Re: var_create_internal

[John Darrington]

On Wed, Jan 07, 2009 at 05:17:40PM -0500, Jason Stover wrote:
     Question:
     
     /* Create a variable to be used for internal calculations only */
     struct variable *
     var_create_internal (int case_idx)
     {
       struct variable *v = var_create ("$internal", 0);
     
       struct vardict_info vdi;
     
       vdi.dict = NULL;
       vdi.dict_index = 0;
       vdi.case_index = case_idx;
     
       var_set_vardict (v, &vdi);
     
       return v;
     }
     
     /* Dictionary data stored in variable. */
     struct vardict_info
       {
         int dict_index;     /* Dictionary index containing the variable. */
         int case_index;     /* Index into case of variable data. */
         struct dictionary *dict;  /* The dictionary containing the variable */
       };
     
     What is the "case of variable data"?

It's the index which needs to be passed to case_data_idx in order to
access the value of this variable.

     All the examples I see in
     src/language/stats show a value of either 0, 1 or 2.

"Internal" variables are a convenience object, typically used when
calculations require, or benefit from,  a temporary casereader.
Copying, and iterating a casereader with a small number of variables
is cheaper than copying and iterating the input data, which can,
potentially have a very large number of variables.  That's why these
indices are small.

J'


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