2 dimension array argument

Question

The function should get as an argument 2-dimension square array of int.so the function signature :

void f (int** arr,int n) // n is a size of each dimension

I have defined the array :

int arr [][5] = {{0,1,1,1,1},{1,1,1,1,1},{0,1,1,0,1},{0,1,1,1,1},{0,1,1,1,0}}

The call :func (arr,5); //<- compilation error

also func ((int**) arr,5); //<- runtime error (*arr == NULL) how should i sole that?

Boris Strandjev · Answer 1 · 2013-11-24T12:43:37.627

3

There is slight difference between int** and what you declare, that is why you can not cast them to each other.

This should work for you:

void f (int arr[][5],int n)

The thing is that when you write arr[i][j] in the body of the function the two indices would need to be translated to a pointer to the memory. For this reason you need the second dimension as the calculation will always be arr[i][j] = arr + i * dimen + j.

edited Nov 24 '13 at 12:43

answered Nov 24 '13 at 12:36

Boris Strandjev

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That won't even compile. – teroi Nov 24 '13 at 12:38
@StoryTeller I am not entirely sure that what you edit is needed (actually I am sure it is not). – Boris Strandjev Nov 24 '13 at 12:42
@teroi why do you think so? – Boris Strandjev Nov 24 '13 at 12:43
@StoryTeller Well have not C++ed for 3 years, but last time I checked what I knew was in synch with this answer: http://stackoverflow.com/a/4802702/1108032 – Boris Strandjev Nov 24 '13 at 12:45
@StoryTeller your experiment was wrong: it must be `arr[][5]` not `[][5]arr`. It might be that I wrote it incorrectly because of my recent java experience. – Boris Strandjev Nov 24 '13 at 12:47
@BorisStrandjev, I copied *your* original code. It was factually wrong. – StoryTeller - Unslander Monica Nov 24 '13 at 12:48
@BorisStrandjev, true. There is still the philosophical debate of using one syntax over another, but that is neither here nor there :) – StoryTeller - Unslander Monica Nov 24 '13 at 12:50

Sergey Kalinichenko · Answer 2 · 2013-11-24T13:08:15.563

The function is declared to take a jagged array, i.e. an array that could have different sizes in each of its rows.

void f (int** arr,int n)

Such an array needs to be created differently - you need to allocate n pointers, and then allocate an individual memory block for each of the array rows:

int **arr = malloc(sizeof(int*)*n);
for (int i = 0; i != n ; i++) {
    // This code uses the same size for each row, but jagged arrays
    // are free of the "rectangular array" limitations.
    arr[i] = malloc(sizeof(int)*n);
}

Once you use such an array, free its rows before freeing the array itself:

for (int i = 0; i != n ; i++) {
    free(arr[i]);
}
free(arr);

Can you explain why what i am doing is not correct?

The reason why what you are doing is incorrect is that you are presenting a rectangular array to a function that expects a jagged array. The two have very different memory structures.

Rectangular array that you initialize statically is a single, contiguous, block of memory of size 25*sizeof(int). The "second dimension" is added to that flat chunk of memory by compiler's knowledge that the second dimension has exactly five elements. That's how the compiler can apply a "cookie cutter" to the block, partitioning it in five equal blocks of five integers. Accessing an element at (i,j) is translated to adding 5*i+j offset to the base address of arr (a single pointer dereference).

Jagged array, on the other hand, is an array of pointers to arrays. It has a size of 5*sizeof(int*). Each of these pointers can point to a separate array. Accessing an element at (i,j) is translated to accessing arr[i] pointer, and then accessing the value at the offset taken from that value (two pointer dereferences).

@Yakov You cannot easily allocate a jagged array statically - you would need to allocate each row separately, or use some other magic to assign pointers to pointers statically. You can use "rectangular" arrays which are easy to initialize, but they must have one of their sides fixed. — Sergey Kalinichenko, Nov 24 '13 at 12:48
@Yakov Here is [a link](http://ideone.com/gRioGo) to an example of making a jagged array statically. Warning: it isn't pretty. — Sergey Kalinichenko, Nov 24 '13 at 12:51
Converting `int[5][7]` to a `std::array` or equivalent statically is not that hard. And `operator int**()` is also easy. Now, why you would want to do this, I am unsure. — Yakk - Adam Nevraumont, Nov 24 '13 at 12:56
@dasblinkenlight - can you explain why what i am doing is not correct? — YAKOVM, Nov 24 '13 at 12:58

score 1 · Answer 3 · answered Nov 24 '13 at 12:52

1

I propose the use of a one-dimensional array which holds the two-dimensional data. This makes handling and passing of the array a lot easier and it's not that difficult to understand in my opinion...

void f (int* arr, int n) // n is a size of each dimension

int arr [25] = {0,1,1,1,1,1,1,1,1,1,0,1,1,0,1,0,1,1,1,1,0,1,1,1,0};

in your function you could traverse the data like this:

...
for (int row = 0; row < n; ++row)
{
    for (int col = 0; col < n; ++col)
    {
        arr[row*n + col] = 1; // or something else...
    }
}

answered Nov 24 '13 at 12:52

xmoex

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Except this `int arr [][5]` doesn't define a one-dimensional array. – StoryTeller - Unslander Monica Nov 24 '13 at 12:52
I have to say, this is a bit silly. It basically is doing the compilers work on your own. Which requires you to understand how multidimensional arrays really work. Which if you do, you won't have a problem to begin with, most probably :) – StoryTeller - Unslander Monica Nov 24 '13 at 12:55
can't see anything silly here... when i look at @BorisStrandjevs solution he is statically nailed to arrays with a width of 5 – xmoex Nov 24 '13 at 12:58
Yeah... but since this is C++. much more efficient and general code can be achieved with a little use of templates and template parameter deduction. Like I said, it's only a little silly. – StoryTeller - Unslander Monica Nov 24 '13 at 13:00
i'd agree to that, but it's pretty far away from the OP's code (which doesn't mean it shouldn't be posted). why don't you show us your solution? – xmoex Nov 24 '13 at 13:03
My solution was already posted by Boris. Your solution got me thinking about template parameter deduction, which is the reason for the up-vote :) – StoryTeller - Unslander Monica Nov 24 '13 at 13:12

2 dimension array argument

3 Answers3