Subtotal of C language -- basic data types and their representations

c The basic data types of the language include integer data 、 Real data and character data .

【1】 Constants and variables

（1） Constant

A constant is an amount whose value cannot be changed while a program is running , It is c One of the basic data objects used in the . Constants are divided into numerical constants and character constants , Numerical constants are divided into integer constants and real constants , Character constants are divided into character constants and string constants .

These types determine the size of the space occupied by various constants and the representation range of numbers .

（2） Symbolic constant

stay c In language , Constants are expressed directly in the form of their own existence , It can also be represented by an identifier , Symbolic constants .

c Language uses macro definition commands to define symbolic constants , Its definition is as follows ：

#define identifier Constant

among #define Is a special definer for macro definition commands , An identifier is the name of a constant , A constant can be any kind of constant .

The functions of macro definitions are ： During compilation preprocessing , Replace all symbolic constants that appear after the macro definition command in the program with the corresponding constants in the macro definition command one by one . Therefore, symbolic constants are also called macro substitution names .

（3） Variable

A variable is an amount whose value can be changed during program operation . Variables here are two completely different concepts from variables in mathematics , stay c In language , Variable is the basic concept of data storage . We know , At the level of computer hardware , When the program is running, the data is stored by internal memory 、 Storage unit 、 Storage address and a series of related mechanisms , The reflection of these mechanisms in the programming language is the concept of variables .

A variable in a program can be regarded as a container for storing data . There are two basic operations on variables ：{1} Put data values into variables , This operation is called ： assignment ;{2} Get the current value of the variable to use , be called ： Value . Variables have the property of holding values , After assignment , You will get this value every time you use this variable .

Programs use variables by their names . stay c In language , The variable name is the identifier of the variable , Its naming rules comply with all provisions of the identifier .

c The basic variable types provided by the language are ： Numerical variable （ It is also divided into integer variables and real variables ）、 Character variables 、 Pointer to the variable .

       { Definition of variables }

c Language program requirements , Each variable must first be defined , That is, to declare its existence first , To be able to use it . Defining variables requires two aspects of information ： Variable name and its type , To determine its storage structure and make c Language compiler allocates storage space for defined variables .

【2】 Shaping data and its representation

（1） Integer types 、 Integer variables           Be careful ： Since the shaping data is represented by binary complement , So the range of negative numbers is larger

plastic                        2 byte             -32768~32767             (short\signed)int   

Long integer                    4 byte              -2147483648~2147483647  long

Unsigned integer             2 byte              0~65535              unsigned

Unsigned long shaping         4 byte               0~4294967295

(2) integer constants

Decimal system

octal ： In numbers 0 start     Such as ：056    It's equivalent to the decimal system 46      017 It's equivalent to the decimal system 15

Hexadecimal ： With 0x perhaps 0X start   hinder a,b,c,d,e,f It can also be capitalized .    Such as ：0x123 The decimal equivalent of 291

（3） Long integer constants

Add... After the constant l perhaps L that will do . for example ：40000 Is an illegal decimal integer constant , but 40000L Is a legal decimal long constant .

【3】 Real data and its representation