X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperldata.pod;h=bc564e650d48f1d5eb0e54e8765c672c0fab9fa2;hb=9bf2270250326fb85445d6849ed84a94434dd12c;hp=5ff97d4bffee347c87283bcc18798b75e0c9cf18;hpb=f61b605bd1c8a7ceeaceb06f66d2a4ccbaa7feaf;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perldata.pod b/pod/perldata.pod index 5ff97d4..bc564e6 100644 --- a/pod/perldata.pod +++ b/pod/perldata.pod @@ -5,6 +5,7 @@ perldata - Perl data types =head1 DESCRIPTION =head2 Variable names +X X X X Perl has three built-in data types: scalars, arrays of scalars, and associative arrays of scalars, known as "hashes". A scalar is a @@ -27,6 +28,7 @@ to locate the namespace in which to look up the final identifier for a simple identifier, an expression that produces a reference to the value at runtime. This is described in more detail below and in L. +X Perl also has its own built-in variables whose names don't follow these rules. They have strange names so they don't accidentally @@ -36,11 +38,13 @@ containing only digits after the C<$> (see L and L). In addition, several special variables that provide windows into the inner working of Perl have names containing punctuation characters and control characters. These are documented in L. +X Scalar values are always named with '$', 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. +X $days # the simple scalar value "days" $days[28] # the 29th element of array @days @@ -50,12 +54,14 @@ single value is expected. Entire arrays (and slices of arrays and hashes) are denoted by '@', which works much like the word "these" or "those" does in English, in that it indicates multiple values are expected. +X @days # ($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 '%': +X %days # (key1, val1, key2, val2 ...) @@ -72,6 +78,7 @@ subroutine name, a format name, or a label. This means that $foo and @foo are two different variables. It also means that C<$foo[1]> is a part of @foo, not a part of $foo. This may seem a bit weird, but that's okay, because it is weird. +X Because variable references always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable @@ -83,6 +90,8 @@ uppercase filehandles also improves readability and protects you from conflict with future reserved words. Case I significant--"FOO", "Foo", and "foo" are all different names. Names that start with a letter or underscore may also contain digits and underscores. +X +X It is possible to replace such an alphanumeric name with an expression that returns a reference to the appropriate type. For a description @@ -96,6 +105,7 @@ significance to Perl. For instance, C<$$> is the current process id.) =head2 Context +X X X The interpretation of operations and values in Perl sometimes depends on the requirements of the context around the operation or value. @@ -148,6 +158,7 @@ for how you would dynamically discern your function's calling context. =head2 Scalar values +X X X X All 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 @@ -172,6 +183,7 @@ 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. +X X X X X There are actually two varieties of null strings (sometimes referred to as "empty" strings), a defined one and an undefined one. The @@ -186,6 +198,7 @@ rare cases of autovivification as explained in L. You can use the defined() operator to determine whether a scalar value is defined (this has no meaning on arrays or hashes), and the undef() operator to produce an undefined value. +X X X X X 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 @@ -220,6 +233,7 @@ Shortening an array this way destroys intervening values. Lengthening an array that was previously shortened does not recover values that were in those elements. (It used to do so in Perl 4, but we had to break this to make sure destructors were called when expected.) +X<$#> X You can also gain some minuscule measure of efficiency by pre-extending an array that is going to get big. You can also extend an array @@ -235,6 +249,7 @@ 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: +X scalar(@whatever) == $#whatever - $[ + 1; @@ -242,6 +257,7 @@ Version 5 of Perl changed the semantics of C<$[>: files that don't set the value of C<$[> no longer need to worry about whether another file changed its value. (In other words, use of C<$[> is deprecated.) So in general you can assume that +X<$[> scalar(@whatever) == $#whatever + 1; @@ -262,6 +278,7 @@ of sixteen buckets has been touched, and presumably contains all 10,000 of your items. This isn't supposed to happen. If a tied hash is evaluated in scalar context, a fatal error will result, since this bucket usage information is currently not available for tied hashes. +X X X You can preallocate space for a hash by assigning to the keys() function. This rounds up the allocated buckets to the next power of two: @@ -269,6 +286,7 @@ This rounds up the allocated buckets to the next power of two: keys(%users) = 1000; # allocate 1024 buckets =head2 Scalar value constructors +X X Numeric literals are specified in any of the following floating point or integer formats: @@ -280,13 +298,14 @@ integer formats: 4_294_967_296 # underscore for legibility 0xff # hex 0xdead_beef # more hex - 0377 # octal + 0377 # octal (only numbers, begins with 0) 0b011011 # binary You are allowed to use underscores (underbars) in numeric literals between digits for legibility. You could, for example, group binary digits by threes (as for a Unix-style mode argument such as 0b110_100_100) or by fours (to represent nibbles, as in 0b1010_0110) or in other groups. +X String literals are usually delimited by either single or double quotes. They work much like quotes in the standard Unix shells: @@ -295,6 +314,7 @@ substitution; single-quoted strings are not (except for C<\'> and C<\\>). The usual C-style backslash rules apply for making characters such as newline, tab, etc., as well as some more exotic forms. See L for a list. +X Hexadecimal, octal, or binary, representations in string literals (e.g. '0xff') are not automatically converted to their integer @@ -310,18 +330,26 @@ 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 $Z<>100." +X $Price = '$100'; # not interpolated print "The price is $Price.