How to convert a (long *) to a 2 dimensional array of longs

Question

I have the following code in C where G is a global variable:

long *G;

int function(long matrix[35][10]){
    G=&matrix[0][0];
}

Once I store the address of matrix[0][0] in G, I can access all the elements of matrix like so:

G[0]=...
G[1]=...
.
.
.
G[349]=...

I want to be able to access all of these elements similar to how I can with matrix, by using the two square brackets from inside of another function. How would I be able to do that? That is, how can I tell the compiler that the offset for the second dimension in this 2 dimensional array is 10?

I did look at this answer: Create a pointer to two-dimensional array , but I'm not sure if the user asked the exact same thing. Please correct me if I'm wrong

@Dabo So if I follow the answer to that question, a 64 bit value will basically be allocated for my global variable right? Just making sure — user3427787, Mar 17 '14 at 07:09
`long *G` is just a pointer, if `sizeof` pointer on your machine is `8` then yes, it will have 64 bit. — Dabo, Mar 17 '14 at 07:24

jfly · Answer 1 · 2014-03-17T07:19:27.203

2

long (*array)[10] = (long(*)[10])G;

array is a pointer to an array of 10 longs.

edited Mar 17 '14 at 07:19

answered Mar 17 '14 at 07:05

jfly

7,715
3
35
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Just to make sure, only 64 bits are allocated for `array` right? The fact that there is a 3 in front of it, makes me think three longs are allocated – user3427787 Mar 17 '14 at 07:07
It depends on the system, `sizeof(long*)` will be allocated for `array`. – jfly Mar 17 '14 at 07:15
2

Or since he wants to use `G` itself like that, rather just declare `G` as `long (* G)[10];` and assign `matrix` to it directly, like `G = matrix;`. – Utkan Gezer Mar 17 '14 at 07:44

Bhavesh Munot · Answer 2 · 2014-03-17T07:10:07.397

-2

Make G a poiter to pointer.

long **G;

Still you will not be able to access directly by G[i] i goes from 0 to 349.

But one easy thing that you can do is converting a number to 2 subscripts.

for example:

G[349] is actually G[34][9].

Hence formula is if you want

G[k]

then access it like this:

G[k/10][k%10];

This is for this particular case.

For general case, the formula to convert 1 subscript to 2 subscripts is:

G[k] is equivalent to G[k/NO_OF_COLUMNS][k%NO_OF_COLUMNS];

edited Mar 17 '14 at 07:10

answered Mar 17 '14 at 07:05

Bhavesh Munot

675
1
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1

this does not work. pointers and arrays are not the same. reading G[34] will get an element from the data buffer of the array and will try to dereference it that obviously causes an access violation – vlad_tepesch Mar 17 '14 at 08:34

How to convert a (long *) to a 2 dimensional array of longs

2 Answers2