Learning Perl notes

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: minute t 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 wocould 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. UseScalarFunction.
• <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 keywordSubDefine 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 completed MED 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 variableIn subrouines. Private variables called lexical variables at any time withMyOperator. 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 shocould process
  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 isStdin, stdout, stderr, argv, Data, argvout. Perl already uses for its own purposes.
• Filehandles name. Before Perl 5.6, all filehandle names wereBarewords, 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 ";