whatis——于查询一个命令执行什么功能
示例1：
# whatis ls
显示ls命令的功能，和执行man命令时NAME信息差不多
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whatis命令
2016-06-13 11:47:00whatis——于查询一个命令执行什么功能 示例1： # whatis ls 显示ls命令的功能，和执行man命令时NAME信息差不多 转载于:https://www.cnblogs.com/zuhua/p/linux-whatis.html...展开全文转载于:https://www.cnblogs.com/zuhua/p/linux-whatis.html
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我怎么知道我服务器上数据库的主机名是什么(How can i know what is the host name of my database on my ...
2021-02-12 03:10:53It looks like you're trying to connect to your database from a different ... Is your PHP script still running on your own PC?The message you get is because you try to reach the database from the c...展开全文It looks like you're trying to connect to your database from a different server. Is your PHP script still running on your own PC?
The message you get is because you try to reach the database from the computer at address 31.170.163.50, which is not allowed. Quite often, when you have 'normal' simple, shared hosting, you are not allowed to use the database from other computers directly. Only the scripts on the server itself are allowed to use the database.
So, the solution would be: Put everything, scripts and database, on your web hosting platform, and use localhost or 127.0.0.1 when your PHP script needs to connect to your database.
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JAVA中 什么是方法签名？what is method signature in java
2017-08-03 17:08:24如题：what is method signature in java？ answer：Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types. 方法声明的两个组件...展开全文如题：what is method signature in java？
answer：Definition: Two of the components of a method declaration comprise the method signature—the method's name and the parameter types.
方法声明的两个组件构成了方法签名 - 方法的名称和参数类型。
举例：
Here is an example of a typical method declaration:(这里是一个典型的方法声明)
public double calculateAnswer(double wingSpan, int numberOfEngines, double length, double grossTons) { //do the calculation here }The signature of the method declared above is:（上面方法的签名是）
calculateAnswer(double, int, double, double)不要再被方法签名迷惑了，之前读Effective Java的时候对这个概念有些疑惑。正好在看oracle的java 练习文档中有明确的定义。
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What is a Parse Tree? 什么是解析树
2010-10-26 16:07:11What is a Parse Tree? Author's name: Carel-Jan Engel Author's Email: cjpengel.dbalert@xs4all.nl ...展开全文What is a Parse Tree?
Author's name: Carel-Jan Engel
Author's Email: cjpengel.dbalert@xs4all.nl
Date written: Mar 24, 2005
Oracle version(s): N/A
In documentation about tuning SQL, I see references to parse trees. What is a parse tree ?
A parse-tree is an internal structure, created by the compiler or interpreter while parsing some language construction. Parsing is also known as 'syntax analysis'.
An example (slightly adapted version of the example found at page 6 of the famous 'Dragon Book', Compilers: principles, techniques and tools, by Alfred V. Aho, Ravi Sethi and Jeffrey D. Ullman, Published by Addison Wesley. My copy is from 1986) will illustrate a parse tree. Rather than dealing with the complexities of a SQL statement, let's take a rather simple language construction: The assignment of the result of an expression to a variable:
result := salary + bonus * 1.10
When the compiler analyzes this statement the resulting parse-tree will look like this :
assignment
________ statement ____
/ | \
/ := \
identifier ___ expression _______
| / | \
result / + \
expression __ expression ___
| / | \
identifier / * \
| expression expression
salary | |
identifier number
| |
bonus 1.10
The picture is an upside-down representation of a tree. The language elements in this small simple assignment are: identifiers (result, salary, bonus), operators (:=, +, *), and a number (1.10). 'Identifier' is the language element that names a variable, function or procedure. 'Operator' is the language element that represents some action to be taken, upon the operands at either end of the operator. Number is a constant, 1.10 in this statement. The syntax rules (' grammar') will specify which 'sentences' are valid.
After successfully decomposing the statement into its internal representation, the compiler or interpreter can 'walk the tree' to create the executable code for the construction. An interpreter will not generate code for the execution, but will invoke built-in executing functions by itself. Let's take the interpreter for the rest of the explanation, execution of the steps is easier to explain than the code-generation of a compiler. For the example I assume the bonus to be 100, and the salary to be 1000. The tree-walk will start at the root of the tree, the assignment statement. The rule for the assignment will tell the interpreter that the right hand has to be evaluated first. This evaluation is also known as 'reduction'. The right hand side of the assignment needs to be reduced to a value, the result of the expression, before it can be assigned to the variable at the left hand side of the statement.
The first node at the right-hand side of the statement contains an expression with a '+' operator. The right hand side of the '+' operator needs to be assigned to the left hand side. So the walk goes on to the next node at the right hand side. There the interpreter will detect the expression with the '*' operator. The left hand side of this operator needs to be multiplied with the right hand side. The interpreter goes on to the right hand side, and detects an expression that consists of a single number: 1.10. This side is fully reduced, the result can be stored and the interpreter walks the tree back up to the '*' operator, and starts evaluating its left hand side. This is an expression that consists of one single identifier, representing a variable, 'bonus'., The memory location represented by this variable is read and it's contents (100) will be multiplied by the right hand side result, 1.10. This expression has been fully reduced to the result 110 now. The interpreter walks up, to the '+' operator, and starts evaluating its left hand side. There it will again detect an identifier, 'salary'. Its location is read (1000) and the expression is reduced to a number, 1000. The right and left hand side will be added, resulting in 1,110. Now the expression at the right hand side of the assignment is fully reduced, and the interpreter walks up the tree, finds the assignment operator ':='. This instructs the interpreter to copy the result of the expression to the left hand side. The left hand side contains an identifier, 'result'. The memory location represented by 'result' will be filled with the result of the expression, 1,110.
It is just a simplified explanation of how an interpreter or compiler uses a parse tree. It's out of scope of this answer to create a complete introduction to compiler building practices. However, it might be clear that creating a parse-tree consumes some resources. Before the language elements can be recognized they must be read character by character, type checking and possible conversion needs to be done, identifiers (tables, columns etc.) need to be identified and checked in the data dictionary, and so on. After this 'hard parse' the parse tree is composed, and is a far cheaper form. to use to execute a statement than doing all this analysis over and over again. Therefore, storing the parse-tree in the SQL-area for future use can save quite some time during the processing of SQL-statements that have come across before.
http://www.jlcomp.demon.co.uk/faq/parse-tree.html
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/35489/viewspace-676754/，如需转载，请注明出处，否则将追究法律责任。
转载于:http://blog.itpub.net/35489/viewspace-676754/
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