[ C 文章收集 ] 2D arrays - dynamic allocation and freeing

來源自 這裡 
Preface: 
If the size is known at compile time; you don't need to allocate anything; let the compiler take care of it (no malloc(), no free()) 

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1.  int a[5][5];  
2. ...  
3. ...  
4.   a[i][j] = xxx;  
5. ...  
6. ...  

2D array allocation: 
Now, if you want a 2D array of int, and the dimensions are not known at compile time, here's a way to do it. Suppose we want an array[size_x][size_y] of int, wheresize_x and size_y are variables. The "array" name will be a "pointer to a pointer to an int". 

1. Allocate memory for size_x pointers to int. The address returned by malloc() will be the "array".
2. For each of the pointers, allocate memory for size_y ints.
Then address the individual elements with array[][] notation. When you don't need it any more:
3. Free the memory allocated for the ints.
4. Free the memory allocated for the pointers.

Example: 
Here's an example (I have given size_x and size_y specific values for purposes of illustration, but these could be obtained from user input or from other program calculations, or whatever.) 

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1. /* illustration of dynamically allocated 2D array */  
2. #include   
3. #include   
4.   
5. int main()  
6. {  
7.   int i; /* general purpose variable used for loop index */  
8.   int j; /* general purpose variable used for loop index */  
9.   
10.   int **a;     /* this is the array name */  
11.   int size_x; /* this variable will be used for the first  dimension */  
12.   int size_y; /* this variable will be used for the second dimension */  
13.   
14.   /* suppose we want an array of int: a[5][3] */  
15.   size_x = 5;  
16.   size_y = 3;  
17.   
18.   /*  allocate storage for an array of pointers */  
19.   a = malloc(size_x * sizeof(int *));  
20.   
21.   /* for each pointer, allocate storage for an array of ints */  
22.   for (i = 0; i < size_x; i++) {  
23.     a[i] = malloc(size_y * sizeof(int));  
24.   }  
25.   
26.   /* just for kicks, show the addresses (note: not all sequential) */  
27.   /* assign an arbitrary value to each element        */  
28.   for (i = 0; i < size_x; i++) {  
29.     for (j = 0; j < size_y; j++) {  
30.       printf("&a[%d][%d] = %p\n", i, j, &a[i][j]); /* show the addresses */  
31.       a[i][j] = i * size_y + j; /* just some unique number for each element */  
32.     }  
33.     printf ("\n");  
34.   }  
35.   
36.   /* now show the contents that were assigned */  
37.   for (i = 0; i < size_x; i++) {  
38.     for (j = 0; j < size_y; j++) {  
39.       printf("a[%d][%d] = %2d\n", i, j, a[i][j]);  
40.     }  
41.     printf ("\n");  
42.   }  
43.   
44.   /* now for each pointer, free its array of ints */  
45.   for (i = 0; i < size_y; i++) {  
46.     free(a[i]);  
47.   }  
48.   /* now free the array of pointers */  
49.   free(a);  
50.   
51.   return 0;  
52. }  

Remember: always free() everything that you got from malloc(), but never anything else. 
C++ Programmers: Do the same, but use new and delete instead of malloc() and free().