Soft test --- fundamentals of programming language (Part 1)

One 、 Low level language and high level language

         Low level language ：

        Machine language and assembly language are called low-level languages ;

        Machine language refers to 0、1 A sequence of machine instructions ;

        Assembly language refers to the language that uses symbols to represent instructions ;

         High-level language ：

        High level language is from the perspective of human logical thinking 、 Programming languages for various applications , It's a lot more abstract , It needs to be compiled into object code on a specific machine to execute . This kind of language is relatively easy to understand , It greatly improves the efficiency of program design ;

 

Two 、 Compilers and interpreters

2.1 Concept

        Programs written in high-level language or assembly language are called Source program ;

        Source program written in assembly language , We need a translator called assembler to translate it into the target program , And then we can execute ;

        Source program written in high-level language , Its translation process is called compile Program ;

        According to the execution order of the source program , A processing program that translates sentence by sentence and executes relevant functions immediately after translation , Call it explain Program ;

2.2 Procedure execution mode

         Compile implementation ：

        There are two stages when a computer executes a high-level language using compilation ：

        （1） Compile phase , Translate the source program into the target program ;

        （2） Operation phase , Execute the compiled object program ;

        advantage ： High execution efficiency 、 Small footprint ;

        shortcoming ： Poor compatibility ;

        

        Explain to perform ：

         Every statement of the source program is executed as soon as it is interpreted ;

        advantage ： Can perform well 、 Fast development speed 、 It is convenient to communicate with users ;

        shortcoming ： Low efficiency ;

3、 ... and 、 Basic principle of compiling system

（1） working process  

        It is mainly divided into 6 Stages ：

        a. Lexical analysis ;

        b. Syntax analysis ;

        c. Semantic analysis ;

        d. Intermediate code generation ;

        e. Code optimization ;

        f. Target code generation ;

（2） Lexical analysis

2.1 Concept

        Input source program , Scan the strings that make up the source program 、 decompose , Recognize every word , And delete useless information , Report errors in the analysis process ;

        The basic grammatical symbols of a programming language are divided into five categories ：

        keyword 、 identifier 、 Constant 、 Operator 、 Boundary symbol ;

        The word symbols output by the lexical analyzer are generally expressed as the following quadratic elements ：

                （ Word type , Attribute values of word symbols ）

        Describing lexical rules is often used ： Normal form 、 Finite automaton ;

        

2.2 State transition diagram

         The state transition diagram is Limited state Of Directed graph , Use a circle to indicate the node state , Between nodes, there are directed edges to represent the state transition , Markable characters on the facing edge , Indicates the state transition of the previous state after accepting a character ;

         Function of state transition diagram ：

        Used to identify a certain string ;

        Requirements of state transition diagram ：

        state （ That is node ） Limited number ;

        At least one initial state , Several termination States ;

        Each side is marked with characters （ It can also be an empty character ）;

        The representation habit of status identifier ：

        The initial state is " ○ " Express ;

        For non terminating status " ○ " Express ;

        The jump between States is used "" （ There is a directional side ） Express ;

        For termination status " ◎ " Express ;

        Read one more character with " * " Express ;

2.3 Normal expression  

         Normal expressions and normal sets （ Definition and operation ）

         State transition diagrams can be used to construct lexical analysis programs , But it belongs to informal description ;

         Normal expression （ Abbreviation: normal form ） It is a formal representation of lexical analysis ;

        The formal method refers to the method of describing the problem with a set of strictly prescribed symbol system ;

        advantage ： Clearer and more accurate ;

        

        Normal expression concept ：

        α ∈ Σ , be α yes Σ A normal form on , The normal set it represents is { α } ;

        An example of a regular to regular rule set ：

         α Normal form , The normal set of its transformation is L(α);

         β Normal form , The normal set of its transformation is L(β);

        ( α | β ) Normal form , The normal set of its transformation is L(α) ∪ L(β);

        ( α • β ) Normal form , The normal set of its transformation is L(α)L(β);

        ( α )* Normal form , The normal set of its transformation is L(α)*;

        

        among , " | " Read as " or "、" • " Read as " Connect "、" * " Read as " Closure "（ Any number of self repeating connections ）;