How to understand the application of multi-dimensional array pointer

Ken H

14y2.3k

Hi all,
How to understand the application of multi-dimensional array pointer in c plus plus.

thanks in advance.

4 Replies

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Vulpes14y ago
Accepted answer
This simple example may help:

// create and initialize a 2 x 2 array of ints
int array2d[2][2];
array2d[0][0] = 1;
array2d[0][1] = 2;
array2d[1][0] = 3;
array2d[1][1] = 4;

// traverse using pointer
int *p = (int *)array2d;
for(int i = 0; i < 4; i++) printf("%d\n", *(p + i)); // 1 2 3 4

// access element [1][1] using pointer
int e = *(p + 2 * 1 + 1);
printf("%d\n", e); // 4
return 0;

Traversing the array using a pointer works because the rows of the multidimensional array are stored one after the other in memory.

In general you can access the [i][j]th element of the array by adding:

(row length) * i + j to a pointer to the first element, and then dereferencing the resulting pointer.
Ken H14y ago
hi sam hobbs,
thanks for your help.
Sam Hobbs14y ago
Multi-dimensional arrays in C can be confusing. I think that if you use one syntax then the items are stored so that all items of a row are contiguous but then if you use a different syntax then all items of a column are contiguous.

A better solution is to use C++ vectors but the C++ standard did not have specific support for multi-dimensional vectors. There are many articles in other sites and forum answers in other forums showing how to do that. I know I asked about this in the MSDN forums.

I don't know about the new C++ language standard but I sure hope that it has better support for multidimensional arrays, although tecnically they are probably multidimensional vectors.

If you must do it the C way, then see Ted Jensen's Tutorial on Pointers and Arrays in C.
Ken H14y ago
thanks!