\n"; # interpolated There is no double interpolation in Perl, so the C<$100> is left as is. +By default floating point numbers substituted inside strings use the +dot (".") as the decimal separator. If C is in effect, +and POSIX::setlocale() has been called, the character used for the +decimal separator is affected by the LC_NUMERIC locale. +See L and L. + 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: +X $who = "Larry"; print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n"; @@ -336,15 +364,12 @@ 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, C<$days{'Feb'}> can be written as C<$days{Feb}> and the quotes will be assumed automatically. But -anything more complicated in the subscript will be interpreted as -an expression. +anything more complicated in the subscript will be interpreted as an +expression. This means for example that C<$version{2.0}++> is +equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>. =head3 Version Strings - -B Version Strings (v-strings) have been deprecated. They will -be removed in some future release after Perl 5.8.1. The marginal -benefits of v-strings were greatly outweighed by the potential for -Surprise and Confusion. +X X X A literal of the form C is parsed as a string composed of characters with the specified ordinals. This form, known as @@ -355,15 +380,14 @@ Unicode strings, and for comparing version "numbers" using the string comparison operators, C, C, C etc. If there are two or more dots in the literal, the leading C may be omitted. - print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}" + print v9786; # prints SMILEY, "\x{263a}" print v102.111.111; # prints "foo" print 102.111.111; # same Such literals are accepted by both C and C for -doing a version check. The C<$^V> special variable also contains the -running Perl interpreter's version in this form. See L. -Note that using the v-strings for IPv4 addresses is not portable unless -you also use the inet_aton()/inet_ntoa() routines of the Socket package. +doing a version check. Note that using the v-strings for IPv4 +addresses is not portable unless you also use the +inet_aton()/inet_ntoa() routines of the Socket package. Note that since Perl 5.8.1 the single-number v-strings (like C) are not v-strings before the C<< => >> operator (which is usually used @@ -374,6 +398,8 @@ Multi-number v-strings like C and C<65.66.67> continue to be v-strings always. =head3 Special Literals +X X<__END__> X<__DATA__> X X +X X X<^D> X<^Z> The special literals __FILE__, __LINE__, and __PACKAGE__ represent the current filename, line number, and package name at that @@ -381,18 +407,19 @@ 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 C directive), __PACKAGE__ is the undefined value. +X<__FILE__> X<__LINE__> X<__PACKAGE__> X X X 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 C, +Text after __DATA__ may be read via the filehandle C, where C is the package that was current when the __DATA__ token was encountered. The filehandle is left open pointing to the contents after __DATA__. It is the program's responsibility to C when it is done reading from it. For compatibility with older scripts written before __DATA__ was introduced, __END__ behaves -like __DATA__ in the toplevel script (but not in files loaded with +like __DATA__ in the top level script (but not in files loaded with C or C) and leaves the remaining contents of the file accessible via C. @@ -403,14 +430,18 @@ as it is seen (during compilation), at which point the corresponding __DATA__ (or __END__) token has not yet been seen. =head3 Barewords +X 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 C pragma or the B<-w> switch, -Perl will warn you about any -such words. Some people may wish to outlaw barewords entirely. If you +Perl will warn you about any such words. Perl limits barewords (like +identifiers) to about 250 characters. Future versions of Perl are likely +to eliminate these arbitrary limitations. + +Some people may wish to outlaw barewords entirely. If you say use strict 'subs'; @@ -421,6 +452,7 @@ end of the enclosing block. An inner block may countermand this by saying C. =head3 Array Joining Delimiter +X X X<$"> Arrays and slices are interpolated into double-quoted strings by joining the elements with the delimiter specified in the C<$"> @@ -447,6 +479,7 @@ which used to be here, that's been moved to L. =head2 List value constructors +X List values are denoted by separating individual values by commas (and enclosing the list in parentheses where precedence requires it): @@ -600,9 +633,9 @@ key/value pairs. That's why it's good to use references sometimes. It is often more readable to use the C<< => >> operator between key/value pairs. The C<< => >> 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 simple -identifier (C<< => >> doesn't quote compound identifiers, that contain -double colons). This makes it nice for initializing hashes: +interpreted as a string if it's a bareword that would be a legal simple +identifier. C<< => >> doesn't quote compound identifiers, that contain +double colons. This makes it nice for initializing hashes: %map = ( red => 0x00f, @@ -634,12 +667,12 @@ of how to arrange for an output ordering. =head2 Subscripts -An array is subscripted by specifying a dollary sign (C<$>), then the +An array is subscripted by specifying a dollar sign (C<$>), then the name of the array (without the leading C<@>), then the subscript inside square brackets. For example: @myarray = (5, 50, 500, 5000); - print "Element Number 2 is", $myarray[2], "\n"; + print "The Third Element is", $myarray[2], "\n"; The array indices start with 0. A negative subscript retrieves its value from the end. In our example, C<$myarray[-1]> would have been @@ -659,6 +692,7 @@ are used. For example: print "Darwin's First Name is ", $scientists{"Darwin"}, "\n"; =head2 Slices +X X X A 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. @@ -735,6 +769,7 @@ hash indicates whether you are getting back a singular value (a scalar) or a plural one (a list). =head2 Typeglobs and Filehandles +X X X<*> Perl uses an internal type called a I to hold an entire symbol table entry. The type prefix of a typeglob is a C<*>, because