?? ch24.htm
字號(hào):
Interpreter</FONT></A></H2>
<P>
An interpreter executes your program without first creating an
executable file. It interprets your program into the language
of the CPU, on-the-fly. Compilers and interpreters do a lot of
the same work. However, siNCe interpreters can't create executable
files, the source code must always be available to users.
<BLOCKQUOTE>
See also <I>Compiler</I>.
</BLOCKQUOTE>
<H2><A NAME="InterprocessCommunication"><FONT SIZE=5 COLOR=#FF0000>
Inter-process Communication</FONT></A></H2>
<P>
You use inter-process communication, or IPC, when two or more
processes need to communicate. The communication can take place
using databases, shared memory, semaphores, or sockets.
<H2><A NAME="IO"><FONT SIZE=5 COLOR=#FF0000>
I/O</FONT></A></H2>
<P>
I/O is an abbreviation for Input/Output.
<H2><A NAME="IPC"><FONT SIZE=5 COLOR=#FF0000>
IPC</FONT></A></H2>
<BLOCKQUOTE>
See <I>Inter-process Communication</I>.
</BLOCKQUOTE>
<H2><A NAME="KeyValuePair"><FONT SIZE=5 COLOR=#FF0000>
Key-Value Pair</FONT></A></H2>
<P>
Each entry in a hash is a key-value pair. The key is used as the
index to retrieve the value.
<H2><A NAME="Label"><FONT SIZE=5 COLOR=#FF0000>
Label</FONT></A></H2>
<P>
You use labels to mark locations in your program to which you
need to return. Typically, you label the outer loop in a nested
series of loops so that you can jump out of the inner loops if
needed.
<H2><A NAME="LF"><FONT SIZE=5 COLOR=#FF0000>
LF</FONT></A></H2>
<P>
LF is the abbreviation for linefeed or newline. An LF is represented
by \n in strings. The linefeed can also be referred to as Ctrl+M,
^M, 0x0d, or as an ASCII value of 13.
<BLOCKQUOTE>
See also <I>ASCII</I> and <I>Control Characters</I>.
</BLOCKQUOTE>
<H2><A NAME="Library"><FONT SIZE=5 COLOR=#FF0000>
Library</FONT></A></H2>
<P>
A library is a file that groups related fuNCtions together. Libraries
are loaded into your program using the <TT>require</TT>
compiler directive. <A HREF="ch15.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch15.htm" >Chapter 15</A>, "Perl Modules," talks
a little bit about libraries.
<H2><A NAME="List"><FONT SIZE=5 COLOR=#FF0000>
List</FONT></A></H2>
<P>
See <I>Array</I>.
<H2><A NAME="Literal"><FONT SIZE=5 COLOR=#FF0000>
Literal</FONT></A></H2>
<P>
A literal is a value that is represented "as is" in
your source code. There are four types of Perl literals: Number,
Strings, Arrays, and Hashes. <A HREF="ch2.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch2.htm" >Chapter 2</A> "Numeric and String
Literals," shows many examples of literals.
<H2><A NAME="Loop"><FONT SIZE=5 COLOR=#FF0000>
Loop</FONT></A></H2>
<P>
A loop is a series of statements that are executed more than oNCe.
Each loop has a control mechanism to stop looping. <A HREF="ch7.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch7.htm" >Chapter 7</A>
"Control Statements," discusses the different types
of looping and controls that are used.
<BLOCKQUOTE>
See also <I>Endless Loop</I>.
</BLOCKQUOTE>
<H2><A NAME="MetaCharacters"><FONT SIZE=5 COLOR=#FF0000>
Meta Characters</FONT></A></H2>
<P>
Meta characters are characters that have more than one meaning
inside regular expressions. <A HREF="ch10.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch10.htm" >Chapter 10</A>, "Regular Expressions,"
has an in-depth discussion of meta-characters.
<BLOCKQUOTE>
See also <I>Regular Expressions</I>.
</BLOCKQUOTE>
<H2><A NAME="Module"><FONT SIZE=5 COLOR=#FF0000>
Module</FONT></A></H2>
<P>
A module is a file that holds a related group of fuNCtions-such
as a library. However, modules are a bit more complex. Modules
can control which fuNCtion and variable names get exported from
the module namespace into the main namespace. See <A HREF="ch15.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch15.htm" >Chapter 15</A>,
"Perl Modules," for more information.
<H2><A NAME="Namespace"><FONT SIZE=5 COLOR=#FF0000>
Namespace</FONT></A></H2>
<P>
Namespaces are used to segregate fuNCtion and variable names.
Each data type has its own namespace. This means that you can
use the same variable name for different data types. For example,
<TT>$foo</TT>, <TT>@foo</TT>,
and <TT>%foo</TT> are different data
types with the same name. You can create your own namespace with
the <TT>Package</TT> keyword. See
<A HREF="ch14.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch14.htm" >Chapter 14</A>, "What Are Objects?" for more information.
<H2><A NAME="Object"><FONT SIZE=5 COLOR=#FF0000>
Object</FONT></A></H2>
<BLOCKQUOTE>
See <I>Class</I>.
