Re: [Help-gsl] Wrong result in vector from gsl_vector_alloc_col_from_matrix

[Peter Johansson]

On 11/14/2012 04:50 PM, Stephan Petzchen wrote:
> Hi Peter,
>
> thanks for your reply.
>
>
>
> Here is my code:
>
> uint coffset=4;
>
> /// 1 1 1 1 1 1 1
> /// 2 2 2 2 2 2 2
> /// 3 3 3 3 3 3 3
> /// 4 4 4 4 4 4 4
> gsl_matrix* m=gsl_matrix_calloc(4,7);
>
> for(int i=0;i < m->rows; i++)
> for(int j=0;j < m->columns; j++)
> m->data[i * m->tda + j]=i+1;
>
> //Expected:
> /// x x x x 1 x x
> /// x x x x 2 x x
> /// x x x x 3 x x
> /// x x x x 3 x x
> gsl_vector* v = gsl_vector_alloc_col_from_matrix(m,coffset);
>
> /// But GSL gives:
> /// x x x x 1 1 1
> /// 1 x x x x x x
> /// x x x x x x x
> /// x x x x x x x
> for(int i=0;i < v->size; i++)
> Assert.AreEqual(m->data[i * m->tda + coffset],v->data[i]);
Here is the problem. Rather than access data in 'v' directly, you should 
access the vector with function

gsl_vector_const_ptr(v, i);

which will take the stride into account which is 7 in this case.

Hope that helps.

Cheers,
Peter