=head2 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.
+Perl has three built-in data types: scalars, arrays of scalars, and
+associative arrays of scalars, known as "hashes". Normal arrays
+are ordered lists indexed by number, starting with 0 and with
+negative subscripts counting from the end. Hashes are unordered
+collections of values indexed by their associated string key.
-Values are usually referred to by name (or through a named reference).
+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
-I<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 C<::> (or by C<'>, 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 L<perlmod/Packages> 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 L<perlref>.
-
-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 C<$> (see L<perlop> and L<perlre>). In addition, several special
-variables that provide windows into the inner working of Perl have names
-containing punctuation characters (see L<perlvar>).
-
-Scalar values are always named with '$', even when referring to a scalar
-that is part of an array. It works like the English word "the". Thus
-we have:
+value to which it refers. Usually this name is a single I<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 C<::> (or by the slightly
+archaic C<'>); all but the last are interpreted as names of packages,
+to locate the namespace in which to look up the final identifier
+(see L<perlmod/Packages> 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 L<perlref>.
+
+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 C<$> (see L<perlop> and L<perlre>).
+In addition, several special variables that provide windows into
+the inner working of Perl have names containing punctuation characters
+and control characters. These are documented in L<perlvar>.
+
+Scalar values are always named with '$', 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
$#days # the last index of array @days
-but entire arrays or array slices are denoted by '@', which works much like
-the word "these" or "those":
+Entire arrays (and slices of arrays and hashes) are denoted by '@',
+which works much like the word "these" or "those" does in English,
+in that it indicates multiple values are expected.
@days # ($days[0], $days[1],... $days[n])
- @days[3,4,5] # same as @days[3..5]
+ @days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
-and entire hashes are denoted by '%':
+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 $foo and @foo are two different variables. It also
-means that C<$foo[1]> is a part of @foo, not a part of $foo. This may
-seem a bit weird, but that's okay, because it is weird.
-
-Because variable and array references always start with '$', '@', 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
-C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using uppercase
-filehandles also improves readability and protects you from conflict
-with future reserved words.) Case I<IS> significant--"FOO", "Foo", and
-"foo" are all different names. Names that start with a letter or
-underscore may also contain digits and underscores.
+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 $foo
+and @foo are two different variables. It also means that C<$foo[1]>
+is a part of @foo, not a part of $foo. This may seem a bit weird,
+but that's okay, because it is weird.
+
+Because variable references always start with '$', '@', or '%', the
+"reserved" words aren't in fact reserved with respect to variable
+names. They I<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
+C<open(LOG,'logfile')> rather than C<open(log,'logfile')>. Using
+uppercase filehandles also improves readability and protects you
+from conflict with future reserved words. Case I<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
+that returns a reference to the appropriate type. For a description
of this, see L<perlref>.
Names that start with a digit may contain only more digits. Names
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
+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
+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".)
+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 E<lt>STDINE<gt>
+the integer operation provides scalar context for the E<lt><gt>
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 E<lt>STDINE<gt>, which
+then the sort operation provides list context for E<lt><gt>, 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 L<perlfunc/wantarray>.
+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 Perl's B<-w> command-line option, you may see warnings
+about useless uses of constants or functions in "void context".
+Void context just means the value has been discarded, such as a
+statement containing only C<"fred";> or C<getpwuid(0);>. It still
+counts as scalar context for functions that care whether or not
+they're being called in list context.
+
+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 L<perlfunc/wantarray>
+for how you would dynamically discern your function's calling
+context.
=head2 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.
+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
+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.
-
-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 B<-w> noises). That's because strings that aren't
-numbers count as 0, just as they do in B<awk>:
+the empty 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 C<"">.
+The undefined version is the value that indicates that 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 or hash. Although in early versions of Perl,
+an undefined scalar could become defined when first used in a
+place expecting a defined value, this no longer happens except for
+rare cases of autovivification as explained in L<perlref>. 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.
+
+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 B<-w> noises). That's because strings
+that aren't numbers count as 0, just as they do in B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
}
-That's usually preferable because otherwise you won't treat IEEE notations
-like C<NaN> or C<Infinity> properly. At other times you might prefer to
-use the POSIX::strtod function or a regular expression to check whether
-data is numeric. See L<perlre> for details on regular expressions.
