Acorn User 10/AU_CD10.iso/Updates/Perl/Non-RPC/Docs/perldata

Text File | 1999-04-17 | 27KB | 680 lines

NAME
    perldata - Perl data types

DESCRIPTION
  Variable names

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

    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 *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 `::' (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 perlmod manpage for details). It's
    possible to substitute for a simple identifier an expression
    that produces a reference to the value at runtime; this is
    described in more detail below, and in the perlref 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 that match parenthesized parts of a
    regular expression are saved under names containing only digits
    after the `$' (see the perlop manpage and the perlre manpage).
    In addition, several special variables that provide windows into
    the inner working of Perl have names containing punctuation
    characters (see the perlvar 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 '%':

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

  Context

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

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

        int( <STDIN> )


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

        sort( <STDIN> )


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

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

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

  Scalar values

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

    Scalars aren't necessarily one thing or another. There's no
    place to declare a scalar variable to be of type "string", or of
    type "number", or type "filehandle", or anything else. Perl is a
    contextually polymorphic language whose scalars can be strings,
    numbers, or references (which includes objects). While strings
    and numbers are considered pretty much 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 defined() operator to 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 -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's usually preferable because otherwise you won't treat IEEE
    notations like `NaN' or `Infinity' properly. At other times you
    might prefer to use the POSIX::strtod function or a regular
    expression to check whether data is numeric. See the perlre
    manpage for details on regular expressions.

        warn "has nondigits"    if     /\D/;
        warn "not a 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 `$#days', as in csh.
    (Actually, it's not the length of the array, it's the subscript
    of the last element, because there is (ordinarily) a 0th
    element.) Assigning to `$#days' changes the length of the array.
    Shortening an array by this method destroys intervening values.
    Lengthening an array that was previously shortened *NO LONGER*
    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 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 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, 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 `$[': files that
    don't set the value of `$[' 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

        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
    that 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.)

    You can preallocate space for a hash by assigning to the keys()
    function. This rounds up the allocated bucked to the next power
    of two:

        keys(%users) = 1000;        # allocate 1024 buckets


  Scalar value constructors

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

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


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

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

        $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 perlmod 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 an empty `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 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 -w switch, Perl will
    warn you about any such words. Some people may wish to outlaw
    barewords entirely. If you say

        use strict 'subs';


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

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

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

        system "echo @ARGV";


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

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

        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;


  List value constructors

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

        (LIST)


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

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


    assigns the entire list value to array foo, but

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


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

    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);


    List 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.

    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 get a null value. This may be useful in a
    local() or my().

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

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


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

    It is often more readable to use the `=>' operator between
    key/value pairs. The `=>' operator is mostly just a more
    visually distinctive synonym for a comma, but it also arranges
    for its left-hand operand to be interpreted as a string--if it's
    a bareword that 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
       );


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

  Typeglobs and Filehandles

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

    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 the section on
    "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 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
    local() operator. These last until their block is exited, but
    may be passed back. For example:

        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 *foo{THING} notation, typeglobs aren't used
    as much for filehandle manipulations, although they're still
    needed to pass brand new file and directory handles into or out
    of functions. That's because *HANDLE{IO} only works if HANDLE
    has already been used as a handle. In other words, *FH can be
    used to create new symbol table entries, but *foo{THING} cannot.

    Another way to create anonymous filehandles is 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 the "open()" entry in the perlfunc
    manpage for an example.

    See the perlref manpage, the perlsub manpage, and the section on
    "Symbol Tables" in the perlmod manpage for more discussion on
    typeglobs and the *foo{THING} syntax.