Common Perl functions 2

Perl Common function set 2

I. Process processing functions 1. Process startup Function Function Name eval Call the syntax eval_r (string) The description regards string as a Perl statement execution. After the command is correctly executed, the system variable $ @ is an empty string. If an error occurs, $ @ is the error message. Example $ print = "Print (/" Hello, world // n /");"; Eval_r ($ print ); Output result Hello, world Function Name: System Call the syntax system (list) The first element in the list is the program name, and the rest are parameters. System starts a process to run the program and waits for it to end. After the program ends, the error code shifts left eight bits to return values. Example @ proglist = ("Echo", "Hello, world! "); System (@ proglist ); Output result Hello, world! Function Name fork Call the syntax procid = fork (); Describe the creation of two copies of the program-parent process and child process-run simultaneously. The child process returns zero, and the parent process returns a non-zero value. This value is the ID of the child process. Example $ retval = fork (); If ($ retval = 0 ){ # This is the child process Exit; # This terminates the child process } Else { # This is the parent process } No result output Function name pipe Call the syntax pipe (infile, OUTFILE ); This topic explains how to use fork to communicate with Parent and Child processes. The information sent to the OUTFILE file variable can be read through the infile variable. Steps: 1. Call Pipe 2. Use fork to divide the program into Parent and Child Processes 3. One process disables infile and the other disables OUTFILE. Example: pipe (input, output ); $ Retval = fork (); If ($ retval! = 0 ){ # This is the parent process Close (input ); Print ("enter a line of input:/N "); $ Line = <stdin>; Print Output ($ line ); } Else { # This is the child process Close (output ); $ Line = <input>; Print ($ line ); Exit (0 ); } Result output $ Program Enter a line of input: Here is a test line Here is a test line $ Function Name Exec Call the exec (list) syntax ); Similar to system, the difference is that the current program is terminated before the new process is started. It is often used with fork. After fork is divided into two processes, the child process starts another program with exec. Example Result output Function Name: syscall Call the syntax syscall (list ); Describe how to call a system function. The first element of the list is the system call name, and the rest are parameters. If the parameter is a number, it is converted to the integer (type INT) of C ). Otherwise, the pointer of the string is passed. For more information, see the Unix help or Perl document. Syscall must contain the syscall. pl file, that is: Require ("syscall. pl "); Example Result output 2. Process Termination Function Function Name: Die Call the syntax die (Message ); Describe terminate the program and output an error message to stderr. Message can be a string or a list. If the last parameter does not contain a line break, the program file name and row number are also output. Example die ("cannot open input file "); Cannot open input file at myprog line 6. Function Name warn Call the syntax warn (Message ); The commentary is similar to die. The difference is that the program is not terminated. Example warn ("Danger! Danger! /N "); Result output danger! Danger! Function Name exit Call the syntax exit (retcode ); Describe terminate the program and specify the return value. Example: exit (2 ); No result output Function Name kill Call the syntax kill (signal, proclist ); Sends signals to a group of processes. Signal is the digital signal sent, and 9 is used to kill the process. Proclist is the list of process IDs. For more information, see kill's Unix help. Example Result output 3. Process Control Functions Function Name sleep Call the syntax sleep (time ); Pause the program for a period of time. Time is the number of seconds to stop. The returned value is the actual number of seconds to stop. Example: Sleep (5 ); No result output Function Name: Wait Call the syntax procid = wait (); Pause the program execution and wait for the sub-process to terminate. No parameter is required. The return value is the sub-process ID. If no sub-process exists,-1 is returned. Example Result output Function Name: waitpid Call the syntax waitpid (procid, waitflag ); Pause the program execution and wait for the specified sub-process to terminate. Procid is the ID of the waiting process Example $ procid = fork (); If ($ procid = 0 ){ # This is the child process Print ("this line is printed first/N "); Exit (0 ); } Else { # This is the parent process Waitpid ($ procid, 0 ); Print ("this line is printed last/N "); } Result output $ Program This line is printed first This line is printed last $ 4. Other control functions Function Name caller Call syntax subinfo = caller (); Returns the caller's program name and row number for Perl debugger. List of three elements returned: 1. Package name at the call 2. Caller's file name 3. the row number at the call Example Result output Function Name chroot Call the syntax chroot (DIR ); For details about how to change the root directory of a program, see the chroot help. Example Result