indexing elements of a 2D array by pointer for transpose

**Gianni Mariani** · Aug 5 '08, 09:35 AM

Re: indexing elements of a 2D array by pointer for transpose

pc_whocares wrote:

My forehead is flat from pounding.

....

As you may suspect, please don't assume I'll just catch a hint.
'splain to me, Lucy!

Without more code, no-one will be able to help you.

**pc_whocares** · Aug 5 '08, 05:35 PM

Re: indexing elements of a 2D array by pointer for transpose

On Aug 4, 6:06 pm, pc_whocares <pc_whoca...@ya hoo.comwrote:

My forehead is flat from pounding.
>
I am building a DLL in VS2005 C++ for use in another software
development platform.
>
I am required to pass my array data in/out of the function via a
pointer, in this case, to double.
>
The simplest way for me to transpose the array is via addressing its
individual elements in the conventional form such as
>
array2dt[i,j] = array2d[j,i];
>
where both variables appear in the call list as
>
long redimSTr(..., double *array2d, double *array2dt)
>
I realize that the algorithm looks like it will likely only work for a
square matrix, but since array2dt is coming back completely unchanged,
I suspect that there is something else fundamentally wrong with my
logic, specifically, the way to directly address individual elements
of a multi dimensional array represented by a pointer in a function.
>
As you may suspect, please don't assume I'll just catch a hint.
'splain to me, Lucy!
>
tanks
>
pc

Thanks for your replies. Let me try again.

I need a way to manipulate a 2D array in a subroutine. What's the
magic thing that will allow me to go from a pointer and base type to a
2D array? I am obviously a bare novice here.

It seems that one cannot dynamically create a 2D array.

tanks

pc

// redim.cpp : Defines the entry point for the DLL application.
//
// Use the defined keyword DLLEXPORT in front of C/C++ functions
// that are meant to be exported for use in calling context
//
#include <stdlib.h>
#include "stdafx.h"

#ifdef WIN32
#ifdef __cplusplus
#define DLLEXPORT extern "C" __declspec(dlle xport)
#else
#define DLLEXPORT __declspec(dlle xport)
#endif
#else
#define DLLEXPORT
#endif

typedef double* dblArrayPtr;

DLLEXPORT long redimSTr(long n, long m, long arraySize, double SF,
double *array, double *array2d)

{
long i;
long j;

dblArrayPtr tmpArray;
// both errors next executable line
// -- apparently can't dynamically create 2D array
// Error 1 error C2540: non-constant
// expression as array bound
// Error 2 error C2440: '=' :
// cannot convert from 'double (*)[1]' to 'dblArrayPtr'
tmpArray = new double[n][m];

// from calling context array2d is m x n
// next for loop simply copies data from
// 1D "array" to 2D "array2d" and scales it
for (i = 0; i < arraySize; i++, array++, array2d++)
{
*array2d = *array * SF;
}

// intention here is to put scaled data into
// an array that I can address by row, column
tmpArray = array2d;

// perform transpose.. will only work for square matrix,
// but that's the least of my worries
for (i = 0; i < n; i++)
{
for (j = 0; j < m; j++)
{
tmpArray[i,j] = array2d[j,i];
}
}

// put transposed data back so I can access
// it from calling function
array = tmpArray;
// clean up
delete [] tmpArray;
// arraySize is unused upon return
return(arraySiz e);
}

**red floyd** · Aug 5 '08, 08:15 PM

Re: indexing elements of a 2D array by pointer for transpose

On Aug 5, 10:32 am, pc_whocares <pc_whoca...@ya hoo.comwrote:

On Aug 4, 6:06 pm, pc_whocares <pc_whoca...@ya hoo.comwrote:
>
>
>

My forehead is flat from pounding.

>

I am building a DLL in VS2005 C++ for use in another software
development platform.

>

I am required to pass my array data in/out of the function via a
pointer, in this case, to double.

>

The simplest way for me to transpose the array is via addressing its
individual elements in the conventional form such as

>

array2dt[i,j] = array2d[j,i];

>

where both variables appear in the call list as

>

long redimSTr(..., double *array2d, double *array2dt)

>

I realize that the algorithm looks like it will likely only work for a
square matrix, but since array2dt is coming back completely unchanged,
I suspect that there is something else fundamentally wrong with my
logic, specifically, the way to directly address individual elements
of a multi dimensional array represented by a pointer in a function.

