Re: [Boost-users] [Boost.MultiArray]: size question

Subject: Re: [Boost-users] [Boost.MultiArray]: size question
From: Larry Evans (cppljevans_at_[hidden])
Date: 2012-12-17 14:00:35

On 12/17/12 11:24, Larry Evans wrote:
> On 12/17/12 09:50, gast128 wrote:
>> Dear all, 
>>
>> we use here the multi_array as a 2 dimensional matrix. Often we want to resize
>> the row or column dependent on some GUI event. Code could be:
>>
>> void Bla::ResizeRows(size_t nRows)
>> {
>>    m_matrix.resize(boost::extents[nRows][m_matrix[0].size()]);
>> }
>>
>> However this uses the 0-index of row count (i.e. 'm_matrix[0].size()') to
>> determine the size of the columns. Will this not crash if you have 0 rows? And
>> if so is there a better alternative?
>>
>> wkr.
> The test driver:
> 
> 
> http://svn.boost.org/svn/boost/sandbox/variadic_templates/sandbox/array_dyn/array_dyn.cpp
> 
> produces output:
> 
> ********reshape demo***********
> dir_op=0
> shape={ 0, 3}
> my_data.size()=0
> ai=
> }
> .
> shape={ 1, 3}
> ai=
> { { 0, 0, 0}
> }
> .
> shape={ 5, 3}
> ai=
> { { 0, 0, 0}
> , { 0, 0, 0}
> , { 0, 0, 0}
> , { 0, 0, 0}
> , { 0, 0, 0}
> }
> .
> 
> Compilation finished at Mon Dec 17 11:11:25
> 
> Which suggests it will work when 1st axis (the row axis) has 0 as it's
> size.  Unfortunately, this is not Boost.MultiArray and it's probably
> a lot slower since the sizes of the dimensions are all runtime values.
> Also, the printout when the shape includes a 0, as in:
> 
>   shape={ 0, 3}
> 
> does not have balanced curly brackets:
> 
>  ai=
>  }
> 
> 
> I'm not sure how to handle that yet.

Addition of print_array<.>::visit_empty and corresponding change
to array_host seems to solve the empty axis print problem:

********reshape demo***********
dir_op=0
shape={ 3, 0}
my_data.size()=0
ai=
{ { }
, { }
, { }
}
.
shape={ 3, 1}
ai=
{ { 0}
, { 1}
, { 2}
}
.
shape={ 3, 5}
ai=
{ { 0, 3, 6, 9, 12}
, { 1, 4, 7, 10, 13}
, { 2, 5, 8, 11, 14}
}
.
shape={ 0, 5}
ai=
{ }
.

Compilation finished at Mon Dec 17 12:55:44

HTH.

-regards,
Larry