[Gretl-devel] Backward compatibility

[Riccardo (Jack) Lucchetti]

On Mon, 5 Nov 2007, Allin Cottrell wrote:

>> 4) enable GRETL_TYPE_STRING as a function argument and store a
>> copy of the string in the global array, only marked at a
>> different level. The copy should be destroyed on function exit.
>
> Yes, IMO this is the only sane way to go.  I've made a start on
> it.  The modifications to the existing code are not too severe.
> I'll test tomorrow than make a snapshot available.

I tested this a little and it seems that the basic functionality is there 
and everything is reasonably solid.
For example, with CVS you can do fancy stuff like

<script>
function ha(string s)
   x = @s
   sprintf res "%g", x
   printf "%s = %s\n", @s, @res
   return scalar x
end function

nulldata 2
set echo off
set messages off

string foo = "3+4"
x = ha(foo)
x = ha("exp(2)")
</script>

and get

<output>
gretl version 1.6.6.pre2
Copyright Ramu Ramanathan, Allin Cottrell and Riccardo "Jack" Lucchetti
This is free software with ABSOLUTELY NO WARRANTY
Current session: 2007/11/06 10:46
? run strfunc.inp
/home/jack/Prove/gretl_test/strfunc.inp opened OK
? function ha(string s)
? nulldata 2
periodicity: 1, maxobs: 2,
observations range: 1-2
? set echo off
3+4 = 7
exp(2) = 7.38906
Done
</output>

This should be enough for the basic needs of user-level function coders. 
In due course, however, we will have to provide proper string handling 
functions, IMO. The ones that spring to my mind are

strlen(s)
left(s,n)
right(s,n)
trim(s)
upper(s)
lower(s)
split(s,c,n)

where s is a string, n is an integer and c is a character. String 
concatenation could conceivably be obtained via a dedicated function, or 
by overloading some operator ("+" and "~" are two of the most prominent 
candidates). The only non-obvious function in the above list is split(); 
an example should suffice:

split("bird,dizzy,monk,mingus", ",", 3)

should yield the string "monk". This is not only cool in itself, but, most 
importantly, it avoids the need for string arrays. Plus, varname(X) should 
be made a proper string function, returning the "label" field of variable 
X. Another one of this kind may be vardesc(X), returning the 
"display_name" field.

The only function that can be added with a small effort is strlen(), 
because it yields a number. All the others involve heavier work, because 
our genr() apparatus needs to be geared so it can handle strings. IMHO, we 
should work on this after release.

Comments?


Riccardo (Jack) Lucchetti
Dipartimento di Economia
Università Politecnica delle Marche

r.lucchetti at univpm.it
http://www.econ.univpm.it/lucchetti