+That method may be best because otherwise you won't treat IEEE
+notations like C<NaN> or C<Infinity> properly. At other times, you
+might prefer to determine whether string data can be used numerically
+by calling the POSIX::strtod() function or by inspecting your string
+with a regular expression (as documented in L<perlre>).
warn "has nondigits" if /\D/;
warn "not a natural number" unless /^\d+$/; # rejects -3
warn "not a C float"
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
-The length of an array is a scalar value. You may find the length of
-array @days by evaluating C<$#days>, as in B<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 C<$#days> changes the
-length of the array. Shortening an array by this method destroys
-intervening values. Lengthening an array that was previously shortened
-I<NO LONGER> recovers the values that were in those elements. (It used to
-in Perl 4, but we had to break this to make sure destructors were
-called when expected.) You can also gain some miniscule measure of efficiency by
-pre-extending an array that is going to get big. (You can also extend
-an array by assigning to an element that is off the end of the array.)
-You can truncate an array down to nothing by assigning the null list ()
-to it. The following are equivalent:
+The length of an array is a scalar value. You may find the length
+of array @days by evaluating C<$#days>, as in B<csh>. Technically
+speaking, this isn't the length of the array; it's the subscript
+of the last element, since there is ordinarily a 0th element.
+Assigning to C<$#days> actually changes the length of the array.
+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.)
+
+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 empty 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:
+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;
scalar(@whatever) == $#whatever + 1;
-Some programmers choose to use an explicit conversion so nothing's
-left to doubt:
+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 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.)
+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 C<"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:
=head2 Scalar value constructors
-Numeric literals are specified in any of the customary floating point or
+Numeric literals are specified in any of the following floating point or
integer formats:
12345
12345.67
- .23E-10
- 0xffff # hex
- 0377 # octal
- 0b111000 # binary
- 4_294_967_296 # underline for legibility
+ .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 shell quotes: double-quoted string
-literals are subject to backslash and variable substitution;
-single-quoted strings are not (except for "C<\'>" and "C<\\>").
-The usual Unix backslash rules apply for making characters such as
-newline, tab, etc., as well as some more exotic forms. See
-L<perlop/"Quote and Quotelike Operators"> 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 L<perlfunc/hex> and
-L<perlfunc/oct> for more details.
+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 "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<perlop/"Quote and Quotelike Operators"> for a list.
+
+Hexadecimal, octal, or binary, representations in string literals
+(e.g. '0xff') are not automatically converted to their integer
+representation. The hex() and oct() functions make these conversions
+for you. See L<perlfunc/hex> and L<perlfunc/oct> 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,
+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."
$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 L<perlmod/Packages>).
-
-Three special literals are __FILE__, __LINE__, and __PACKAGE__, which
+As in some shells, you can enclose the variable name in braces to
+disambiguate it from following alphanumerics. 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 $whospeak, a
+C<$who::0>, and a C<$who's> variable. The last two would be the
+$0 and the $s variables in the (presumably) non-existent package
+C<who>.
+
+In fact, an identifier within such curlies is forced to be a string,
+as is any simple identifier within a hash subscript. Neither need
+quoting. Our earlier example, C<$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.
+
+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 C<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 L<SelfLoader> 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
+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__ in the main program, __DATA__ in a separate
+module. See L<SelfLoader> 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
end of the enclosing block. An inner block may countermand this
by saying C<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 C<$">
-variable (C<$LIST_SEPARATOR> in English), space by default. The following
-are equivalent:
+Arrays and slices are interpolated into double-quoted strings
+by joining the elements with the delimiter specified in the C<$">
+variable (C<$LIST_SEPARATOR> in English), space by default. The
+following are equivalent:
- $temp = join($",@ARGV);
+ $temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish substitution)
-there is a bad ambiguity: Is C</$foo[bar]/> to be interpreted as
+there is an unfortunate ambiguity: Is C</$foo[bar]/> to be interpreted as
C</${foo}[bar]/> (where C<[bar]> is a character class for the regular
expression) or as C</${foo[bar]}/> (where C<[bar]> is the subscript to array
@foo)? If @foo doesn't otherwise exist, then it's obviously a
character class. If @foo exists, Perl takes a good guess about C<[bar]>,
and is almost always right. If it does guess wrong, or if you're just
plain paranoid, you can force the correct interpretation with curly
-brackets as above.
+braces as above.
-A line-oriented form of quoting is based on the shell "here-doc"
+A line-oriented form of quoting is based on the shell "here-document"
syntax. Following a C<E<lt>E<lt>> you specify a string to terminate
the quoted material, and all lines following the current line down to
the terminating string are the value of the item. The terminating
ABC
+ 20;
+If you want your here-docs to be indented with the
+rest of the code, you'll need to remove leading whitespace
+from each line manually:
+
+ ($quote = <<'FINIS') =~ s/^\s+//gm;
+ The Road goes ever on and on,
+ down from the door where it began.
