All numbers have the same format internally in Prel. in fact the Perl use double-precision floating-point value to store numbers.
Creating charaters by code point: chr(), and ord() metheds.
e.g.
$alef = chr(0x05df) ;$code_point = ord('d') ;
In Perl you may write func() wit or without the parentheses. This is a general rule in Perl: except in cases whtere is changes the meaning to remove them, parentheses are always optional.
Undef is a good thing.
Array & List:
- A list or array may hold numbers , string, undef value, or any mixture of different scalar values. in other words, can hold different type scalar value in list or array.
- If the subscript indicates an element that would be beyond the ene of the array, the corresponding value will be undef.
- How to get the last element index in an array? $#array_name is the last element index. OR -1 beause, negative array indices conut from the end of the array.
- push,pop and shift,unshift. end & begin.
- splice @array_name, start, length, replacement ; the third and fourth arguments is not essential. and length allow is zero.
- foreach: foreach $var (list) {...}. It not same as Java the $var is not a copy of the list element-it actually is the list element. That is, if you modify the control variable($var) inside the loop, you modify the element itself.
- Deault:$_
- revers,sort
Scalar and List Context:
- Expressing in Perl always return the appopriate value for their context. In other words, a same thing in defferent context that it's meaning is defferent.
- Using List-Producing expressions in Scalar Context.@people in a list context, it gives the list of elemetns. But in a scalar context, it returns the nunber of elements in the array.sortalways returns undef.reverse
returns a reversed string.
- Using Scalar-producing expresions in List Context.always get a list. e.g. @william = undef; # Gets the one-element list (undef) @betty = (); #A correct way to emtyp an array.
- Forcing Scalar Context in List Context. use scalar function.
- <STDIN> in List Context: returns all of the remaining lines up to the end-of-file. each line as a separate element of the list. use EOF finishing the input. in linux is Ctrl+D, in wiondws is Ctrl+Z.
Subroutines
- Definition. use the keyword sub define your own subroutine. e.g. sub fuckk {...}
- Invoking. using the subroutine name that you want invoked with the ampersand&. e.g.&fuckk ;
- Return Values. all Perl subroutines have a return value--Whatever calcuation is last performed in a subroutine is automatically also the return value.
- Arguments. Perl has subroutine arguments, to pass an argument list to the subroutne,after the subroutine invocation. e.g.$n = &fuckk(arg1, arg2...). Perl automatically stores the paramenter list in the special array variable named
@_. The @_ is private to the subroutine.
- Overwrite. if you define two same-name subroutines, the last will overwrite the first.
- Private Variable in Subrouines. Private variables called lexical variables at any time with the my operator. e.g. my($n1 $n2) ; by the way, By default,in Perl all variables are global variable. attenionmy
has lexical scope. like in C local variable.
- The use strict pragma. impose a little discipline.
- The return operator. returns a value from a subroutine immediately.
- state variales. Persistent Private Variables pretty like static in C, but state is a priavet variable. you can make any varibale(scalar and list) type a state variable; but you can't initalize a state variable in list contexts. e.g. state
@array = qw(a b c); # error
Input and Output
- Input to Standard Input. STDIN this is standard input filehandle. the <filehandle> of <STDIN> is line_input operator and gives you the next line with \n.
- - the hyphen. If you use a hyphen as one of the argumets pass in perl, that the hyphen means standard input as well. If you just only a hyphen argument you can omiss it. because if no invocation arguments, the program should process the
standard input stream.
- The @ARGV array. This is a special array that is preset by Perl interpreter as the list of the invocation arguments. In other words, It store command arguments. but is can bu changed.
- <> the Diamond Operator. read line data from the @ARGV array. It also is line_input operator.
while (<>) { chomp ;...}
- Formatted Output with printf. e.g.printf "Hello,%s your age is %d.\n", "Tom", 18 ; the % sign is called conversions. print a number in what's generally a good way, use %g.decimal use %d, foalt use %f, strng use %s.
- Arrays and print. It is defferent print @arry and print "@arry". the print "@array" has a whilespace speasue two elements but print @array is not.
- Filehandles. a filehandle is the name in Perl program for an I/O connection between your Perl process and the outside world. In fact,It's just the name of connection, not necessarily the name of files.
- The six special built-in filehandlle name. Its is STDIN, STDOUT, STDERR, ARGV, DATA, ARGVOUT. Perl already uses for its own purposes.
- Filehandles name. Before Perl 5.6, all filehandle names were barewords, and Perl 5.6 added the ability to store a filehandle reference in a normal scalar variable.
- Opening a filehandle. If you use bareword as a filehandle name, the open styanx is open LOG, '[< | > | >>] file' ; or open LOG, ' < | > | >>', 'fle' ; in 5.6 add "three-argument" open. the file can be a scalar. if the file is a scalar you
must use "..." around arugments. In default open a file is only read, if you like this invocate open LGO, 'file' ; If filehandles in a Scalar. e.g. open my $log_fh, '<', 'file' ;
- Specify an file encoding. If you use the three-arugments open you can specify an encoding. e.g. open LOG, '<:encoding(UTF-8)', 'file' ; you can get a list of all of the encodings that perl understands with a Perl command: # perl -MEncode
-le "print for Encode->encodings(':all')"
- use binmode func tell output Unicode to STDOUT. e.g. binmode STDOUT, ':encoding(UTF-8)' ; if you don't do this, you might get a warning "Wide charater in print at test line .."
- Use :crlf Specify file line endings. in DOS the file each line ends with CR-LF pair.(also as "\r\n"). Unix line endings only use the LF. The :crlf encoding can translate a newline to \r\n. e.g. open LOG, '<:crlf', 'file' ; this is open
a only read DOS file Prel can translate crfl to newline. e.g. open LOG, '>:crlf', 'file'; this is open a write file, write this file each lien endings with crlf. (from newlne to crlf).
- Closeing a Filehandle. close operator close it. e.g. close LOG ;
- Use die Show Fatal Errors. e.g. die "Has a error: $!\n" ; the $! stroe a human-readable complaint error mssage form system. and it well exit the program immidealy.
- Use warn show a Warning message. the warn show a message to STDERR, but don't exit the program.
- Automaticlly die-ing. Starting with Perl 5.10, the autodie pragma is part of the Standard Libaray. So you can use autodie ; with open LOG, '>>' , 'file' ; don't write die message.
- Changing the Default STD flehandle. There are some ways to change the Default STD filehandle. one is you cat useingselect
operator chanages default output filehandle. e.g select LOG ; select return currently output filehanlds. the two way is reopen a Standard filehandle e.g. open STDERR, ">>", "error.log" ;
