Ptrs to Multi-Arrays
Author
Greetings,
I have started coding the game Connect 4. Although, I keep running into errors when I try to use a ptr to my multi-array. My c++ books has NO documentation on ptrs to matrix. Any explanation on it and iterating through it is appreciated. Thanks in advance!
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"There are ones that say they can and there are those who actually do."
"...u can not learn programming in a class, you have to learn it on your own."
Edited by - cMADsc on December 26, 2001 9:10:53 PM
An array size s of ''type'', let''s call it ''xyz'', is defined like this:
type xyz;<br><br>In memory, that looks like:<br><br>+—+ +——-+<br>|xyz—->|xyz[0] |<br>+—+ +——-+<br> |xyz[1] |<br> +——-+<br> |xyz[2] |<br> +——-+<br> |xyz[3] |<br> +——-+<br> </pre> <br><br>An array is, in fact, equivalent to a pointer; the variable ''xyz'' is a pointer to the first element in the array. If you put brackets after that, xyz[n] is the nth element of the array. A 2-D array is an array of arrays, which looks something like this:<br><br><pre><br>type xyz[s1][s2];<br><br><br>+—-+ +——-+ +———+———+———+<br>|xyz—>|xyz[0]—>|xyz[0][0]|xyz[0][1]|xyz[0][2]|<br>| | | | +———+———+———+<br>+—-+ | |<br> +——-+ +———+———+———+<br> |xyz[1]—>|xyz[1][0]|xyz[1][1]|xyz[1][2]|<br> | | +———+———+———+<br> | |<br> +——-+ +———+———+———+<br> |xyz[2]—>|xyz[2][0]|xyz[2][1]|xyz[2][2]|<br> | | +———+———+———+<br> | |<br> +——-+ +———+———+———+<br> |xyz[3]—>|xyz[3][0]|xyz[3][1]|xyz[3][2]|<br> | | +———+———+———+<br> | |<br> +——-+<br> </pre> <br><br>To get from xyz to an actual piece of data, we have to follow pointers (<I>dereference</I>) more than once. The first dereferencing, we go from, for example, xyz to xyz[0], the second dereferencing gets us from xyz[0] to xyz[0][1]. The number of pointers to be followed are called <I>levels of indirection</I>. xyz has two levels of indirection. A pointer to X always has one more level of indirection than X does, so a pointer to xyz has three levels of indirection; that is, it is a pointer to a pointer to a pointer. So, you would declare a pointer to a 2-D array named foo with the line:<br><br><pre><br>type ***foo;<br><br>+—-+ +—-+ +——-+ +———+———+———+<br>|foo—->|xyz—>|xyz[0]—>|xyz[0][0]|xyz[0][1]|xyz[0][2]|<br>| | | | | | +———+———+———+<br>+—-+ +—-+ | |<br> +——-+ +———+———+———+<br> |xyz[1]—>|xyz[1][0]|xyz[1][1]|xyz[1][2]|<br> | | +———+———+———+<br> | |<br> +——-+ +———+———+———+<br> |xyz[2]—>|xyz[2][0]|xyz[2][1]|xyz[2][2]|<br> | | +———+———+———+<br> | |<br> +——-+ +———+———+———+<br> |xyz[3]—>|xyz[3][0]|xyz[3][1]|xyz[3][2]|<br> | | +———+———+———+<br> | |<br> +——-+<br> </pre> <br><br>To get from foo to xyz[0][1], you would first dereference foo once (*foo), then give two subscripts, to make (*foo)[0][1].<br><br>As you can see, having many levels of indirection can get messy. Another common problem is going out of array bounds. If I have an array X with 5 elements, and then I try to refer to X[7], that is an error and will crash. In C, it is customary to pass size information around separately; in C++, you might make container classes. (Which, by the way, is a different and complex topic).<br><br>(I hope those pointer graphs came out OK. If they didn''t, I''ll try to edit it until they do, but barring that, copy it into an editor with a monospace font.) <br><br>—————<br>A picture is worth 1000 words, 500 longwords, or 250 dwords. 
