001 basic knowledge (unfinished)

Catalog

Basic data types

sizeof  Data type size

Variable

         Create variables

         classification

         Scope

         Life cycle

Constant

         Literal constants

        const Modified constant variable

        # define Defined identifier constant

         Enumeration constants

Characters and strings （ A character array ）

         character

        character string

         A character array

         String length

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Basic data types

         Data types are used to define variables

        char                Character data type

        short              Short

        int                  integer

        long                Long integer

        long long        Longer integers

        float              Single-precision floating-point

        double          Double precision floating point

sizeof  Data type size

        sizeof yes C A key word of language , Calculate the size of the data type , Unit is byte

            sizeof(char)                  1

            sizeof(short)                 2

            sizeof(int)                     4

            sizeof(long)                  4

            sizeof(long long)          8

            sizeof(float)                  4

            sizeof(double)              8

        A byte is 8 position , Each can represent 0 and 1 Two figures

Variable

          Used to describe data that can be changed

         Create variables

                   When creating variables, it's best to initialize

                        char ch = ‘a’;                          The variable type is char, The name is ch, The content is a character a

                        int age = 18;            

                        float pi = 3.1415926;     

         classification

         Variables are divided into global variables and local variables

                   Global variables         { } Curly braces are used to define  

                    local variable         { } Curly braces are used to define  

         When local variables and global names conflict , Local variables take precedence

         for example ：

#include <stdio.h>

int a = 100;

int main(void)

{

    int a = 10;

    printf(“%d”, a);

    return 0

}

The screen printing result of the program is 10

         Scope

         Local variable scope ： The local scope of the variable , That is, inside the current brace

         Global variable scope ： The whole project

         Life cycle

         The life cycle of a variable is the period between the creation of a variable and its destruction
        1. The life cycle of a local variable is ： Enter the scope lifecycle begins , Out of scope life cycle ends .
        2. The life cycle of global variables is ： The whole life cycle of the program .

Constant

         Describe immutable data

         Literal constants

                3.14
                10
                 'a'
                "abcde"

        const Modified constant variable

                const int num = 10;        //num It's a constant variable -- Has constant properties （ Properties that cannot be changed ）
                num = 20;        // This is the wrong statement , The statement is in the VS2022 Direct error reporting in ,num Do not modify the
                printf("%d", num);

                int arr[10] = { 0 };
                const int n = 10;        //n The essence is still a variable , It just has constant properties ,n The value of cannot be modified
                int arr2[n] = { 0 };        // This is the wrong statement ,n It's a constant variable

        # define Defined identifier constant

                # define M 200           //M It's a constant , The value is 200
                M = 100;                    // The statement is incorrect ,M It's a constant , You can't change it like this
                printf("M = %d\n", M);

         Enumeration constants

         Constants that can be enumerated one by one

            enum Date
            {
                DAY1 = 1,// Assign initial value to 1, The default is 0
                DAY2 = 3,// Assign initial value to 3
                DAY3,// There is no initial value , Its value is the above value +1, That is to say 4
                DAY4,
                DAY5
            };

            enum Date day1 = DAY1;
            //DAY1 = 1// The statement is incorrect ,DAY1 Is a constant
            printf("%d ", day1);//1
            printf("%d ", DAY2);//3
            printf("%d ", DAY3);//4
            printf("%d ", DAY4);//5
            printf("%d ", DAY5);//6

Characters and strings （ A character array ）

         character

                 The characters are enclosed in single quotes
                 'a'
                 'b'
                 'c'

        character string

                A string is a string of characters , In double quotation marks , The end of a string is a \0 The escape character of .

                "abcdef";
                "123456";

         A character array

                 The string hides a... At the end \0 The characters of
                char arr1[ ] = "hehe";
                char arr2[ ] = {'a', 'b', 'c'};          // error , Should be changed to char arr2[] = {'a', 'b', 'c', '\0'};

                printf("%s\n", arr1);         // Print hehe
                printf("%s\n", arr2);        // Print out garbled code , because arr2 No, \0

         String length

                 When calculating the length of a string \0 It's the end sign , It doesn't count as string content . 

                printf("%d\n", strlen("haha"));         // Print 4

                printf("%d\n", strlen(arr1)); 
                printf("%d\n", strlen(arr2));         // Print garbled code , I don't know where the string ends

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