Basic knowledge of C language

1. Constant
2. Variable
3. Type conversion

1. Constant

During program operation , The amount whose value will not be changed

1.1 character constants

‘a’  ‘b’  ‘c’

1.2 integer constants

Binary system ：0110

octal ：066

Decimal system ：24

Hexadecimal ：0x33

Be careful ： By default , The integer constant is A signed Of int type （long、longlong、short）

Unsigned int type ：66U

Long integer ：66L

Long integer without sign ：66UL

1.3 Floating point constants

Decimal form ：1.23   0.00001         100000（ Floating point constants contain integer constants ）

Exponential form ：0.0001 （1e-4）   100000 （1e+5）

1.4 String constant

“hello” Several bytes

Be careful ： String is based on ’\0’ As an end sign

1.5 Identity constant （ macro ）

#define Macro name expression

Be careful ：（1） Macro names are usually capitalized , Lowercase is fine , Because in order to distinguish from variables , So use capital

        （2） There is no semicolon after the macro

Macros are simply replacements

Macro functions （ It has the characteristics of both functions and macros ）

#define  Function name ( Shape parameter ) The body of the function

Be careful ： Macros are simply symbolic substitutions , The priority of operators will not be considered , So we need to give each shape （）, The entire expression is also bracketed

2、 Variable

Definition ： It means opening up space in memory

c Memory allocation diagram of language ：

How to define a variable ？

Storage type   data type   Variable name ;

Storage type ： Determines which partition of the opened space is in the memory

data type ： Determines the size of memory space to be opened

Variable name ： The name of the opened memory space

2.1 local variable

Defined in function body （ Any function body ） Internal variables

2.2 Global variables

Variables defined outside the function

2.3 Storage type

Storage type ：auto、extern、static、register

auto： Decorated variables are stored in the stack , Only local variables can be modified

extern： Decorated variables are stored in the static area （.bss and .data Collectively referred to as static areas ）, Only global variables can be modified

static： Decorated variables are stored in the static area , Both local and global variables can be modified

register： Decorated variables are stored in registers , Only local variables can be modified

summary ：

1. except static and register Modified local variables , There are stack areas for others
2. Static area exists in global variables
3. Static variables have static areas

2.4 initialization

initialization ： Assign values to variables when defining variables

1. int a = 10; // initialization
2. int a;

a = 10; // assignment

summary ：（1） The global variable is not initialized, and its value defaults to 0

     （2） Local variables are not initialized, and their values default to random values

auto： Modify local variables , Store in stack area

register： Modify local variables , Stored in a register , It is recommended to store variables in registers , Can improve the running speed of the program , Whether it is finally stored in the register , Depends on the compiler , If there are no registers , There is a stack area

extern： Modify global variable , Store in static area

The program can consist of multiple .C The composition of the document , however A program can only have one main function

effect ： Tell compiler , This global variable has been defined in other files

static： Decorated variables are stored in the static area , You can modify both local variables and global variables

1. static Modify local variables , Prolongs the life cycle of local variables , If the local variable is not initialized , Its value is 0, If initialization is only assigned once
2. static Modify global variable , Only in this document （ Limits the scope of global variables ）

2.5 Lifecycle and scope

Life cycle

From when to open up space to when to release space

Scope

Scope of use

local variable ：

              Life cycle ： Start with the definition , To module （ Curly braces ） end

              Scope ： In braces

static Modified local variables ：

              Life cycle ： Start with the definition , By the end of the program

              Scope ： In braces

Global variables ：

              Life cycle ： Start with the definition , By the end of the program

              Scope ： Whole procedure

static Decorated global variables ：

              Life cycle ： Start with the definition , By the end of the program

              Scope ： In this document

3、 Data type conversion

3.1 Cast （ We turn by ourselves ）

3.2 Implicit type conversion （ The compiler automatically transfers ）

Horizontal arrow ： Whether we have mixed operations or not , Are bound to be converted

Be careful ：float When using, follow double To use the

         Empathy ：char、short Is in accordance with the int To use the

Vertical arrow ： Only in the case of mixed operations , To switch