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Basic framework:

#include<stdio.h>    //编译预处理指令
 
int main()           //程序的入口主函数main
{                    //程序（函数、功能）结束标志
  
  return 0;          //程序退出前返回给调用者（操作系统）的值
}                    //程序（函数、功能）结束标志

variable:

The emphasis is on change. Its value will change during the program running.

Four characteristics: variable name, variable value, storage unit, variable type; define first and then use.

(1) Variable name/identifier:

It consists of letters, numbers and underscores, and can only start with an underscore or a letter, not a number. It is case sensitive.

Naming method: As the name implies, use English naming. Camel case naming, int secondPreYear. Function names are often connected by underscores.

(2) Variable type:

To keep it simple, in operating systems (Windows, Linux), integers are integers (4 bytes = 32 bits), floating-point types are decimals (float: 4 bytes = 32 bits), and characters are ASCII codes (char: 1 byte = 8 bits) (which can be represented by integers)

Note: 32 bits is 2 to the power of 32

In a single-chip microcomputer, an integer is 2 bytes = 16 bits, which means that the maximum number that can be represented is 2 to the 16th power, or 65536.

Operators:

The operators in C language are very rich and can be roughly divided into the following categories:

1. Arithmetic operators: Used to perform basic mathematical operations.

   • +addition
   • -Subtraction
   • *multiplication
   • /division
   • %Find the remainder (modulus)
   • ++Auto-increment
   • --Decrement

2. Relational operators: Used to compare two values.

   • ==equal
   • !=not equal to
   • >more than the
   • <Less than
   • >=greater or equal to
   • <=Less than or equal to

3. Logical Operators: Used to perform logical operations.

   • &&Logical AND
   • ||Logical OR
   • !Logical NOT

4. Bitwise Operators: Used to operate on the binary bits of integers.

   • &Bitwise AND
   • |Bitwise OR
   • ^Bitwise XOR
   • ~Bitwise inversion
   • <<Shift Left
   • >>Shift Right

5. Assignment Operators: Used for assignment.

   • =Assignment
   • +=Accumulation Assignment
   • -=Cumulative decrement assignment
   • *=Accumulated multiplication assignment
   • /=Cumulative division assignment
   • %=Remainder Assignment
   • <<=Left shift assignment
   • >>=Right shift assignment
   • &=Bitwise AND assignment
   • |=Bitwise OR assignment
   • ^=Bitwise XOR assignment

6. Conditional Operators(also known as the ternary operator):

   • ? :Conditional operators, such asa > b ? a : bIfa > bIf established, the result isa, otherwiseb。

7. Comma operator：

   • ,The comma operator is used to execute multiple expressions in sequence, but the result of the entire expression is the result of the last expression.

8. sizeof Operator：

   • sizeofOperators are used to get the number of bytes that a data type or variable occupies in memory.

9. Type conversion operators：

   • (type)The type conversion operator is used to convert the value of an expression to a specified type.

10. Subscript OperatorandStructure/union member access operators：

    • []Subscript operator is used to index into an array and to access the offset of an element pointed to by a pointer.
    • .Member access operator for a structure or union.
    • ->Access members of a structure or union through a pointer.

11. Pointer Operators：

    • &Address-of operator.
    • *Pointer dereference operator (also the multiplication operator, but context determines its meaning).

These operators play a very important role in C language and are the basis for building various complex expressions and statements.