发信人: saka.bbs@bbs.neu.edu.cn (机器猫), 信区: cnlinux
标 题: perl(23)
发信站: 白山黑水BBS (Wed Apr 2 17:41:20 1997)
转信站: Lilac!ustcnews!ustcnews!sjtunews!neubbs
出 处: conger.neu.edu.cn
--------------6167540147EE
Content-Type: text/plain; charset=us-ascii; name="perlsyn.txt"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="perlsyn.txt"
NAME
perlsyn - Perl syntax
---------------------------------------------------------------------------
DESCRIPTION
A Perl script consists of a sequence of declarations and statements. The
only things that need to be declared in Perl are report formats and
subroutines. See the sections below for more information on those
declarations. All uninitialized user-created objects are assumed to start
with a null or 0 value until they are defined by some explicit operation
such as assignment. (Though you can get warnings about the use of undefined
values if you like.) The sequence of statements is executed just once,
unlike in sed and awk scripts, where the sequence of statements is executed
for each input line. While this means that you must explicitly loop over
the lines of your input file (or files), it also means you have much more
control over which files and which lines you look at. (Actually, I'm
lying--it is possible to do an implicit loop with either the -n or -p
switch. It's just not the mandatory default like it is in sed and awk.)
---------------------------------------------------------------------------
Declarations
Perl is, for the most part, a free-form language. (The only exception to
this is format declarations, for obvious reasons.) Comments are indicated
by the ``#'' character, and extend to the end of the line. If you attempt
to use /* */ C-style comments, it will be interpreted either as division or
pattern matching, depending on the context, and C++ // comments just look
like a null regular expression, so don't do that.
A declaration can be put anywhere a statement can, but has no effect on the
execution of the primary sequence of statements--declarations all take
effect at compile time. Typically all the declarations are put at the
beginning or the end of the script. However, if you're using
lexically-scoped private variables created with my() , you'll have to make
sure your format or subroutine definition is within the same block scope as
the my if you expect to to be able to access those private variables.
Declaring a subroutine allows a subroutine name to be used as if it were a
list operator from that point forward in the program. You can declare a
subroutine without defining it by saying just
sub myname;
$me = myname $0 or die "can't get myname";
Note that it functions as a list operator though, not as a unary operator,
so be careful to use or instead of || there.
Subroutines declarations can also be loaded up with the require statement
or both loaded and imported into your namespace with a use statement. See
the perlmod manpage for details on this.
A statement sequence may contain declarations of lexically-scoped
variables, but apart from declaring a variable name, the declaration acts
like an ordinary statement, and is elaborated within the sequence of
statements as if it were an ordinary statement. That means it actually has
both compile-time and run-time effects.
---------------------------------------------------------------------------
Simple statements
The only kind of simple statement is an expression evaluated for its side
effects. Every simple statement must be terminated with a semicolon, unless
it is the final statement in a block, in which case the semicolon is
optional. (A semicolon is still encouraged there if the block takes up more
than one line, since you may eventually add another line.) Note that there
are some operators like eval {} and do {} that look like compound
statements, but aren't (they're just TERMs in an expression), and thus need
an explicit termination if used as the last item in a statement.
Any simple statement may optionally be followed by a SINGLE modifier, just
before the terminating semicolon (or block ending). The possible modifiers
are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics, presuming you're a
speaker of English. The while and until modifiers also have the usual
``while loop'' semantics (conditional evaluated first), except when applied
to a do-BLOCK (or to the now-deprecated do-SUBROUTINE statement), in which
case the block executes once before the conditional is evaluated. This is
so that you can write loops like:
do {
$line = <STDIN>;
...
} until $line eq ".\n";
See do . Note also that the loop control statements described later will
NOT work in this construct, since modifiers don't take loop labels. Sorry.
You can always wrap another block around it to do that sort of thing.
---------------------------------------------------------------------------
Compound statements
In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case of a
required file, or the program as a whole), and sometimes a block is
delimited by the extent of a string (in the case of an eval).
But generally, a block is delimited by curly brackets, also known as
braces. We will call this syntactic construct a BLOCK.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not
statements. This means that the curly brackets are required--no dangling
statements allowed. If you want to write conditionals without curly
brackets there are several other ways to do it. The following all do the
same thing:
if (!open(FOO)) { die "Can't open $FOO: $!"; }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
# a bit exotic, that last one
The if statement is straightforward. Since BLOCKs are always bounded by
curly brackets, there is never any ambiguity about which if an else goes
with. If you use unless in place of if, the sense of the test is reversed.