output Function name local Call the syntax local ($ variable ); The definition of local variables in the statement block (a set of statements surrounded by braces) is only used in the statement block. changes to these variables do not affect the variables with the same name outside the block. Never use it in a loop. Otherwise, a new local variable is defined for each loop! Example Result output Function name Times Call syntax timelist = times Returns the time consumed by the program and all sub-processes. List of four floating point numbers returned: 1. User time consumed by the program 2. System Time consumed by the program 3. User time consumed by sub-Processes 4. System Time consumed by sub-Processes Example Result output Ii. mathematical functions Function Name sin Call the syntax retval = sin (value ); The comment parameter is a radian value. Function name cos Call the syntax retval = cos (value ); The comment parameter is a radian value. Function Name atan2 Call the syntax retval = atan2 (value1, value2 ); Describe the operation and return the arctan value of value1 divided by value2. Unit: radian, range:-pi ~ Pi. Example: Converts an angle to a radian subprogram. Sub degrees_to_radians { Local ($ degrees) = @_; Local ($ radians); 11: $ Radians = atan2 (1, 1) * $ degrees/45; } Function Name SQRT Call the syntax retval = SQRT (value ); Describe the square root function. Value is not negative. Function Name exp Call the syntax retval = exp (value ); Returns the value power of E. Function Name Log Call the syntax retval = Log (value ); Describe the natural logarithm Based on E. Function Name ABS Call syntax retval = ABS (value ); Describe the absolute value function. (Not in Perl 4) Function Name Rand Call the syntax retval = rand (Num ); Returns a floating point number between 0 and an integer num. Function Name srand Call the syntax srand (value ); Describe the random number generator initialization. Make sure that Rand is randomly called each time. Iii. string processing functions Function name index Call the syntax position = index (string, substring, position ); Returns the position of the substring in the string. If not, returns-1. The position parameter is optional, indicating the number of characters skipped before matching, or starting from this position. Function Name rindex Call the syntax position = rindex (string, substring, position ); The description is similar to the index, with the difference being matching from the right side. Function Name Length Call the syntax num = length (string ); Returns the length of a string, or the number of characters. Function Name POS Call syntax offset = pos (string ); Returns the position of the last pattern match. Function Name substr Call the syntax substr (expr, skipchars, length) Describe the substring In the extracted string (or the string generated by the expression) expr, skip the skipchars character, or extract the substring from the position skipchars (the first character is 0 ), the length of the substring is length. This parameter can be ignored, meaning that all the remaining characters are taken. When this function appears on the left of the equation, expr must be a variable or an array element. In this case, the molecular string in the middle of the function is replaced by the value on the right of the equation. Function Name Study Call the syntax Study (scalar ); An internal format is used to increase the access speed of a variable. At the same time, it only takes effect for one variable. Function Name LC UC Call the syntax retval = Lc (string ); Retval = UC (string ); Converts all strings to uppercase or lowercase letters. Function Name lcfirst Ucfirst Call the syntax retval = lcfirst (string ); Retval = ucfirst (string ); The description converts the first letter into uppercase or lowercase letters. Function Name quotameta Call the syntax newstring = quotemeta (oldstring ); Add a backslash (/) to the front of a non-word letter (/). Statement: $ string = quotemeta ($ string ); Equivalent to: $ string = ~ S/(/W) // $1/g; It is often used in pattern matching to ensure that no character in the string is considered as a matching operator. Function Name join Call the syntax join (joinstr, list ); Comments: Combine the string list (array) into a long string and insert the string joinstr between each two list elements. Function Name sprintf Call the syntax sprintf (string, fields ); Similar to printf, the difference is that the result is not output to the file, but is assigned to the variable as the return value. Example $ num = 26; $ Outstr = sprintf ("% d = % x hexadecimal or % O octal/N", $ num ); Print ($ outstr ); Output 26 = 1a hexadecimal or 32 octal Iv. scalar conversion functions Function Name: Chop Call syntax $ lastchar = chop (VAR ); Description: VaR can be a variable or array. When VaR is a variable, the last character is deleted and assigned to $ lastchar. When VaR is an array or list, the last character of all elements is deleted, and the last letter of the last element is assigned to $ lastchar. Function Name chomp Call syntax result = chomp (VAR ); Check whether the last character of the element in the string or string list is a line separator defined by the system variable $/. If yes, delete it. The return value