Passing a pointer to two-dimension array in a function

I am having no problem in passing a pointer to a one dimension array in a function in my code. However, when I tried passing a pointer to a two-dimension array im receiving an error. Can somebody help or explain?


Code using one-dimension array:

Code:

#include <iostream>


void InitializeArrays(double *array1, double *array2)
{

}


int main()
{

double Array_A[5], Array_B[5];

InitializeArrays(Array_A, Array_B);


return 0;

}

Code using two-dimension array:

Code:

#include <iostream>


void InitializeArrays(double *array1, double *array2)
{

}


int main()
{

double Array_A[5][5], Array_B[5][5];

InitializeArrays(Array_A, Array_B);


return 0;

}

Error encountered: error C2664: 'InitializeArrays' : cannot convert parameter 1 from 'double [5][5]' to 'double *'

When passed as an argument, a double[5][5] becomes a pointer to a double[5], which is quite different from a pointer to a double. A solution is to change the function to:

Code:

void InitializeArrays(double (*array1)[5], double (*array2)[5])

you should probably listen to laserlight, but something like this would work also:

Code:

void InitializeArrays(double **array1, double **array2,size_t a1l1,size_t a1l2,size_t a2l1,size_t a2l2)
{

}

Originally Posted by m37h0d

you should probably listen to laserlight, but something like this would work also:

Code:

void InitializeArrays(double **array1, double **array2,size_t a1l1,size_t a1l2,size_t a2l1,size_t a2l2)
{

}

No, because you do not have a pointer to a pointer in the first place, you have the address of a two-dimensional array. To make that work, you'd have to create a one-dimensional array of pointers that point to the first element of each row of data.

--
Mats

I was having trouble cutting my grass by swinging my lawnmower blade about...it worked much better in the lawnmower.

Code:

class Matrix {
    double[5][5];
public:
    Matrix() { // initialize array here }
};

Better would be vector of vector and pass by reference.
Then you wouldn't have to guess what type must be placed in the function declaration.

Originally Posted by matsp

No, because you do not have a pointer to a pointer in the first place, you have the address of a two-dimensional array. To make that work, you'd have to create a one-dimensional array of pointers that point to the first element of each row of data.

--
Mats

yes, you're correct. that's what i ended up having to do when i was in this sort of position...

is there any other way to pass a multidimensional array without specifying the lengths as a literals?

Originally Posted by m37h0d

is there any other way to pass a multidimensional array without specifying the lengths as a literals?

No, because the compiler wouldn't be able to generate the correct code, then.
But it's possible with classes and objects, say a vector of vector.

Originally Posted by Elysia

No, because the compiler wouldn't be able to generate the correct code, then.
But it's possible with classes and objects, say a vector of vector.

It is possible if you degenerate the array to a single dimension array [which is essentially how the compiler goes about solving that problem].

It is not "possible with objects" - but objects can hold information to describe their own contents, so you can hide the information about the array dimensions inside the class.

--
Mats

Originally Posted by matsp

It is possible if you degenerate the array to a single dimension array [which is essentially how the compiler goes about solving that problem].

True, but this would also make it a 1D array. But a 2D array can also be represented as a 1D array, so it's possible, but it isn't exactly the same.

It is not "possible with objects" - but objects can hold information to describe their own contents, so you can hide the information about the array dimensions inside the class.

That's more to the point I was referring to. Classes can hide things such as their dimensions (such as vector) and enable passing themselves in such a way that they can remain 2D-arrays without passing the actual dimensions.

it is curious to me that you can treat a pointer-to-pointer like a two dimensional array (provided it's been properly initialized), but the inverse isn't true.

They are of different nature.
A dynamic 2D array is like a linked list:

Code:

A*:
    B*:
        C
        D
    E*:
        F
        G

But 2D arrays are sequential:

Code:

A
B
C
D
E
F
G

So it wouldn't work.
In the dynamic array, you'd dereference the first point with the first dimension which in turn would have more pointers to the second dimension.
But in normal 2D arrays, there are no pointers to dereference.