Re: [Boost-users] [MultiArray] subscript to index

Subject: Re: [Boost-users] [MultiArray] subscript to index
From: Sami (randomaccessiterator_at_[hidden])
Date: 2010-05-04 20:16:36

Hi Larry,

On Tue, May 4, 2010 at 1:16 AM, Larry Evans <cppljevans_at_[hidden]>wrote:

> On 05/03/10 22:20, Sami wrote:
>
>>
>> Hmm... I puzzled why you think Sami's method won't work.
> For example, given the strides, s[ndim] and indices, i[ndim], and
> a.origin(), then the distance between a.origin() and the element
> at indices, i[0...ndim-1] is simply the sum of the vector product
> of s prepended with 1 and i.  e.g.,
> For a[2][2][2], the strides would be:
>   s={2,4,8},
>

actually that would be {4,2,1} for default C (row-major) storage ordering.

> and for i={1,1,1}, then
>  offset (of a[1]][1][1]) = 1*i[0]+s[0]*i[1]+s[1]*i[2]
> Wouldn't that work?
>
>
wouldn't work when any of index_bases() is non-zero. You'd need the
following to make it right:

// using inner_product instead of transform/accumulate
collection index = {i,j,k}; // looking for offset of this from data()
size_t base_offset = std::inner_product(
      a.index_bases(), a.index_bases()+a.num_dimensions(),
      a.strides(),
      0);
size_t offset = std::inner_product( index.begin(), index.end(), a.strides(),
-base_offset );