home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Skunkware 5
/
Skunkware 5.iso
/
man
/
cat.1
/
perldata.1
< prev
next >
Wrap
Text File
|
1995-07-25
|
26KB
|
595 lines
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
NNNNAAAAMMMMEEEE
perldata - Perl data structures
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
VVVVaaaarrrriiiiaaaabbbblllleeee nnnnaaaammmmeeeessss
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
Page 1 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
open(LOG,'logfile') rather than open(log,'logfile'). Using
uppercase filehandles also improves readability and protects
you from conflict with future reserved words.) Case _I_S
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 _p_e_r_l_r_e_f 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.)
CCCCoooonnnntttteeeexxxxtttt
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
Page 2 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
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 _p_e_r_l_f_u_n_c manpage.
SSSSccccaaaallllaaaarrrr vvvvaaaalllluuuueeeessss
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
_d_e_f_i_n_e_d() 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 ccccsssshhhh.
(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 _N_O _L_O_N_G_E_R 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
Page 3 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
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.)
SSSSccccaaaallllaaaarrrr vvvvaaaalllluuuueeee ccccoooonnnnssssttttrrrruuuuccccttttoooorrrrssss
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 "\'"
Page 4 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
and "\\"). The usual Unix backslash rules apply for making
characters such as newline, tab, etc., as well as some more
exotic forms. See the qq entry in the _p_e_r_l_o_p manpage 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. Also
note that a single-quoted string must be separated from a
preceding word by a space, since single quote is a valid
(though discouraged) character in an identifier (see the
Packages entry in the _p_e_r_l_m_o_d manpage).
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 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 ----wwww 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
Page 5 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
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!
Page 6 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
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;
LLLLiiiisssstttt vvvvaaaalllluuuueeee ccccoooonnnnssssttttrrrruuuuccccttttoooorrrrssss
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 an 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:
Page 7 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
@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 _N_O_T interpolate, see the _p_e_r_l_r_e_f 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'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
The final element may be an array or a hash:
Page 8 (printed 6/30/95)
PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLDDDDAAAATTTTAAAA((((1111))))
($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 _l_o_c_a_l() or _m_y().
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.
Page 9 (printed 6/30/95)