</BLOCKQUOTE>
<H2><A NAME="Octal"><FONT SIZE=5 COLOR=#FF0000>
Octal</FONT></A></H2>
<P>
Octal refers to numbers using base 8.
<H2><A NAME="Operator"><FONT SIZE=5 COLOR=#FF0000>
Operator</FONT></A></H2>
<P>
The operators in a computer language tell the computer what actions
to perform. For example, the plus sign (+) is an operator.
<H2><A NAME="Parameter"><FONT SIZE=5 COLOR=#FF0000>
Parameter</FONT></A></H2>
<P>
Some fuNCtions need outside information before they can perform
their tasks. The outside information is called a parameter. For
example, the <TT>print()</TT> fuNCtion
needs to know what it should print and where.
<H2><A NAME="Polymorphism"><FONT SIZE=5 COLOR=#FF0000>
Polymorphism</FONT></A></H2>
<P>
Polymorphism is a term from the object-oriented world. It means
that a child class can redefine a method already defined in the
parent class. <A HREF="ch14.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch14.htm" >Chapter 14</A>, "What Are Objects?" discusses
polymorphism.
<H2><A NAME="Port"><FONT SIZE=5 COLOR=#FF0000>
Port</FONT></A></H2>
<P>
A port is the address of a socket on an Internet server. In addition
to the server address, each socket also needs a port number. The
port number is added to the end of the server address to create
a full address. For example, www.locked.com:80 is a full Internet
address that specifies a port number of 80.
<H2><A NAME="PrecedeNCe"><FONT SIZE=5 COLOR=#FF0000>
PrecedeNCe</FONT></A></H2>
<P>
Every Perl operator and fuNCtion has an associated priority. This
priority or precedeNCe level tells Perl which operators should
be evaluated first. <A HREF="ch4.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch4.htm" >Chapter 4</A> "Operators," lists all
of the operators and their priorities.
<H2><A NAME="Procedure"><FONT SIZE=5 COLOR=#FF0000>
Procedure</FONT></A></H2>
<P>
FuNCtions, procedures, routines, and subroutines are all basically
the same thing-a set of statements that are grouped together for
a common cause. If you like to be picky, fuNCtions are routines
that return values while subroutines don't return values. Procedure
is the generic name used to refer to both fuNCtions and subroutines.
<H2><A NAME="Protocol"><FONT SIZE=5 COLOR=#FF0000>
Protocol</FONT></A></H2>
<P>
A protocol is a set of agreed-upon commands and responses. The
Internet has a plethora of protocols that you can use. See Chapter
22, "Internet Resources," for information about how
to find more information.
<H2><A NAME="Range"><FONT SIZE=5 COLOR=#FF0000>
Range</FONT></A></H2>
<BLOCKQUOTE>
See <I>Array Range</I>.
</BLOCKQUOTE>
<H2><A NAME="Record"><FONT SIZE=5 COLOR=#FF0000>
Record</FONT></A></H2>
<BLOCKQUOTE>
See <I>Database</I>.
</BLOCKQUOTE>
<H2><A NAME="RefereNCe"><FONT SIZE=5 COLOR=#FF0000>
RefereNCe</FONT></A></H2>
<P>
A refereNCe is a scalar value that points to a memory location
that holds some type of data. See <A HREF="ch8.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch8.htm" >Chapter 8</A> "RefereNCes,"
for more information.
<H2><A NAME="RegularExpression"><FONT SIZE=5 COLOR=#FF0000>
Regular Expression</FONT></A></H2>
<P>
A Regular Expression is used to find patterns in strings. See
<A HREF="ch10.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch10.htm" >Chapter 10</A>, "Regular Expressions," for more information.
<H2><A NAME="ReturnValue"><FONT SIZE=5 COLOR=#FF0000>
Return Value</FONT></A></H2>
<P>
All Perl fuNCtions return a value when they are finished. The
return value is the value of the last executed statement or you
can use the <TT>return()</TT> to explicitly
state it. You may always choose to ignore the return value by
not assigning the fuNCtion call to a variable.
<H2><A NAME="RunTimeError"><FONT SIZE=5 COLOR=#FF0000>
Run-Time Error</FONT></A></H2>
<P>
Run-time errors happen while your program is executing. Run-time
errors are logic errors and therefore usually harder to track
down than compile-time errors.
<H2><A NAME="Scalar"><FONT SIZE=5 COLOR=#FF0000>
Scalar</FONT></A></H2>
<P>
A scalar variable can hold one string or number value at a time.
<A HREF="ch3.htm" tppabs="http://cheminf.nankai.edu.cn/~eb~/Perl%205%20By%20Example/ch3.htm" >Chapter 3</A> "Variables," shows you how scalars can be
used.
<H2><A NAME="ScalarContext"><FONT SIZE=5 COLOR=#FF0000>
Scalar Context</FONT></A></H2>
<BLOCKQUOTE>
See <I>Context (Array & Scalar)</I>.