The while statement executes the block as long as the expression is true
(does not evaluate to the null string or 0 or ``0''). The LABEL is
optional, and if present, consists of an identifier followed by a colon.
The LABEL identifies the loop for the loop control statements next , last ,
and redo . If the LABEL is omitted, the loop control statement refers to
the innermost enclosing loop. This may include dynamically looking back
your call-stack at run time to find the LABEL. Such desperate behavior
triggers a warning if you use the -w flag.
If there is a continue BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
for loop in C. Thus it can be used to increment a loop variable, even when
the loop has been continued via the next statement (which is similar to the
C continue statement).
---------------------------------------------------------------------------
Loop Control
The next command is like the continue statement in C; it starts the next
iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The last command is like the break statement in C (as used in loops); it
immediately exits the loop in question. The continue block, if any, is not
executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The redo command restarts the loop block without evaluating the conditional
again. The continue block, if any, is not executed. This command is
normally used by programs that want to lie to themselves about what was
just input.
For example, when processing a file like /etc/termcap. If your input lines
might end in backslashes to indicate continuation, you want to skip ahead
and get the next record.
while (<>) {
chomp;
if (s/\\$//) {
$_ .= <>;
redo unless eof();
}
# now process $_
}
which is Perl short-hand for the more explicitly written version:
LINE: while ($line = <ARGV>) {
chomp($line);
if ($line =~ s/\\$//) {
$line .= <ARGV>;
redo LINE unless eof(); # not eof(ARGV)!
}
# now process $line
}
Or here's a a simpleminded Pascal comment stripper (warning: assumes no {
or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo LINE;
}
}
}
print;
}
Note that if there were a continue block on the above code, it would get
executed even on discarded lines.
If the word while is replaced by the word until, the sense of the test is
reversed, but the conditional is still tested before the first iteration.
In either the if or the while statement, you may replace ``(EXPR)'' with a
BLOCK, and the conditional is true if the value of the last statement in
that block is true. While this ``feature'' continues to work in version 5,
it has been deprecated, so please change any occurrences of ``if BLOCK'' to
``if (do BLOCK)''.
---------------------------------------------------------------------------
For Loops
Perl's C-style for loop works exactly like the corresponding while loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
Besides the normal array index looping, for can lend itself to many other
interesting applications. Here's one that avoids the problem you get into
if you explicitly test for end-of-file on an interactive file descriptor
causing your program to appear to hang.
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
---------------------------------------------------------------------------
Foreach Loops
The foreach loop iterates over a normal list value and sets the variable
VAR to be each element of the list in turn. The variable is implicitly
local to the loop and regains its former value upon exiting the loop. If
the variable was previously declared with my , it uses that variable
instead of the global one, but it's still localized to the loop. This can
cause problems if you have subroutine or format declarations within that
block's scope.
The foreach keyword is actually a synonym for the for keyword, so you can
use foreach for readability or for for brevity. If VAR is omitted, $_ is
set to each value. If LIST is an actual array (as opposed to an expression
returning a list value), you can modify each element of the array by
modifying VAR inside the loop. That's because the foreach loop index
variable is an implicit alias for each item in the list that you're looping
over.
Examples:
for (@ary) { s/foo/bar/ }
foreach $elem (@elements) {
$elem *= 2;
}
for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
print $count, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
}
Here's how a C programmer might code up a particular algorithm in Perl:
for ($i = 0; $i < @ary1; $i++) {
for ($j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more confortable with the idiom might
do it:
OUTER: foreach $wid (@ary1) {
INNER: foreach $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and faster. It's cleaner
because it's less noisy. It's safer because if code gets added between the
inner and outer loops later, you won't accidentally excecute it because
you've explicitly asked to iterate the other loop rather than merely
terminating the inner one. And it's faster because Perl executes a foreach
statement more rapidly than it would the equivalent for loop.
---------------------------------------------------------------------------
Basic BLOCKs and Switch Statements
A BLOCK by itself (labeled or not) is semantically equivalent to a loop
that executes once. Thus you can use any of the loop control statements in
it to leave or restart the block. The continue block is optional.
