?\d+$/; warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; 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 You can also gain some miniscule 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 an array in 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, nor of built-in functions, which return whatever they feel like returning.) The following is always true: scalar(@whatever) == $#whatever - $[ + 1; Version 5 of Perl changed the semantics of scalar(@whatever) == $#whatever + 1; Some programmers choose to use an explicit conversion so as to leave nothing to doubt: $element_count = scalar(@whatever); If you evaluate a hash in scalar context, it returns false if the
hash is empty. If there are any key/value pairs, it returns true;
more precisely, 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 internal 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 You can preallocate space for a hash by assigning to the keys(%users) = 1000; # allocate 1024 buckets
Scalar value constructorsNumeric literals are specified in any of the following floating point or integer formats: 12345 12345.67 .23E-10 # a very small number 4_294_967_296 # underline for legibility 0xff # hex 0377 # octal 0b011011 # binary String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
representation. The 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 or hash 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.'' $Price = '$100'; # not interpreted print "The price is $Price.\n"; # interpreted As in some shells, you can enclose the variable name in braces to disambiguate it from following alphanumerics (and underscores). You must also do this when interpolating a variable into a string to separate the variable name from a following double-colon or an apostrophe, since these would be otherwise treated as a package separator: $who = "Larry"; print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n"; print "We use ${who}speak when ${who}'s here.\n"; Without the braces, Perl would have looked for a $whospeak, a
In fact, an identifier within such curlies is forced to be a string,
as is any simple identifier within a hash subscript. Neither need
quoting. Our earlier example, A literal of the form print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}" print v102.111.111; # prints "foo" print 102.111.111; # same Such literals are accepted by both The special literals __FILE__, __LINE__, and __PACKAGE__
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 an empty The two control characters ^D and ^Z, and 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. Text after __DATA__ but may be read via the filehandle See the SelfLoader 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 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 Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the $temp = join($", @ARGV); system "echo $temp"; system "echo @ARGV"; Within search patterns (which also undergo double-quotish substitution)
there is an unfortunate ambiguity: Is A line-oriented form of quoting is based on the shell ``here-document''
syntax. Following a print <<EOF; The price is $Price. EOF print << "EOF"; # same as above The price is $Price. EOF print << `EOC`; # execute commands echo hi there echo lo there EOC print <<"foo", <<"bar"; # you can stack them I said foo. foo I said bar. bar myfunc(<< "THIS", 23, <<'THAT'); Here's a line or two. THIS and here's another. THAT Just don't forget that you have to put a semicolon on the end to finish the statement, as Perl doesn't know you're not going to try to do this: print <<ABC 179231 ABC + 20; If you want your here-docs to be indented with the rest of the code, you'll need to remove leading whitespace from each line manually: ($quote = <<'FINIS') =~ s/^\s+//gm; The Road goes ever on and on, down from the door where it began. FINIS If you use a here-doc within a delimited construct, such as in s/this/<<E . 'that' the other E . 'more '/eg; you have to write s/this/<<E . 'that' . 'more '/eg; the other E If the terminating identifier is on the last line of the program, you must be sure there is a newline after it; otherwise, Perl will give the warning Can't find string terminator ``END'' anywhere before EOF.... Additionally, the quoting rules for the identifier are not related to
Perl's quoting rules -- print << "abc\"def"; testing... abc"def Finally, quoted strings cannot span multiple lines. The general rule is that the identifier must be a string literal. Stick with that, and you should be safe.
List value constructorsList 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 what appears to be a list literal is simply 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 the scalar variable $foo. Note that the value of an actual array in 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, ); To use a here-document to assign an array, one line per element, you might use an approach like this: @sauces = <<End_Lines =~ m/(\S.*\S)/g; normal tomato spicy tomato green chile pesto white wine End_Lines LISTs do automatic interpolation of sublists. That is, when a LIST is evaluated, each element of the list is evaluated in list context, and the resulting list value is interpolated into LIST just as if each individual element were a member of LIST. Thus arrays and hashes lose their identity in a LIST--the list (@foo,@bar,&SomeSub,%glarch) contains all the elements of @foo followed by all the elements of @bar, followed by all the elements returned by the subroutine named SomeSub called in list context, followed by the key/value pairs of %glarch. 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. This interpolation combines with the facts that the opening
and closing parentheses are optional (except necessary for
precedence) and lists may end with an optional comma to mean that
multiple commas within lists are legal syntax. The list 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]; Lists may be assigned to only when each element of the list is itself legal to assign to: ($a, $b, $c) = (1, 2, 3); ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00); An exception to this is that you may assign to ($dev, $ino, undef, undef, $uid, $gid) = stat($file); List assignment in 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 $x to f()'s return count This is handy when you want to do a list assignment in a Boolean context, because most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE. The final element may be an array or a hash: ($a, $b, @rest) = split; my($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 become
undefined. This may be useful in a A hash can be initialized using a literal list holding pairs of items 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 often 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 %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 sort in the perlfunc manpage for examples of how to arrange for an output ordering.
