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carrayspointers
dspverden
dspverden

Reputation: 31

C: Pointer to 2-dimensional array of pointers

I am trying to solve the following issue but could not succeed yet:

I have a two-diwmensional array of pointers:

int* a[16][128];

Now I want to make a pointer to this array in that way that I can use pointer arithmetic on it. Thus, something like this:

ptr = a;
if( ptr[6][4] == NULL )
  ptr[6][4] = another_ptr_to_int;

I tried already some variations but it either fails then on the first line or on the if condition.

So, how can it be solved? I would like to avoid template classes etc. Code is for a time critical part of an embedded application, and memory is very limited. Thus, I would like ptr to be only sizeof(int*) bytes long.

Upvotes: 0

Views: 639

Answers (3)

MountainLogic
MountainLogic

Reputation: 308

To start with array pointer basics for a 1D array, [tutorialspoint][1] has a very easy to ready description. From their example:

    #include <stdio.h>

int main () {

   /* an array with 5 elements */
   double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
   double *p;
   int i;

   p = balance;                                 //Here the pointer is assign to the start of the array

   /* output each array element's value */
   printf( "Array values using pointer\n");

   for ( i = 0; i < 5; i++ ) {
      printf("*(p + %d) : %f\n",  i, *(p + i) );
   }

   printf( "Array values using balance as address\n");

   for ( i = 0; i < 5; i++ ) {
      printf("*(balance + %d) : %f\n",  i, *(balance + i) );    // Note the post increment
   }

   return 0;
}

There are a couple of relavent Stack overflow answers that describe 2D arrays: How to use pointer expressions to access elements of a two-dimensional array in C?

Pointer-to-pointer dynamic two-dimensional array

how to assign two dimensional array to **pointer ?

Representing a two-dimensional array assignment as a pointer math?

  [1]: https://www.tutorialspoint.com/cprogramming/c_pointer_to_an_array.htm

Upvotes: 0

yuri kilochek
yuri kilochek

Reputation: 13589

Thing you seem to want:

int* (*ptr)[128] = a;

Actual pointer to the array:

int* (*ptr)[16][128] = &a;

Upvotes: 1

molbdnilo
molbdnilo

Reputation: 66459

A pointer to the first element of the array (which is what you want) could be declared as 

int* (*ptr)[128];

A pointer to the array itself would be

int* (*ptr)[16][128];

and is not what you're looking for.

Upvotes: 2

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