The BLOCK construct is particularly nice for doing case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
There is no official switch statement in Perl, because there are already
several ways to write the equivalent. In addition to the above, you could
write
SWITCH: {
$abc = 1, last SWITCH if /^abc/;
$def = 1, last SWITCH if /^def/;
$xyz = 1, last SWITCH if /^xyz/;
$nothing = 1;
}
(That's actually not as strange as it looks once you realize that you can
use loop control ``operators'' within an expression, That's just the normal
C comma operator.)
or
SWITCH: {
/^abc/ && do { $abc = 1; last SWITCH; };
/^def/ && do { $def = 1; last SWITCH; };
/^xyz/ && do { $xyz = 1; last SWITCH; };
$nothing = 1;
}
or formatted so it stands out more as a ``proper'' switch statement:
SWITCH: {
/^abc/ && do {
$abc = 1;
last SWITCH;
};
/^def/ && do {
$def = 1;
last SWITCH;
};
/^xyz/ && do {
$xyz = 1;
last SWITCH;
};
$nothing = 1;
}
or
SWITCH: {
/^abc/ and $abc = 1, last SWITCH;
/^def/ and $def = 1, last SWITCH;
/^xyz/ and $xyz = 1, last SWITCH;
$nothing = 1;
}
or even, horrors,
if (/^abc/)
{ $abc = 1 }
elsif (/^def/)
{ $def = 1 }
elsif (/^xyz/)
{ $xyz = 1 }
else
{ $nothing = 1 }
A common idiom for a switch statement is to use foreach's aliasing to make
a temporary assignment to $_ for convenient matching:
SWITCH: for ($where) {
/In Card Names/ && do { push @flags, '-e'; last; };
/Anywhere/ && do { push @flags, '-h'; last; };
/In Rulings/ && do { last; };
die "unknown value for form variable where: `$where'";
}
Another interesting approach to a switch statement is arrange for a do
block to return the proper value:
$amode = do {
if ($flag & O_RDONLY) { "r" }
elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "w" : "a" }
elsif ($flag & O_RDWR) {
if ($flag & O_CREAT) { "w+" }
else { ($flag & O_APPEND) ? "r+" : "a+" }
}
};
---------------------------------------------------------------------------
Goto
Although not for the faint of heart, Perl does support a goto statement. A
loop's LABEL is not actually a valid target for a goto ; it's just the name
of the loop. There are three forms: goto-LABEL, goto-EXPR, and goto-&NAME.
The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that requires
initialization, such as a subroutine or a foreach loop. It also can't be
used to go into a construct that is optimized away. It can be used to go
almost anywhere else within the dynamic scope, including out of
subroutines, but it's usually better to use some other construct such as
last or die. The author of Perl has never felt the need to use this form of
goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved
dynamically. This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is highly magical, and substitutes a call to the named
subroutine for the currently running subroutine. This is used by AUTOLOAD()
subroutines that wish to load another subroutine and then pretend that the
other subroutine had been called in the first place (except that any
modifications to @_ in the current subroutine are propagated to the other
subroutine.) After the goto , not even caller() will be able to tell that
this routine was called first.
In almost cases like this, it's usually a far, far better idea to use the
structured control flow mechanisms of next , last , or redo insetad
resorting to a goto . For certain applications, the catch and throw pair of
eval{} and die() for exception processing can also be a prudent approach.
---------------------------------------------------------------------------
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with source code. If
while expecting the beginning of a new statement, the compiler encounters a
line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with =cut will be ignored. The format of the intervening text is
described in the perlpod manpage .
This allows you to intermix your source code and your documentation text
freely, as in
=item snazzle($)
The snazzle() function will behave in the most spectacular
form that you can possibly imagine, not even excepting
cybernetic pyrotechnics.
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
my $thingie = shift;
.........
}
Note that pod translators should only look at paragraphs beginning with a
pod diretive (it makes parsing easier), whereas the compiler actually knows
to look for pod escapes even in the middle of a paragraph. This means that
the following secret stuff will be ignored by both the compiler and the
translators.
$a=3;
=secret stuff
warn "Neither POD nor CODE!?"
=cut back
print "got $a\n";
You probably shouldn't rely upon the warn() being podded out forever. Not
all pod translators are well-behaved in this regard, and perhaps the
compiler will become pickier.
--------------6167540147EE--
--
※ 来源:．白山黑水站 bbs.neu.edu.cn．[FROM: ygh@rose.dlut.edu.cn]