SlicesA common way to access an array or a hash is one scalar element at a time. You can also subscript a list to get a single element from it. $whoami = $ENV{"USER"}; # one element from the hash $parent = $ISA[0]; # one element from the array $dir = (getpwnam("daemon"))[7]; # likewise, but with list A slice accesses several elements of a list, an array, or a hash simultaneously using a list of subscripts. It's more convenient than writing out the individual elements as a list of separate scalar values. ($him, $her) = @folks[0,-1]; # array slice @them = @folks[0 .. 3]; # array slice ($who, $home) = @ENV{"USER", "HOME"}; # hash slice ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice Since you can assign to a list of variables, you can also assign to an array or hash slice. @days[3..5] = qw/Wed Thu Fri/; @colors{'red','blue','green'} = (0xff0000, 0x0000ff, 0x00ff00); @folks[0, -1] = @folks[-1, 0]; The previous assignments are exactly equivalent to ($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/; ($colors{'red'}, $colors{'blue'}, $colors{'green'}) = (0xff0000, 0x0000ff, 0x00ff00); ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]); Since changing a slice changes the original array or hash that it's
slicing, a foreach (@array[ 4 .. 10 ]) { s/peter/paul/ } foreach (@hash{keys %hash}) { s/^\s+//; # trim leading whitespace s/\s+$//; # trim trailing whitespace s/(\w+)/\u\L$1/g; # "titlecase" words } A slice of an empty list is still an empty list. Thus: @a = ()[1,0]; # @a has no elements @b = (@a)[0,1]; # @b has no elements @c = (0,1)[2,3]; # @c has no elements But: @a = (1)[1,0]; # @a has two elements @b = (1,undef)[1,0,2]; # @b has three elements This makes it easy to write loops that terminate when a null list is returned: while ( ($home, $user) = (getpwent)[7,0]) { printf "%-8s %s\n", $user, $home; } As noted earlier in this document, the scalar sense of list assignment is the number of elements on the right-hand side of the assignment. The null list contains no elements, so when the password file is exhausted, the result is 0, not 2. If you're confused about why you use an '@' there on a hash slice instead of a '%', think of it like this. The type of bracket (square or curly) governs whether it's an array or a hash being looked at. On the other hand, the leading symbol ('$' or '@') on the array or hash indicates whether you are getting back a singular value (a scalar) or a plural one (a list).
Typeglobs and FilehandlesPerl uses an internal type called a typeglob to hold an entire
symbol table entry. The type prefix of a typeglob is a The main use of typeglobs in modern Perl is create symbol table aliases. This assignment: *this = *that; makes $this an alias for $that, @this an alias for @that, %this an alias for %that, &this an alias for &that, etc. Much safer is to use a reference. This: local *Here::blue = \$There::green; temporarily makes $Here::blue an alias for $There::green, but doesn't make @Here::blue an alias for @There::green, or %Here::blue an alias for %There::green, etc. See Symbol Tables in the perlmod manpage for more examples of this. Strange though this may seem, this is the basis for the whole module import/export system. Another use for typeglobs is to pass filehandles into a function or to create new filehandles. 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; See the perlsub manpage for examples of using these as indirect filehandles in functions. Typeglobs are also a way to create a local filehandle using the sub newopen { my $path = shift; local *FH; # not my! open (FH, $path) or return undef; return *FH; } $fh = newopen('/etc/passwd'); Now that we have the All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and sub myopen { open my $fh, "@_" or die "Can't open '@_': $!"; return $fh; } { my $f = myopen("</etc/motd"); print <$f>; # $f implicitly closed here } Another way to create anonymous filehandles is with the Symbol module or with the IO::Handle module and its ilk. These modules have the advantage of not hiding different types of the same name during the local(). See the bottom of open() in the perlfunc manpage for an example.
SEE ALSOSee the perlvar manpage for a description of Perl's built-in variables and
a discussion of legal variable names. See the perlref manpage, the perlsub manpage,
and Symbol Tables in the perlmod manpage for more discussion on typeglobs and
the
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