指向指向c中結構的指針

Question

我有一個簡單的結構。

struct grades 
{ 
int lowest; 
int highest; 
}; 

然後，我要創建一個函數，該函數返回一個初始化結構的指針。

struct *grades setGrades(int low, int high) 
{ 
    struct *ptr = malloc(sizeof(struct grades)); 
    ptr->lowest = low; 
    ptr->highest = high; 
    return ptr; 
} 

現在我應該使一個函數定義與struct **grades random(int size); 我應該爲指針數組以grade結構具有等於size元件的數量分配空間。當創建指針數組時，我想將數組中的每個指針設置爲一個新創建的數據結構，該結構的低變量等於10，高變量等於100，然後返回指針。

在這一點上，我真的迷失了，因爲我在網上查找結構的雙指針，但沒有找到任何可以幫助我清楚理解的例子。我在想，如果有人能夠向我解釋結構的雙重指針是如何工作的，它會給我一個正確方向上的好開始。

Answer 1

你稱之爲「雙指針」只是一個指向指針的指針。就是說，struct grade example是一個包含lowest和highest的變量，如您所定義的那樣。一個指針，struct grade *example是一個變量，用於存儲相同結構的內存地址。指向指針的指針struct grade **example是一個變量，用於存儲存儲結構內存地址的變量的內存地址。更詳細的解釋可以參見here。不管怎麼說，回答您的具體問題，一個函數是：

struct grades** random(int size) { 
    struct grades** result = malloc(sizeof(struct grades*) * size); //here you are 
                //allocating enough space for an 
                //array of pointers 
    int i; 
    for(i = 0; i < size; i++) { 
     result[i] = setGrades(10, 100); //here you are setting each pointer to one 
             //grade through the function you've already 
             //defined 
    } 

    return result; 
} 

Answer 2

struct grades { int lowest; int highest; }; 

struct grades * createGradeSet(int low, int high) //careful: return type is struct grades * 
{ 
    // variable name: ptr, type: struct grades * 
    struct grades * ptr = malloc(sizeof(struct grades)); 
    ptr->lowest = low; 
    ptr->highest = high; 
    return ptr; 
} 

struct grades ** random(int size) 
{ 
    // Create a pointer to an array of struct grades pointers 
    // the size of the array is `size` x the size of a struct grades pointer 
    struct grades ** ptr_arr = malloc(sizeof(struct grades *) * size); 
    for (unsigned int i = 0; i < size; i++) 
    ptr_arr[i] = createGradeSet(10, 100); // assign a newly created Gradeset to every cell 
    return ptr_arr; 
} 

Answer 3

請嘗試以下

struct grades ** random(size_t size) 
{ 
    if (size == 0) return NULL: 

    struct grades **p = malloc(size * sizeof(struct grades *)); 

    if (p != NULL) 
    { 
     size_t i = 0; 
     do 
     { 
      p[i] = malloc(sizeof(struct grades)); 
      if (p[i] != NULL) 
      { 
       p[i]->lowest = 10; 
       p[i]->highest = 100; 
      } 
     } while (p[i] != NULL && ++i < size); 

     if (i != size) 
     { 
      for (size_t j = 0; j < i; j++) free(p[i]); 
     } 
     free(p); 
     p = NULL;  
    } 

    return p; 
}  

功能setGrades應該寫成

struct *grades setGrades(int low, int high) 
{ 
    struct *p = malloc(sizeof(struct grades)); 

    if (p != NULL) 
    { 
     p->lowest = low; 
     p->highest = high; 
    } 

    return p; 
} 

在這種情況下，while循環了嗎在上面的函數中可以寫成

 do 
     { 
      p[i] = setGrades(10, 100); 
     } while (p[i] != NULL && ++i < size);