// a multi-arrayint board[6][4];// point to the beginningint main(int argc, char** args){ int* pBoard = &board[0][0]; int x; x = pBoard[0]; // x = board[0][0]; x = pBoard[3]; // x = board[0][3]; x = pBoard[7]; // x = board[1][1]; return 0;} I think that should work, but I could be wrong
No, HTML is not an OO language.
Author
i have my matrix declared in main and trying to pass it by refrences to a function....
int Move(char *);
int main()
{
const int row=6, col=7;
char Matrix[row][col];
char *pMatrix= &matrix[0][0];
.
.
.
return 0;
}
Matrix
==========
Before inserting a peice i need to check each col to see if its value is '\0', if so insert 'x' or 'o'. each 'x' or 'o' will be
places at the bottom. can i iterate through the matrix with a FOR loop like
{
damn mental block!!!
Edited by - cMADsc on December 27, 2001 12:34:11 AM
int Move(char *);
int main()
{
const int row=6, col=7;
char Matrix[row][col];
char *pMatrix= &matrix[0][0];
.
.
.
return 0;
}
Matrix
==========
| | | | | | | |+---+---+---+---+---+---|---+| | | | | | | |+---+---+---+---+---+---|---+| | | | | | | |+---+---+---+---+---+---|---+| | | | | | | |+---+---+---+---+---+---|---+| | o | | | | | |+---+---+---+---+---+---|---+| x | x | | o | | | |+---+---+---+---+---+---|---+ 1 2 3 4 5 6 7 Before inserting a peice i need to check each col to see if its value is '\0', if so insert 'x' or 'o'. each 'x' or 'o' will be
places at the bottom. can i iterate through the matrix with a FOR loop like
========================| |==================================== for (int i=0; i<7; i++)|or| do i have use pointer arithmetic? pMatrix[row]; | | *pMatrix++;========================| |==================================== {
damn mental block!!!
Edited by - cMADsc on December 27, 2001 12:34:11 AM
yes you can use a for loop...
for(int i=0; i<rows; i++)for(int j=0; <cols; j++)if(0==matrix[i][j])matrix[i][j]=''x'';
To pass the matrix[row][col] to Move(), it should be declared as:
int Move (char matrix[][col]);
Then to call Move() from main() you just do:
Move (matrix);
and matrix can be accessed with the usual [row][col] syntax from within Move()
Ok, now I''m going to try to explain this... (somebody please correct me if I''m wrong)
There are two types of multi-dimensional arrays in C++. When an array is allocated dynamically, it is declared as something like
char **array;
and when an element accessed, the compiler creates code which dereferences twice;
If an array is declared statically, such as:
char array[5][10];
then it is created in a single block of memory and to access an element, only one dereference needs to be done. For example, to access element [2][3] from array[5][10], the compiler does this:
array[2 * 10 + 3];
This is why when passing a statically declared array to a function, you don''t need to pass the number inside the first [], because the compiler doesn''t need this in order to access the array elements.
int Move (char matrix[][col]);
Then to call Move() from main() you just do:
Move (matrix);
and matrix can be accessed with the usual [row][col] syntax from within Move()
Ok, now I''m going to try to explain this... (somebody please correct me if I''m wrong)
There are two types of multi-dimensional arrays in C++. When an array is allocated dynamically, it is declared as something like
char **array;
and when an element accessed, the compiler creates code which dereferences twice;
If an array is declared statically, such as:
char array[5][10];
then it is created in a single block of memory and to access an element, only one dereference needs to be done. For example, to access element [2][3] from array[5][10], the compiler does this:
array[2 * 10 + 3];
This is why when passing a statically declared array to a function, you don''t need to pass the number inside the first [], because the compiler doesn''t need this in order to access the array elements.
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