NAME

perldata - Perl data structures

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.

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.

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. 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 only contain more digits. Names which 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 wantarray .

Scalar values

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.

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.

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, since 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 make to make sure destructors were called when expected.) 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.) 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.) 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 just assume that

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

If you evaluate a hash in a scalar context, it returns a value which 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 only useful 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.)

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 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 qq for a list.

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, identifiers 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 identifier 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, since single quote is a valid (though deprecated) character in an identifier (see Packages ).

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 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, 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 blank line--see the 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;    # Legal but discouraged.  Use <<"".
    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
        myfunc(<<"THIS", 23, <<'THAT'');
    Here's a line
    or two.
    THIS
    and here 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 to $foo the value 3:

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

You may have an optional comma before the closing parenthesis of an 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 lose their identity in a LIST--the list

(@foo,@bar,&SomeSub)

contains all the elements of @foo followed by all the elements of @bar , followed by all the elements returned by the subroutine named SomeSub. 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. Examples:

# Stat returns list value. $time = (stat($file))[8]; # 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 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);

The final element may be an array or a hash:

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

You can actually put an array anywhere in the list, but the first array 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);

It is often more readable to use the => operator between key/value pairs (the => operator is actually nothing more than a more visually distinctive synonym for a comma):

%map = ( 'red' => 0x00f, 'blue' => 0x0f0, 'green' => 0xf00, );

Array 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

This is very handy when you want to do a list assignment in a Boolean context, since most list functions return a null list when finished, which when assigned produces a 0, which is interpreted as FALSE.