Data Types and Objects

Perl has three data types: scalars, arrays of scalars, and associative arrays of scalars. Normal arrays are indexed by number, and associative arrays by string.

The interpretation of operations and values in perl sometimes depends on the requirements of the context around the operation or value. There are three major contexts: string, numeric and array. Certain operations return array values in contexts wanting an array, and scalar values otherwise. (If this is true of an operation it will be mentioned in the documentation for that operation.) Operations which return scalars don't care whether the context is looking for a string or a number, but scalar variables and values are interpreted as strings or numbers as appropriate to the context. A scalar is interpreted as TRUE in the boolean sense if it is not the null string or 0. Booleans returned by operators are 1 for true and 0 or '' (the null string) for false.

There are actually two varieties of null string: defined and undefined. Undefined null strings 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 string may become defined the first time you access it, but prior to that you can use the defined() operator to determine whether the value is defined or not.

References to scalar variables always begin with '$', even when referring to a scalar that is part of an array. Thus:

    $days		# a simple scalar variable
    $days[28]		# 29th element of array @days
    $days{'Feb'}	# one value from an associative array
    $#days		# last index of array @days

    @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'})

    %days		# (key1, val1, key2, val2 ...)

Any of these eight constructs may serve as an lvalue, that is, may be assigned to. (It also turns out that an assignment is itself an lvalue in certain contexts--see examples under s, tr and chop.) 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 an array context.

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, since there is (ordinarily) a 0th element.) Assigning to $#days changes the length of the array. Shortening an array by this method does not actually destroy any values. Lengthening an array that was previously shortened recovers the values that were in those elements. You can also gain some measure of efficiency by preextending 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. This differs from assigning to $#whatever in that intervening values are set to null rather than recovered.) You can truncate an array down to nothing by assigning the null list () to it. The following are exactly equivalent

	@whatever = ();
	$#whatever = $[ - 1;

	scalar(@whatever) == $#whatever - $[ + 1;

Multi-dimensional arrays are not directly supported, but see the discussion of the $; variable later for a means of emulating multiple subscripts with an associative array. You could also write a subroutine to turn multiple subscripts into a single subscript.

Every data type has its own namespace. You can, without fear of conflict, use the same name for a scalar variable, an array, an associative array, a filehandle, a subroutine name, and/or a label. Since variable and array references always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character. Hint: you could say open(LOG,'logfile') rather than open(log,'logfile'). Using uppercase filehandles also improves readability and protects you from conflict with future reserved words.) Case IS significant--"FOO", "Foo" and "foo" are all different names. Names which start with a letter may also contain digits and underscores. Names which do not start with a letter are limited to one character, e.g. "$%" or "$$". (Most of the one character names have a predefined significance to perl. More later.)

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

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

	\t		tab
	\n		newline
	\r		return
	\f		form feed
	\b		backspace
	\a		alarm (bell)
	\e		escape
	\033		octal char
	\x1b		hex char
	\c[		control char
	\l		lowercase next char
	\u		uppercase next char
	\L		lowercase till \E
	\U		uppercase till \E
	\E		end case modification

    $Price = '$100';			# not interpreted
    print "The price is $Price.\n";	# interpreted

Two special literals are __LINE__ and __FILE__, which represent the current line number and filename at that point in your program. They may only be used as separate tokens; they will not be interpolated into strings. In addition, the token __END__ 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 the DATA filehandle. (The DATA filehandle may read data only from the main script, but not from any required file or evaluated string.) The two control characters ^D and ^Z are synonyms for __END__.

A word that doesn't have any other interpretation in the grammar will be treated as if it had single quotes around it. For this purpose, a word consists only of alphanumeric characters and underline, and must start with an alphabetic character. As with filehandles and labels, a bare word 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.

