Thread: Pointer to 2d Array

Code:

#include <stdio.h>

int main()
{

	int dat[20][20] = {
		{8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5 ,7 ,78 ,52 ,12 ,50 ,77 ,91 ,8},
		{49 ,49 ,99 ,40 ,17 ,81 ,18 ,57 ,60 ,87 ,17 ,40 ,98 ,43 ,69 ,48 ,4 ,56 ,62 ,0},
		{81 ,49 ,31 ,73 ,55 ,79 ,14 ,29 ,93 ,71 ,40 ,67 ,53 ,88 ,30 ,3 ,49 ,13 ,36 ,65},
		{52 ,70 ,95 ,23 ,4 ,60 ,11 ,42 ,69 ,24 ,68 ,56 ,1 ,32 ,56 ,71 ,37 ,2 ,36 ,91},
		{22 ,31 ,16 ,71 ,51 ,67 ,63 ,89 ,41 ,92 ,36 ,54 ,22 ,40 ,40 ,28 ,66 ,33 ,13 ,80},
		{24 ,47 ,32 ,60 ,99 ,3 ,45 ,2 ,44 ,75 ,33 ,53 ,78 ,36 ,84 ,20 ,35 ,17 ,12 ,50},
		{32 ,98 ,81 ,28 ,64 ,23 ,67 ,10 ,26 ,38 ,40 ,67 ,59 ,54 ,70 ,66 ,18 ,38 ,64 ,70},
		{67 ,26 ,20 ,68 ,2 ,62 ,12 ,20 ,95 ,63 ,94 ,39 ,63 ,8 ,40 ,91 ,66 ,49 ,94 ,21},
		{24 ,55 ,58 ,5 ,66 ,73 ,99 ,26 ,97 ,17 ,78 ,78 ,96 ,83 ,14 ,88 ,34 ,89 ,63 ,72},
		{21 ,36 ,23 ,9 ,75 ,0 ,76 ,44 ,20 ,45 ,35 ,14 ,0 ,61 ,33 ,97 ,34 ,31 ,33 ,95},
		{78 ,17 ,53 ,28 ,22 ,75 ,31 ,67 ,15 ,94 ,3 ,80 ,4 ,62 ,16 ,14 ,9 ,53 ,56 ,92},
		{16 ,39 ,5 ,42 ,96 ,35 ,31 ,47 ,55 ,58 ,88 ,24 ,0 ,17 ,54 ,24 ,36 ,29 ,85 ,57},
		{86 ,56 ,0 ,48 ,35 ,71 ,89 ,7 ,5 ,44 ,44 ,37 ,44 ,60 ,21 ,58 ,51 ,54 ,17 ,58},
		{19 ,80 ,81 ,68 ,5 ,94 ,47 ,69 ,28 ,73 ,92 ,13 ,86 ,52 ,17 ,77 ,4 ,89 ,55 ,40},
		{4 ,52 ,8 ,83 ,97 ,35 ,99 ,16 ,7 ,97 ,57 ,32 ,16 ,26 ,26 ,79 ,33 ,27 ,98 ,66},
		{88 ,36 ,68 ,87 ,57 ,62 ,20 ,72 ,3 ,46 ,33 ,67 ,46 ,55 ,12 ,32 ,63 ,93 ,53 ,69},
		{4 ,42 ,16 ,73 ,38 ,25 ,39 ,11 ,24 ,94 ,72 ,18 ,8 ,46 ,29 ,32 ,40 ,62 ,76 ,36},
		{20 ,69 ,36 ,41 ,72 ,30 ,23 ,88 ,34 ,62 ,99 ,69 ,82 ,67 ,59 ,85 ,74 ,4 ,36 ,16},
		{20 ,73 ,35 ,29 ,78 ,31 ,90 ,1 ,74 ,31 ,49 ,71 ,48 ,86 ,81 ,16 ,23 ,57 ,5 ,54},
		{1 ,70 ,54 ,71 ,83 ,51 ,54 ,69 ,16 ,92 ,33 ,48 ,61 ,43 ,52 ,1 ,89 ,19 ,67 ,48},
	};

	int (*matrix_ptr)[20] = dat;

	return 0;
}

When declaring a pointer to a two dimensional array, why must I include the subscript? And when I dereference the pointer twice (**matrix_ptr) I get the first element of the first array; that is dat[0][0], or 8. What am I getting when I dereference the pointer only once (*matrix_ptr)?

Originally Posted by deadrabbit

When declaring a pointer to a two dimensional array, why must I include the subscript? And when I dereference the pointer twice (**matrix_ptr) I get the first element of the first array; that is dat[0][0], or 8. What am I getting when I dereference the pointer only once (*matrix_ptr)?

So the program knows what to initialize the row pointers, to. They have to point to the address of the first row's element, and you can't do that, if you don't know how many columns are in a row.

When you dereference **matrix, you get the address of the matrix array (the whole thing pointer if you will). When you dereference *matrix, you get the address of the first row's, first element. As you see, they both happen to be the same address - but they're not the same things.

Originally Posted by deadrabbit

When declaring a pointer to a two dimensional array, why must I include the subscript? And when I dereference the pointer twice (**matrix_ptr) I get the first element of the first array; that is dat[0][0], or 8. What am I getting when I dereference the pointer only once (*matrix_ptr)?

Perhaps, writing it out symbolically may help:

Code:

    matrix == dat;         /*   dat same as &dat[0]                  */
    *matrix == dat[0];     /*   dat[0] same as &dat[0][0]            */
    **matrix == dat[0][0]  /*   and, *&dat[0][0] same as dat[0][0]   */