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cpointersmultidimensional-array
Alex Q
Alex Q

Reputation: 57

Dereferencing an array of pointers to two dimensional arrays in C?

I was wondering if it is possible to dereference a pointer to a two dimensional array in C:

int matrix1[2][2] = {{1,2},{3,4}};
int matrix2[2][2] = {{4,5},{6,7}};

I am trying to create an array of pointers, where the first pointer points to matrix1 and the second one to matrix2. I thought of

int *(pt[2]);
pt[0] = &matrix1

My goal would be to be able to access any element in one of the two array with something like:

pt[0][n][m];

Or for the second matrix:

pt[1][n][m]

But as far as I know this doesn't work. Is the pointer declaration wrong?

Upvotes: 3

Views: 900

Answers (2)

Kariem
Kariem

Reputation: 780

The answer provided is correct but lacks explanation. I think it is more helpful if you understand why your approach failed.

Using array indexes on pointers works by incrementing the pointer using the size of the data type.

For example if we define an array as follows:

int a[2] = {1,2};

and assume that the array was created in memory at location x

a[0]; // points at x
a[1]; //points at x + sizeof(int)

now lets take the example of two dimensional array

int a[2][2] = {{1,2},{3,4}}

again assume location x

a[0]; //points at x
a[1]; //points at x + sizeof(int [2]) because int[2] is the data type
a[0][1]; // points at x + sizeof(int)
a[1][1]; // points at x + sizeof(int[2]) + sizeof(int)

Now lets take your example

int matrix1[2][2] = {{1,2},{3,4}};
int matrix2[2][2] = {{4,5},{6,7}};

int *pt[2];
pt[0] = &matrix1; //doesn't compile

First, that won't really compile because the type of pt is pointer to int and matrix1 is a pointer to int[2]. However, you can force it by casting like this

pt[0] = (int*)&matrix1;

But it still won't work because the following is not valid either

pt[0][n][m]; //fails to compile

This is because pt is an array of pointer to int, so pt[0][n] points at an integer, not an array. As such you cannot use apply an index to it

However, if you define it as follows

int (*pt[2])[2] = {&matrix1,&matrix2}

pt is then an array of pointers to two dimensional arrays. As such, assuming matrix1 defined at x1 and matrix2 defined at x2,

pt[1][1][1]; //points to x2 + sizeof(int[2]) + sizeof(int)

Upvotes: 0

haccks
haccks

Reputation: 106012

You have to declare pt as an array of pointers to array. 

 int (*pt[2])[2] = { matrix1, matrix2 };

Demo.

Upvotes: 3

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