</BLOCKQUOTE>
<H2><A NAME="Scope"><FONT SIZE=5 COLOR=#FF0000>
Scope</FONT></A></H2>
<P>
Normal Perl variables can be used by any fuNCtion and therefore
are said to have a global visibility or scope. You can create
variables that are local to a particular fuNCtion or block of
code with the <TT>my()</TT> fuNCtion.
These variables have a local scope.
<H2><A NAME="ShortCircuitOperators"><FONT SIZE=5 COLOR=#FF0000>
Short-Circuit Operators</FONT></A></H2>
<P>
The <TT>&&</TT> and <TT>||</TT>
operators are considered short-circuit operators because the second
operand might not be evaluated. For example, in the statement
<TT>0 && die();</TT> the <TT>die()</TT>
fuNCtion will not be executed. However, in the statement <TT>0
|| die();</TT> the <TT>die()</TT>
fuNCtion will be executed.
<H2><A NAME="Signals"><FONT SIZE=5 COLOR=#FF0000>
Signals</FONT></A></H2>
<P>
A signal is a message sent to your program by the operating system.
When a signal is received by your program, it interrupts the normal
flow of execution. If you don't have a signal handler fuNCtion
defined, default internal fuNCtions will be called. See Chapter
13, "Handling Errors and Signals," for more information.
<H2><A NAME="Slice"><FONT SIZE=5 COLOR=#FF0000>
Slice</FONT></A></H2>
<BLOCKQUOTE>
See <I>Array Slice</I>.
</BLOCKQUOTE>
<H2><A NAME="Socket"><FONT SIZE=5 COLOR=#FF0000>
Socket</FONT></A></H2>
<P>
A socket is the end link of a connection between two computers.
The first step to using any of the Internet protocols is to create
a connection to another computer using the socket fuNCtions. Then,
you can send and receive information over the sockets. See Chapter
18, "Using Internet Protocols," for more information.
<H2><A NAME="Splice"><FONT SIZE=5 COLOR=#FF0000>
Splice</FONT></A></H2>
<BLOCKQUOTE>
See <I>Array Splice</I>.
</BLOCKQUOTE>
<H2><A NAME="Stack"><FONT SIZE=5 COLOR=#FF0000>
Stack</FONT></A></H2>
<P>
A stack is a data structure that has the same properties as a
stack of potato chips in a Pringles can. Only the top chip is
accessible. And, therefore, two operations are possible: add a
chip or remove a chip. A stack works exactly the same way. You
can push a new item onto the stack or you can pop an item off
the stack.
<H2><A NAME="Statement"><FONT SIZE=5 COLOR=#FF0000>
Statement</FONT></A></H2>
<P>
A statement is an expression with a semicolon at the end. The
semicolon transforms an expression into an executable statement.
<H2><A NAME="STDERRSTDINandSTDOUT"><FONT SIZE=5 COLOR=#FF0000>
STDERR, STDIN, and STDOUT</FONT></A></H2>
<P>
<TT>STDERR</TT>, <TT>STDIN</TT>,
and <TT>STDOUT</TT> are predefined
filehandles that every program can use. You use <TT>STDERR</TT>
to display error messages, usually on the computer's monitor.
You use <TT>STDIN</TT> to get input,
usually from the keyboard. And you use <TT>STDOUT</TT>
to display messages, usually on the computer's monitor.
<H2><A NAME="Subroutine"><FONT SIZE=5 COLOR=#FF0000>
Subroutine</FONT></A></H2>
<BLOCKQUOTE>
See <I>Procedure</I>.
</BLOCKQUOTE>
<H2><A NAME="TextMode"><FONT SIZE=5 COLOR=#FF0000>
Text Mode</FONT></A></H2>
<P>
When using files, you can use either binary mode or text mode.
Binary mode means that Perl will not change your input or output
in any way. This is my preferred mode of operation, by the way.
Text mode-only available on some operating systems like Windows
95 and Windows NT-will convert newline/carriage return character
pairs into a single newline. It will also interpret any byte that
has a value of 26 as the end-of-file marker.
<H2><A NAME="UndefinedValue"><FONT SIZE=5 COLOR=#FF0000>
Undefined Value</FONT></A></H2>
<P>
The undefined value (<TT>undef</TT>)
can be returned by fuNCtions to indicate an error condition. It
is also the value returned when a nonexistent hash entry is accessed.
<H2><A NAME="Variable"><FONT SIZE=5 COLOR=#FF0000>
Variable</FONT></A></H2>
<P>
A variable is a changeable piece of information used in computer
programs. Typically, variables have a name and a data type. Perl
variables can be scalars, arrays, or hashes. Every variable has
a life-cycle. It gets created, used, and is then destroyed. Regular
Perl variables are created when they are initialized and destroyed
when the program ends. The <TT>my()</TT>
fuNCtion can create a variable that only exists inside a fuNCtion
or code block.
<H2><A NAME="Whitespace"><FONT SIZE=5 COLOR=#FF0000>
Whitespace</FONT></A></H2>
<P>
Whitespace is a term that refers to space, tab, and newline characters.
These characters create white space on a page when printed. You
can use the <TT>\s</TT> symbolic character
class in patterns to match whitespace characters.
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