struct with a pointer to an array
Author
Hey All. think this is my first post on the forum.
I'm trying to make a tetris clone.
pointers is not my strong side. tried looking in some tuts. but cant grasp this one by myself.
i have struct with a pointer
int *pBlock;
its planned to be pointing at a 3D array.
pBlock = &iBlock[0][0][0];
but i'm having some trouble using the array through the pointer. can someone show me how.
thanks in advance.
Regards Morten Due
You probably tried to use the pointer like this:
Hope some of this helps.
edit: oh I left out pointer to a 4x4 2D array. That's probably my least favorite solution though. I think it would look like this:
pBlock[0][1][2] = 1;struct TetrisBlock{ int Block[4][4][4];};TetrisBlock iBlock;// codeTetrisBlock * pBlock = &iBlockpBlock->Block[0][1][2] = 1;int iBlock[4*4*4];int x = 0;int y = 1;int z = 2;int *pBlock = iBlock;pBlock[x+(4*y)+(4*4*z)] = 1;Hope some of this helps.
edit: oh I left out pointer to a 4x4 2D array. That's probably my least favorite solution though. I think it would look like this:
int * pBlock[4][4];
Author
thanks for the help, the struct option you mentioned first, should be all i need.
so think im going for that one.
so think im going for that one.
if you need two (or more) dimensional array, you need to define pointer to pointers
int **arr;
//int arr[m][n]
arr = (int**)malloc(sizeof(int*)*m);
for(i=0; i<m; i++){
arr = (int*)malloc(sizeof(int)*n);
}
and this generates a m*n integer array for you. first it creates a m sized int pointer array, after for each element of this array, it creates a n sized integer array
int **arr;
//int arr[m][n]
arr = (int**)malloc(sizeof(int*)*m);
for(i=0; i<m; i++){
arr = (int*)malloc(sizeof(int)*n);
}
and this generates a m*n integer array for you. first it creates a m sized int pointer array, after for each element of this array, it creates a n sized integer array
Quote:Original post by shultays
if you need two (or more) dimensional array, you need to define pointer to pointers
If you know all but the first dimensions at compile-time, you can create multidimensional arrays dynamically without pointers to pointers:
void do_something(int number_of_lines){ typedef int row[10]; /* <--- note compile-time constant */ row *board = malloc(number_of_lines * sizeof(row)); /* <--- note variable */ board[0][0] = 42; /* ... */ free(board);}Quote:Original post by shultays
if you need two (or more) dimensional array, you need to define pointer to pointers
int **arr;
//int arr[m][n]
arr = (int**)malloc(sizeof(int*)*m);
for(i=0; i<m; i++){
arr = (int*)malloc(sizeof(int)*n);
}
and this generates a m*n integer array for you. first it creates a m sized int pointer array, after for each element of this array, it creates a n sized integer array
No, this is a bad way to do things (and is not actually making an "m*n integer array"). It incurs extra overhead, and implies things about the storage that are not true (namely, that it would make sense to vary the "row sizes" individually.
A proper rectangular array is a single storage block. Because the size in each dimension is known, there is no need to hold a separate allocation for each "row"; internally, a little arithmetic is done to translate indices into a single index.
In C and C++, multi-dimensional arrays sized at compile time (e.g. int x[4][4]) do this automatically. Otherwise, It Depends(TM). In C++, it is often appropriate to use std::vector, boost::multi_array, or boost::array - sometimes more than one of those, in varying configurations. In C, you normally have to roll it yourself, but blindly choosing the "dynamically allocate each row" solution is sloppy coding.
But to answer the OP's question, it is, actually, possible, with a pointer-to-array type. I recommend the use of typedefs to ensure getting them right.
#include <stdio.h>int main() { int test[7][4][4]; typedef int subarray[4][4]; subarray* test_ptr = test; /* the pointer-to-array type. */ int i, j, k, counter = 0, *verifier; for (i = 0; i < 7; ++i) { for (j = 0; j < 4; ++j) { for (k = 0; k < 4; ++k) { test_ptr[j][k] = counter++; } } } for (verifier = &(test[0][0][0]), i = 0; i < 7 * 4 * 4; ++verifier, ++i) { printf("%d\n", *verifier); }}This topic is closed to new replies.
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