2D array

Decay就是當變數要傳給function的時候
像是a[N]  傳進去function只要傳他的第一個位置
int (*)[N]的意思是 有一個poionter指向有[N]個的int
int* (*)[N]有一個pointer指向有[N]個int *

char g[8]="456";
char (*ptr2)[8]="234"; -->(*ptr2)是一個指標指向一個長度為8的char (這邊指派一個唯讀的"234"給他)

ptr2=&g; -->因為ptr2是指標 所以可以

但是如果一開始只有宣告char (*ptr2)[8];
是不能做
(*ptr2)[0]='G';
必須要是
char t='G';
(*ptr2)[0]=t;  -->要給一個實體記憶體位置




So the following all hold true:

Declaration        Expression        Type             Decays to
-----------        ----------        ----             ---------
     T a[N]                 a        T [N]            T *
                           &a        T (*)[N]     
                           *a        T
                         a[i]        T

  T a[M][N]                 a        T [M][N]         T (*)[N]
                           &a        T (*)[M][N]  
                           *a        T [N]            T *
                         a[i]        T [N]            T *
                        &a[i]        T (*)[N]      
                        *a[i]        T
                      a[i][j]        T

T a[M][N][O]                a        T [M][N][O]      T (*)[M][N]
                           &a        T (*)[M][N][O]
                           *a        T [M][N]         T (*)[N]
                         a[i]        T [M][N]         T (*)[N]
                        &a[i]        T (*)[M][N]  
                        *a[i]        T [N]            T *
                      a[i][j]        T [N]            T *
                     &a[i][j]        T (*)[N]
                     *a[i][j]        T
                   a[i][j][k]        T

The pattern should be clear for higher-dimensional arrays.
So let's analyze your dictionary:

/* list of words and meanings */         

char  *dic[][40] = {         
    "atlas", "A volume of maps.",         
    "car", "A motorized vehicle.",         
    "telephone", "A communication device.",         
    "airplane", "A flying machine.",         
    "", ""  /* null terminate the list */         
};

This isn't going to set up your dictionary the way you want; you've basically set this up as a 1-element array of 40 pointers to char. If you want an array of pairs of strings, then the declaration should look like this:

char *dic[][2] = 
{
  {"atlas", "A volume of maps"},
  {"car", "A motorized vehicle"},
  {"telephone", "A communication device"},
  {"airplane" , "A flying machine"},
  {NULL, NULL} // empty strings and NULLs are different things.  
}; 

The type of dic is "5-element array of 2-element arrays of pointer to char", or char *[5][2]. Going by the rules above, the expression dic should decay to char *(*)[2] -- a pointer to a 2-element array of pointer to char.
A function to search this dictionary would then look like this:

char *definition(char *term, char *(*dictionary)[2]) // *NOT* char ***dictionary
{
  while ((*dictionary)[0] != NULL && strcmp((*dictionary)[0], term) != 0)
    dictionary++;
  return (*dictionary)[1];
}

and you would call it from your main function like

char *def = definition(term, dic);

Note that we have to use parentheses around the *dictionary expression in the function. The array subscript operator [] has higher precedence than the dereference operator *, and we don't want to subscript into dictionary directly, we want to subscript into the array that dictionary points to.






------------------------
How to create an array size 5 containing pointers to characters:

char *array_of_pointers[ 5 ];        //array size 5 containing pointers to char
char m = 'm';                        //character value holding the value 'm'
array_of_pointers[0] = &m;           //assign m ptr into the array position 0.
printf("%c", *array_of_pointers[0]); //get the value of the pointer to m

How to create a pointer to an array of characters:

char (*pointer_to_array)[ 5 ];        //A pointer to an array containing 5 chars
char m = 'm';                         //character value holding the value 'm'
*pointer_to_array[0] = m;             //dereference array and put m in position 0
printf("%c", (*pointer_to_array)[0]); //dereference array and get position 0

How to create an 2D array containing pointers to characters:

char *array_of_pointers[5][2];          
//An array size 5 containing arrays size 2 containing pointers to char

char m = 'm';                           
//character value holding the value 'm'

array_of_pointers[4][1] = &m;           
//Get position 4 of array, then get position 1, then put m ptr in there.

printf("%c", *array_of_pointers[4][1]); 
//Get position 4 of array, then get position 1 and dereference it.

How to create a pointer to an 2D array of characters:

char (*pointer_to_array)[5][2];
//A pointer to an array size 5 each containing arrays size 2 which hold chars

char m = 'm';                            
//character value holding the value 'm'

(*pointer_to_array)[4][1] = m;           
//dereference array, Get position 4, get position 1, put m there.

printf("%c", (*pointer_to_array)[4][1]); 
//dereference array, Get position 4, get position 1

---------------------

Here's an example if char a[N][3]:



+===========================+===========================+====
|+--------+--------+-------+|+--------+--------+-------+|
|| a[0,0] | a[0,1] | a[0,2]||| a[1,0] | a[1,1] | a[1,2]|| ...
|+--------+--------+-------+++--------+--------+-------++ ...
|            a[0]           |            a[1]           |
+===========================+===========================+====
                                    a
^^^
||+-- &a[0,0]
|+-----&a[0]
+-------&a

&a: address of the entire array of arrays of chars, which is a char[N][3]
&a[0], same as a: address of the first element, which is itself a char[3]
&a[0][0]: address of the first element of the first element, which is a char

-----------------

http://stackoverflow.com/questions/6599707/2d-character-array-initialization-in-c

http://stackoverflow.com/questions/3920729/in-c-c-is-char-arrayname-a-pointer-to-a-pointer-to-a-pointer-or-a-pointe

http://stackoverflow.com/questions/12674094/array-to-pointer-decay-and-passing-multidimensional-arrays-to-functions?answertab=oldest#tab-top