Re: [Boost-users] [Boost][multi_array] copying a float array to a particular dimension of a multi_array

From: Ronald Garcia (garcia_at_[hidden])
Date: 2006-08-12 12:39:29

Hello Sebastien,

On Aug 11, 2006, at 9:31 AM, Sebastien Fortier wrote:

> I would like to know if it is possible to use memcpy to copy a  
> float array to a particular dimension of a multi_array.

In some situations you can, but not as your array is currently  
specified.

>
> ex.:
> I declare a 3 dimension multi_array of floats
>
> typedef boost::multi_array < float, 3 > array_type3D_f_t;
> int dim1 = 2;
> int dim2 = 2;
> int dim3 = 4;
> array_type3D_f_t valueMatrix(boost::extents[dim1][dim2][dim3]);
>
> then for each 3rd dimension I would like to copy a float array into  
> it using memcpy
> (the final real multi_array will contain approx 21 000 000  
> elements, and thts why I do wish to iterate ).
>

If you change your multi_array so that the third dimension is the  
first dimension, then the dim1*dim2 elements will be stored  
contiguously in the array and you will then be able to memcpy them  
(or pass a pointer to the array buffer and avoid copying altogether).
If you require the dimensions of the multi_array to be as you  
specified, then you will need to specify a different storage order  
for the array in order to make the dim1*dim2 elements contiguous.

for example:
/* switch the dimensions of the array */
array_type3D_f_t valueMatrix(boost::extents[dim3][dim1][dim2]);

/* ... */
for (int i=0; i != dim3; ++i) {
   function(valueMatrix[i].origin()); // directly passes the  
multi_array buffer to the function
}

HTH,
ron

> for (int i = 0; i < dim3; ++i) { // iterate over each 3rd dimension  
> of the multi_array
>     float *arr = NULL;
>     arr = (float*)malloc(sizeof(float) * dim1 * dim2);
>     function(&arr) // fills in the array (2D contiguous (dim1 * dim2))
>     memcpy(&valueMatrix[something...], arr, sizeof(float) * dim1 *  
> dim2) // I want to copy the vector into the multi_array's 3rd  
> dimension
> }
>
> then when I want to access an element I simply do
>
> valueMatrix[1][1][1];
>
> what i used to do is
>
>     int level = 0;
>    while(it1 != it2){ // it1 and it2 are iterators to a list of  
> records holding the data
>         float* arr;
>         readData(it1, &arr); // read the data into the array
>         for (int j = 0; j < dim2; ++j){
>             for (int i = 0; i < dim1; ++i){
>                 valueMatrix[i][j][level] = datavec[ j * dim1+i ];
>             }
>         }
>         ++it1;
>         // change level
>         ++level;
>     }
>
> I would like to replace the nested loops by a memcpy, is this  
> possible?
>
> -- 
> Sébastien Fortier
>
>
>
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