Embedded-c Language-4

One 、 Array

1.1 Computer programs actually play with memory , To play memory, you have to allocate it first , At present, there is only one way to allocate memory ： Defining variables .

for example ：  int a;

             int b;

             int c;     ........

          Data types are consistent , And a large number of distribution , This allocation method will make the code extremely cumbersome .

ask ： Is there a way to solve such problems : It can ensure the consistency of data types , It can allocate a large amount of memory at one time ？

answer ： Yes ,C The method of allocating memory for language ： Array

1.2. Definition of array ： Is a method of allocating memory , The allocated memory can store multiple Data of the same type

1.3. Array of Advantages and disadvantages ：

       advantage ： Can allocate a large amount of memory

       shortcoming ： The data type must be the same , Because some occasions require different data types , for example ： fraction ( Floating point numbers ) And age ( Integers )

                 Must be allocated at one time , You can't use it at any time and distribute it at any time

1.4. Define the syntax format of the array

       Element data type   Array name [ length ( Also known as the number of array elements )] ={ Initialization list ( If there are more than one , Comma , Separate )};

       for example ：int   a[5] = {1,2,3,4,5};

       semantics ： Continuous distribution 5 Memory space of elements , And the data type of each element is int, Each element accounts for 4 Byte memory space , So finally, the continuous distribution 20 Bytes of memory , And the value of each element is ：1,2,3,4,5   

      similar ：int a = 1, b = 2, c = 3, d = 4, e = 5;   

1.5. Array characteristics

      1. The memory allocated by the array is continuous

      2. The array name is the first address of the entire array , It's equal to array number 0 The first address of an element

      3. The length of an array is also called the number of array elements , Instead of the size of the entire array allocated memory

          for example ：int a[5]; The length of the array = Number of array elements =5, The memory size allocated is 20 byte

      4. Array subscript is the number of elements in the array , from 0 Start

      5. The elements in the array are accessed through the operator "[]" And subscripts to access

          Format ： Array elements = Array name [ Subscript ]

          for example ：int   a[5] = {1,2,3,4,5};

          The first 0 Elements ：a[0] = 1

          The first 1 Elements ：a[1] = 2

          The first 2 Elements ：a[2] = 3

          The first 3 Elements ：a[3] = 4

          The first 4 Elements ：a[4] = 5

          Print page 3 The value of the elements ：printf("%d\n", a[3]);

          Will be the first 3 Change the value of the elements to 250：a[3] = 250;

      6. Pay attention to the cross-border access of the array ( The written examination questions are required )

          Accessed memory that should not be accessed

          for example ：int   a[5] = {1,2,3,4,5}; // Currently, the effective memory allocated by the operating system is 20 byte ,5 Elements

         printf("%d\n", a[4]); // Print page 4 The value of the elements , legal

         printf("%d\n", a[5]); // Printing does not exist 5 The value of the elements , Theoretically, it is beyond the limit , But the program didn't crash

           // It is not clear what value to print , It doesn't crash because it just looks at the memory information

         a[5] = 250; // A cross-border visit , Now the program crashes ,250 The number may have modified the core program of the operating system

1.6. Array definition form ：

form 1：int a[5] = {1,2,3,4,5};

form 2：int a[5] = {1,2,3}; a[0]=1,a[1]=2,a[2]=3,a[3]=a[4]=0, If there is no initialization, give it to 0

form 3：int a[5] = {0}; Is full of 0

form 4：int a[5] = {}; Is full of 0

form 5：int a[5]; Only defined but not initialized , The results are random numbers

form 6：int a[] = {1,2,3,4,5}; The number of elements is 5 individual

form 7：int a[5] = {1,2,3,4,5,6,7,8}; gcc Ignore invalid elements directly ,6,7,8 Don't

Be careful ： Wrong form ：int a[]; // Compile not pass , Report errors

1.7. Formula for finding the number of array elements ： Element number =sizeof( Array name )/sizeof( The first of an array of 0 Elements )

       for example ：int a[5] = {1,2,3,4,5};

       Array first address =a=&a[0]

       The amount of memory the array occupies =sizeof(a)=20

       The first 0 Memory size occupied by elements =sizeof(a[0])=4

       Element number =sizeof(a)/sizeof(a[0])=5 individual

1.8. Variable length array

    Definition ： The number of data elements is variable

    for example ： int a[5]; // Fixed length array , Fixed length ,gcc Continuous distribution in the future 20 Bytes of memory

   int len;

   int a[len]; // Variable length array , Uninitialized

   scanf("%d", &len);