Array values are interpolated into double-quoted strings by joining all the elements of the array with the delimiter specified in the $" variable, space by default. (Since in versions of perl prior to 3.0 the @ character was not a metacharacter in double-quoted strings, the interpolation of @array, $array[EXPR], @array[LIST], $array{EXPR}, or @array{LIST} only happens if array is referenced elsewhere in the program or is predefined.) The following are equivalent:

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

	system "echo @ARGV";

A line-oriented form of quoting is based on the shell here-is 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 blank line--see Merry Christmas example below.) The terminating string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line.

	print <<EOF;		# same as above
The price is $Price.
EOF

	print <<"EOF";		# same as above
The price is $Price.
EOF

	print << x 10;		# null identifier is delimiter
Merry Christmas!

	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

	(LIST)

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

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

    @foo = ('cc', '-E', $bar);
    $foo = @foo;		# $foo gets 3

    @foo = (
	1,
	2,
	3,
    );

A list value may also be subscripted like a normal array. Examples:

	$time = (stat($file))[8];	# stat returns array value
	$digit = ('a','b','c','d','e','f')[$digit-10];
	return (pop(@foo),pop(@foo))[0];

Array lists may be assigned to if and only if each element of the list is an lvalue:

    ($a, $b, $c) = (1, 2, 3);

    ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);

    ($a, $b, @rest) = split;
    local($a, $b, %rest) = @_;

An associative array 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);

	$x = (($foo,$bar) = (3,2,1));	# set $x to 3, not 2

There are several other pseudo-literals that you should know about. If a string is enclosed by backticks (grave accents), it first undergoes variable substitution just like a double quoted string. It is then interpreted as a command, and the output of that command is the value of the pseudo-literal, like in a shell. In a scalar context, a single string consisting of all the output is returned. In an array context, an array of values is returned, one for each line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command is returned in $? (see Predefined Names for the interpretation of $?). Unlike in csh, no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a $ through to the shell you need to hide it with a backslash.

Evaluating a filehandle in angle brackets yields the next line from that file (newline included, so it's never false until EOF, at which time an undefined value is returned). Ordinarily you must assign that value to a variable, but there is one situation where an automatic assignment happens. If (and only if) the input symbol is the only thing inside the conditional of a while loop, the value is automatically assigned to the variable "$_". (This may seem like an odd thing to you, but you'll use the construct in almost every perl script you write.) Anyway, the following lines are equivalent to each other:

    while ($_ = <STDIN>) { print; }
    while (<STDIN>) { print; }
    for (;<STDIN>;) { print; }
    print while $_ = <STDIN>;
    print while <STDIN>;

If a <FILEHANDLE> is used in a context that is looking for an array, an array consisting of all the input lines is returned, one line per array element. It's easy to make a LARGE data space this way, so use with care.

The null filehandle <> is special and can be used to emulate the behavior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the ARGV array is checked, and if it is null, $ARGV[0] is set to '-', which when opened gives you standard input. The ARGV array is then processed as a list of filenames. The loop

	while (<>) {
		...			# code for each line
	}

	unshift(@ARGV, '-') if $#ARGV < $[;
	while ($ARGV = shift) {
		open(ARGV, $ARGV);
		while (<ARGV>) {
			...		# code for each line
		}
	}

You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as if the input was one big happy file. (But see example under eof for how to reset line numbers on each file.)

If you want to set @ARGV to your own list of files, go right ahead. If you want to pass switches into your script, you can put a loop on the front like this:

	while ($_ = $ARGV[0], /^-/) {
		shift;
	    last if /^--$/;
		/^-D(.*)/ && ($debug = $1);
		/^-v/ && $verbose++;
		...		# other switches
	}
	while (<>) {
		...		# code for each line
	}

If the string inside the angle brackets is a reference to a scalar variable (e.g. <$foo>), then that variable contains the name of the filehandle to input from.

If the string inside angle brackets is not a filehandle, it is interpreted as a filename pattern to be globbed, and either an array of filenames or the next filename in the list is returned, depending on context. One level of $ interpretation is done first, but you can't say <$foo> because that's an indirect filehandle as explained in the previous paragraph. You could insert curly brackets to force interpretation as a filename glob: <${foo}>.

Example:

	while (<*.c>) {
		chmod 0644, $_;
	}

	open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'");
	while (<foo>) {
		chop;
		chmod 0644, $_;
	}

	chmod 0644, <*.c>;