Re: [Help-gsl] Vector and matrix views

[Dimitris Papavasiliou]

Brian Gough wrote:

> In C a const object has to be initialised in a single step (because
> you can't modify it afterwards).  And initialisers can only contain
> constants, not expressions.  So to create a const object whose members
> depend on other variables you have to go through an indirection --
> create a non-const object first, then make it const.  That is where
> the struct comes from.  It's const on the outside but non-const on the
> inside.
>
> It would be cool to discover other ways to do it but I think that this
> might be the only way in C.
>  
Are you sure about that constant initialiser part?  That is necessary 
with "static" variables and for good reason: the intial values of static 
vars (which live in the heap) are written into the data section of the 
executable at compile time.  Therefore the compiler has to know what to 
write.

But the same shouldn't hold for const vars, unless I'm getting the whole 
thing wrong.  If I understand you correctly and you just want a function 
that returns "const gsl_vector" from an array then wouldn't this work:

+====8<== cut here ==8<====

#include <gsl/gsl_vector.h>

const gsl_vector _gsl_vector_const_view_array(double *BASE, int n)
{
   const gsl_vector v = {n, 1, BASE, NULL, 0};

   return v;
}

int main()
{
   double v_c[3] = {1, 2, 3};
   const gsl_vector v_g = _gsl_vector_const_view_array(v_c, 3);

   gsl_vector_fprintf(stdout, &v_g, "%f");

   /* uncomment this to make sure the compiler will complain */
   /* v_g.data = NULL; */
  
   exit (0);
}

+====8<== cut here ==8<====

I'm not sure wherher I can/should attach this so I'm sending this 
in-line.  Just cut and paste it into a file, say foo.c and compile with 
something along the lines of

gcc -ansi foo.c `gsl-config --cflags --libs`

in UNIX.  I've included the -ansi switch to gcc to make sure it won't 
work just because of some GNU extension. 

This works as expected (or at least as I expected) and is functionally 
equivalent to the standard gsl function.  The only difference is the 
absence of the buffering struct.

Am I missing sometinhg?

Dimitris P.

PS: Thanks for the tip James!  It certainly looks better than my approach.