1.9. Multidimensional arrays

a) Definition of 2D array ： An array consisting of multiple one-dimensional arrays , Each element of a two-dimensional array is an array , The essence of two-dimensional array is one-dimensional array , It's just a formal grouping of one-dimensional arrays

b) Define two-dimensional array syntax ： Array element type 2D array name [ Two dimensional array length ][ One dimensional array length ] = { Initialize the table };

    for example ：int a[5][10];

    semantics ：

   1.a The representative is a containing 5 Array of elements , Each of these elements is a containing 10 Array of elements , The data type of each element is int, therefore a Is the first address of the two-dimensional array

   2.a[0] representative a Of the 0 Elements , And the first 0 Elements are also a one-dimensional array , therefore a[0] It's a one-dimensional array , And is the first address of a one-dimensional array

   3.a[0][0] representative a Of the 0 The number of one-dimensional array elements 0 Elements ,  therefore a[0][0] Is an element in a two-dimensional array

   4.a[i][j] representative a Of the i The number of one-dimensional array elements j Elements

c) Define the form of a two-dimensional array ：

form 1：int a[3][4]; // Define only and do not initialize , random number

form 2：int a[3][4] = { {1,2,3,4},{4,3,2,1},{2,1,3,4}};

form 3：int a[3][4] = {1,2,3,4,5,6,7,8,9,10,11,12};

form 4：int a[3][4] = {0};

form 5：int a[3][4] = { {1,2},{0}};

form 6：int a[][4] = {1,2,3,4,5,6,7,8}; Equivalent to int a[2][4]

form 7:  int a[2][3] = { {1,2},{3,4},{5,6}}; // error , Transboundary

Be careful ： The length of two-dimensional array can be omitted , But the length of one-dimensional array cannot be omitted

for example ：

int a[3][4] = {
   {1,2,3,4},{5,6,7,8},{9,10,11,12}};

printf("%d\n", a[2][3]); //12

printf("%d\n", a[0][3]); //4

printf("%d\n", a[1][2]); //7

a[2][2] = 250; //11->250

d) Two dimensional array induction ：

    for example ：int a[2][3] = {0};

   a Is the first address of a two-dimensional array =&a[0][0]

   sizeof(a)： Get the memory size occupied by the entire two-dimensional array

   sizeof(a[0]): Get the second... In the two-dimensional array 0 Memory size occupied by one-dimensional array elements

   sizeof(a[0][0]): Get the second... In the two-dimensional array 0 The... In a one-dimensional array 0 Memory size occupied by elements

   sizeof(a)/sizeof(a[0]): Get the length of the two-dimensional array

   sizeof(a[0])/sizeof(a[0][0]): Get the length of the one-dimensional array

2.0.goto sentence ( Go back where you came from )

2.1. effect ： It can make CPU Jump to any statement to execute

       application ：linux The kernel operating system is heavily used goto sentence

2.2. grammar ：

      goto Tag name ; // The tag name specifies the place to jump

       Be careful ： The tag name is determined according to the identifier naming rules

2.3. Use form ： Two kinds of

       form 1：    

 Tag name ：
       sentence 
      ...
      goto  Tag name ;
      ....

      for example ：

label：
printf("1\n");
     goto label;
     printf("2\n");

     

      form 2：  

   goto   Tag name ;
     ....
      Tag name ：
      sentence ;
     ...

      for example ：     

goto label;
     printf("2\n");
label：
     printf("1\n");

2.4.linux Kernel source code goto Examples of classic use of statements ：

      Start a program , Allocate three blocks of memory , As long as one memory allocation fails , Program end , And return the memory release that was successfully allocated to linux operating system

3.0. Empty statement

a) grammar ： Just one ; sentence

    for example ：

   int a = 1;

   printf("a  = %d\n", a);// Empty statement , It plays the role of delay

b) Application scenarios ： Used to implement an empty loop

     for example ：

    for(;;); // Empty loop , Give Way CPU Stop running down here

    int i = 10000;

    for(; i >= 0; i--); // Empty cycle 10000 Time , Every time CPU It takes time , This code acts as a delay , But it's not accurate .

c) Error paradigm ：

    int i;
    for(i = 0; i < 5; i++) ;  // A valid loop becomes an empty loop 
       printf("i = %d\n", i);
    int i = 0;   
    while(i  < 2); // A valid loop becomes an empty loop 
       ....

    Bear in mind ： stay while/for The parentheses in the loop are misspelled ;, Unexpectedly, it becomes an empty loop