is the number of characters actually deleted. Function Name crypt Call syntax result = crypt (original, salt ); Describe how to use the DES algorithm to encrypt a string. Original is the string to be encrypted, and salt is a two-character string. It defines how to change the DES algorithm to make decoding more difficult. The returned value is an encrypted string. Function Name hex Call the syntax decnum = hex (hexnum ); Converts a hexadecimal number (in string form) into a decimal number. Function Name int Call the syntax intnum = int (floatnum ); Converts the rounded decimal part of a floating point number to an integer. Function Name Oct Call the syntax decnum = OCT (octnum ); Converts an octal number (in string form) or a hexadecimal number ("0x .." form) into a decimal number. Function Name ORD Call syntax asciival = ord (char ); Returns the ASCII value of a single character, similar to the function of the same name in Pascal. Function Name Chr Call syntax $ char = CHR (asciival ); Returns the corresponding characters of the ASCII value, similar to the functions with the same name in Pascal. Function Name pack Call the syntax formatstr = pack (packformat, list ); A list or array is converted (packaged) into a simple variable in the format used by programming languages such as machine storage or C. The packformat parameter contains one or more formatting characters. Each element in the list corresponds to one. The format characters can be separated by spaces or tabs, because pack ignores spaces. In addition to the formats A, A, and @, you can add an integer after the format is repeated multiple times. For example: $ Twoints = pack ("I2", 103,241 ); Apply the same format to all elements with a * number, for example: $ Manyints = pack ("I *", 14, 26, 11, 83 ); For A and A, the following integer indicates the length of the string to be created. The repeat method is as follows: $ Strings = pack ("A6" X 2, "test1", "Test2 "); The format @ is special. An integer must be added after it. This number indicates the length required by the string. If the length is not enough, use null to supplement it. For example: $ Output = pack ("A @ 6 A", "test", "Test2 "); The most common purpose of the pack function is to create data that can interact with the C program. For example, strings in the C language end with null. You can create such data as follows: $ Cstring = pack ("ax", $ mystring ); The following table shows the equivalence relationship between some format characters and Data Types in C: Character equivalent c Data Type C char D double F float I int I unsigned int (or unsigned) L long L unsigned long S short S unsigned short The complete format characters are shown in the following table. Format Character Description A string supplemented by null A string supplemented by Space B-Bit String, before Low Position B-bit string with a high position in front C-Signed characters (usually-128 ~ 127) C unsigned characters (usually 8 characters) D double-precision floating point number F single-precision floating point number H hexadecimal number string, low front H hexadecimal number string with a high position in front I-signed integer I unsigned integer L signed long integer L unsigned long integer N short integer in network order N Network order long integer P string pointer S signed short integer S unsigned short integer Convert u to uuencode format V vax short integer V vax ordinal long integer X an empty byte X returns one byte. @ Fill with NULL bytes Function Name unpack Call syntax @ list = unpack (packformat, formatstr ); The unpack function is opposite to the pack function. The values stored in the machine format are converted into a list of Perl values. The format characters are basically the same as those of Pack (that is, the preceding table). The difference is that a format converts the machine format string to a Perl string and removes all spaces or empty characters at the end; X skips one byte. @ skips some bytes to the specified position. For example, @ 4 skips four bytes. Here is an example of @ and X contract: $ longrightint = unpack ("@ * X4 L", $ packstring ); This statement converts the last four bytes into unsigned long integers. Here is an example of decoding uuencode files: 1 :#! /Usr/local/bin/perl 2: 3: open (codedfile, "/u/janedoe/codefile") | 4: Die ("can't open input file "); 5: open (OUTFILE, "> OUTFILE") | 6: Die ("can't open output file "); 7: While ($ line = <codedfile> ){ 8: $ decoded = unpack ("u", $ line ); 9: Print OUTFILE ($ decoded ); 10 :} 11: Close (OUTFILE ); 12: Close (codedfile ); When using pack and unpack for uuencode, remember that although they are the same as the uuencode and uudecode tools in UNIX, they do not provide the first and last lines, if you want to use uudecode to decode the File Created by pack output, you must also output the first line and the last line (see uuencode help in UNIX ). Function Name VEC Call the syntax retval = VEC (vector, index, BITs ); As the name suggests, VEC is a vector function. It regards the value of a simple variable vector as multiple pieces of (dimension) data. Each piece contains a certain number of BITs, which together form a vector data. Each call accesses a piece of data, which can be read or written. The index