>

As you may suspect, please don't assume I'll just catch a hint.
'splain to me, Lucy!

>

tanks

>

pc

>
Thanks for your replies. Let me try again.
>
I need a way to manipulate a 2D array in a subroutine. What's the
magic thing that will allow me to go from a pointer and base type to a
2D array? I am obviously a bare novice here.
>
It seems that one cannot dynamically create a 2D array.
>
tanks
>
pc
>
// redim.cpp : Defines the entry point for the DLL application.
//
// Use the defined keyword DLLEXPORT in front of C/C++ functions
// that are meant to be exported for use in calling context
//
#include <stdlib.h>
#include "stdafx.h"
>
#ifdef WIN32
#ifdef __cplusplus
#define DLLEXPORT extern "C" __declspec(dlle xport)
#else
#define DLLEXPORT __declspec(dlle xport)
#endif
#else
#define DLLEXPORT
#endif
>
typedef double* dblArrayPtr;
>
DLLEXPORT long redimSTr(long n, long m, long arraySize, double SF,
double *array, double *array2d)
>
{
long i;
long j;
>
dblArrayPtr tmpArray;
// both errors next executable line
// -- apparently can't dynamically create 2D array
// Error 1 error C2540: non-constant
// expression as array bound
// Error 2 error C2440: '=' :
// cannot convert from 'double (*)[1]' to 'dblArrayPtr'
tmpArray = new double[n][m];
>
// from calling context array2d is m x n
// next for loop simply copies data from
// 1D "array" to 2D "array2d" and scales it
for (i = 0; i < arraySize; i++, array++, array2d++)
{
*array2d = *array * SF;
}
>
// intention here is to put scaled data into
// an array that I can address by row, column
tmpArray = array2d;
>
// perform transpose.. will only work for square matrix,
// but that's the least of my worries
for (i = 0; i < n; i++)
{
for (j = 0; j < m; j++)
{
tmpArray[i,j] = array2d[j,i];

this line does not do what you think it does. It actually does
tmpArray[(i,j)] = array2d[(i,j)]

}
}
>
// put transposed data back so I can access
// it from calling function
array = tmpArray;
// clean up
delete [] tmpArray;
// arraySize is unused upon return
return(arraySiz e);
>
}
>
>

**Kevin Frey** · Aug 6 '08, 12:25 AM

Re: indexing elements of a 2D array by pointer for transpose

In C++, for an array having multiple dimensions, two adjacent elements in
the last dimension of the array are arranged contiguously in memory *, and
you work back from that.

* I say contiguously - but this will include allowances for issues such as
structure padding and alignment etc. The important point is that the
elements are effectively "next to each other" in memory.

You *must* know the dimensions of the array.

So an array like a[5,10] will have p0=a[0,0], p1=a[0,1], p2=a[0,2], ...
p10=a[1,0] etc, where p0 is a memory location, and p1 is the next memory
location etc. (Next contiguous memory location based on the SIZE of the
elements, that is).

You can write a simple class to handle the access to a multi-dimensional
array, so it does the conversions for you.

For example (I am writing this off the top of my head, so it may not compile
cleanly):

template< class T >
class TwoDimArray
{
public:
TwoDimArray( T* _Buffer, int _FirstDimension , int _SecondDimensio n )
{
m_Buffer = _Buffer;
m_FirstDim = _FirstDimension ;
m_SecondDim = _SecondDimensio n;
}

T& Item( int d1, int d2 )
{
// you could do assertions etc here to check that d1, d2 are
"in-range"

int sequentalPositi on = ( d1 * m_SecondDim ) + d2;
return m_Buffer[ sequentialPosit ion ];
}

T* m_Buffer;
int m_FirstDim;
int m_SecondDim;
};

int main( )
{
double* buffer = new buffer [ 100 ];

TwoDimArray arr( buffer, 10, 10 );

arr.Item( 0, 0 ) = 100.0;
arr.Item( 1, 7 ) = 200.0;
}

indexing elements of a 2D array by pointer for transpose

indexing elements of a 2D array by pointer for transpose

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