+ FINIS
=head2 List value constructors
(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,
+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
+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:
+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; # $foo gets 3
3,
);
+To use a here-document to assign an array, one line per element,
+you might use an approach like this:
+
+ @sauces = <<End_Lines =~ m/(\S.*\S)/g;
+ normal tomato
+ spicy tomato
+ green chile
+ pesto
+ white wine
+ End_Lines
+
LISTs do automatic interpolation of sublists. That is, when a LIST is
-evaluated, each element of the list is evaluated in a list context, and
+evaluated, each element of the list is evaluated in list context, and
the resulting list value is interpolated into LIST just as if each
individual element were a member of LIST. Thus arrays and hashes lose their
identity in a LIST--the list
contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub
-called in a list context, followed by the key/value pairs of %glarch.
+called in list context, followed by the key/value pairs of %glarch.
To make a list reference that does I<NOT> interpolate, see L<perlref>.
-The null list is represented by (). Interpolating it in a list
+The empty list is represented by (). Interpolating it in a list
has no effect. Thus ((),(),()) is equivalent to (). Similarly,
interpolating an array with no elements is the same as if no
array had been interpolated at that point.
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
-You may assign to C<undef> in a list. This is useful for throwing
-away some of the return values of a function:
-
- ($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:
+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'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
-List assignment in a scalar context returns the number of elements
+An exception to this is that you may assign to C<undef> in a list.
+This is useful for throwing away some of the return values of a
+function:
+
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+
+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 $x to f()'s return count
-This is very handy when you want to do a list assignment in a Boolean
-context, because most list functions return a null list when finished,
+This is handy when you want to do a list assignment in a Boolean
+context, because most list functions return a empty list when finished,
which when assigned produces a 0, which is interpreted as FALSE.
The final element may be an array or a hash:
my($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().
+in the list will soak up all the values, and anything after it will become
+undefined. This may be useful in a my() or local().
-A hash literal contains pairs of values to be interpreted
-as a key and a value:
+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 usually interchangeable, that's
+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
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+=head2 Slices
+
+A common way 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
+ $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+
+A slice accesses several elements of a list, an array, or a hash
+simultaneously using a list of subscripts. It's a more convenient
+that writing out the individual elements as a list of separate
+scalar values.
+
+ ($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], $folks[-1]);
+
+Since changing a slice changes the original array or hash that it's
+slicing, a C<foreach> construct will alter through some--or even
+all--of the values of the array or hash.
+
+ foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
+
+ foreach (@hash{keys %hash}) {
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
+ }
+
+You couldn't just loop through C<values %hash> to do this because
+that function produces a new list which is a copy of the values,
+so changing them doesn't change the original.
+
+As a special rule, if a slice would produce a list consisting entirely
+of undefined values, the empty list is produced instead. This makes
+it easy to write loops that terminate when an empty list is returned:
+
+ while ( ($home, $user) = (getpwent)[7,0]) {
+ printf "%-8s %s\n", $user, $home;
+ }
+
+As noted earlier in this document, the scalar sense of list assignment
+is the number of elements on the right-hand side of the assignment.
+The empty list contains no elements, so when the password file is
+exhausted, the result is 0, not 2.
+
+If you're confused about why you use an '@' there on a hash slice
+instead of a '%', think of it like this. The type of bracket (square
+or curly) governs whether it's an array or a hash being looked at.
+On the other hand, the leading symbol ('$' or '@') on the array or
+hash indicates whether you are getting back a singular value (a
+scalar) or a plural one (a list).
+
=head2 Typeglobs and Filehandles
Perl uses an internal type called a I<typeglob> to hold an entire
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
+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:
sub newopen {
my $path = shift;
- local *FH; # not my!
+ 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
+Now that we have the C<*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
-L<perlfunc/open()> for an example.
-
-See L<perlref>, L<perlsub>, and L<perlmod/"Symbol Tables"> for more
-discussion on typeglobs and the *foo{THING} syntax.
+C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
+In other words, C<*FH> must be used to create new symbol table entries;
+C<*foo{THING}> cannot. When in doubt, use C<*FH>.
+
+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 L<perlfunc/open()> for an
+example.
+
+=head1 SEE ALSO
+
+See L<perlvar> for a description of Perl's built-in variables and
+a discussion of legal variable names. See L<perlref>, L<perlsub>,
+and L<perlmod/"Symbol Tables"> for more discussion on typeglobs and
+the C<*foo{THING}> syntax.
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