parameter is like an array subscript. It indicates which block to access, where 0 is the first block, and so on. Note that the access order is from right to left, that is, the first block is on the rightmost side. The bits parameter specifies the number of digits in each block, which can be 1, 2, 4, 8, 16, or 32. Example 1 :#! /Usr/local/bin/perl 2: 3: $ vector = pack ("B *", "11010011 "); 4: $ val1 = VEC ($ vector, 0, 4 ); 5: $ val2 = VEC ($ vector, 1, 4 ); 6: Print ("high-to-low order values: $ val1 and $ val2/N "); 7: $ vector = pack ("B *", "11010011 "); 8: $ val1 = VEC ($ vector, 0, 4 ); 9: $ val2 = VEC ($ vector, 1, 4 ); 10: Print ("low-to-high order values: $ val1 and $ val2/N "); Result: high-to-low order values: 3 and 13. Low-to-high order values: 11 and 12 Function Name Defined Call the syntax retval = defined (expr ); Judge whether an element of a variable, array, or array has been assigned a value. Expr is a variable name, array name, or an array element. If defined, true is returned. Otherwise, false is returned. Function Name UNDEF Call the syntax retval = UNDEF (expr ); Describe how to cancel definitions of variables, arrays, array elements, and even subprograms and reclaim their space. The returned value is always an undefined value, which is equivalent to an empty string. 5. array and list Functions Function Name grep Call the syntax @ foundlist = grep (pattern, @ searchlist ); Similar to the Unix search tool with the same name, the grep function extracts elements that match the specified pattern in the list. The pattern parameter is the pattern to be searched, and the return value is the list of matching elements. Example @ list = ("this", "is", "A", "test "); @ Foundlist = grep (/^ [TT]/, @ list ); Result @ foundlist = ("this", "test "); Function Name splice Call the syntax @ retval = splice (@ array, slipelements, length, @ newlist ); The commenting concatenation function can insert an element into the List (array), delete a sublist, or replace a sublist. The skipelements parameter is the number of elements skipped before splicing, length is the number of elements to be replaced, and newlist is the list to be spliced in. When the length of newlist is greater than the length, the subsequent elements are automatically moved backward, and vice versa. Therefore, when length = 0, it is equivalent to inserting an element into the list, which is like a statement. Splice (@ array,-1, 0, "hello "); Add an element to the end of the array. When newlist is empty, it is equivalent to deleting the sublist. If length is empty, it is deleted from the skipelements element, and the last element is deleted: splice (@ array,-1); in this case, the returned value is the list of deleted elements. Function name shift Call the syntax element = shift (@ arrayvar ); Describe how to delete the first element of the array, move the remaining element forward, and return the deleted element. If no parameter is added, @ argv is operated by default. Function Name unshift Call the syntax COUNT = unshift (@ arrayver, elements ); In contrast to shift, adding one or more elements at the beginning of array arrayvar returns the length of the result (list. Equivalent to splice (@ array, 0, 0, elements ); Function Name push Call the syntax push (@ arrayvar, elements ); Add one or more elements to the end of the array. Equivalent to slice (@ array, @ array, 0, elements ); Function name pop Call the syntax element = POP (@ arrayvar ); Reverse to push, delete the last element of the list and use it as the return value. When the list is empty, "undefined value" (that is, an empty string) is returned ). Function Name split Call syntax @ list = Split (pattern, String, maxlength ); Describe how to split a string into a list of elements. Each time a pattern is matched, a new element is started, but the pattern itself is not included in the element. Maxlength is optional. When it is specified, it is not separated when it reaches this length. Function Name sort Call the syntax @ sorted = sort (@ list ); The comments are sorted in alphabetical order. Function Name reverse Call the syntax @ reversed = reverse (@ list ); The comments are sorted in alphabetical order. Function Name Map Call the syntax @ resultlist = map (expr, @ list ); This function is defined in perl5. Each element in the list can be used as the operand of the expression expr. It does not change and the result is returned. In the expression expr, the system variable $ _ represents each element. Example 1: @ list = (100,200,300 ); @ Results = map ($ _ + 1, @ list ); 2. @ Results = map (& mysub ($ _), @ list ); Result 1: (101,201,301) 2. None Function Name wantarray Call syntax result = wantarray (); In Perl, the behavior of some built-in functions varies depending on whether simple variables are processed or arrays, such as chop. User-Defined subprograms can also define these two actions. When a subroutine is expected to return a list, the return value of this function is a non-zero value (true); otherwise, it is a zero value (false ). Example 1 :#! /Usr/local/bin/perl 2: 3: @ array = & mysub (); 4: $ scalar = & mysub (); 5: 6: Sub mysub { 7: If (wantarray ()){ 8: Print ("True/N "); 9:} else { 10: Print ("false/N "); 11 :} 12 :} Result $ Program True False $