String variables

[Ethan M. <merritt@u.washington.edu>]

On Thursday 08 July 2004 03:27 am,  
Hans-Bernhard Broeker <br...@ph...> wrote:
>
> > I also have some largish patches corresponding to implementations
> > of several different flavors of user-accessible string variables.
> > These have been discussed off and on over the last year or so,
> > but I never sensed a real consensus as to which approach was
> > best.
>
> I think that means we'll need a summary of their differences to
> start off that discussion again.  As far as I remember, there
> were several open issues we never decided upon conclusively:

I need to clean them up, modify them to apply against the current
cvs source, and generally re-familiarize myself with the code.  Then
I'll update or replace the descriptions on SourceForge and post them
to the list as well.

> 1) how deep down into the expression evaluator do we put them, i.e. do
> we only need string-valued variables controlled by a 'set' or 'let'
> command of their own, or should 'string' become a third native type of
> expression, in addition to integer and complex, meaning that we could
> have string operators, string arguments and results in both internal and
> user-defined functions?

That would be the hard part, yes.

> 3) As a follow-up to 2), how to expand variables into strings? 
> Bash-like $var, or C-like sprintf() style? 

I don't quite see how the latter would work in general.  Do you mean

    set label 1 "After fitting, A = "+printf(A)+"  B = "+ printf(B)

I find that incredibly ugly and unreadable compared to

    set label 1 "After fitting, A = $A  B = $B"

Or did you mean

    set label 1 sprintf( "After fitting, A = %3.2f, B = %3.2f", A, B)

That variant does have the virtue of allowing format specifiers.

But I don't really see how either of these would work on the
command line. The existing "userstrings" patch allows

    STYLE1 = "1:4 with lines lw 2"
    plot 'datafile' using $STYLE1

or even

    Source1 = " 'data1' using 1:4"
    Source2 = " 'data2' using 2:3"
    plot $Source1 with lines, $Source2 with points

How does printf/sprintf fit in here? 

Admittedly the userstrings patch would probably better be
described as implementing macro definitions rather string variables,
but it raises the same issues of acceptable syntax.

> $ already being used for at 
> least two entirely unrelated things in gnuplot

The primary use is for column numbers.
The second use I know about is for the command line 
variables ($1, $2, $3...).  

But of those are easily avoided simply by requiring that string
variables start with a letter, which we would want to do
anyhow to avoid parsing problems.

Are there any other uses?

> I would prefer the latter.

I prefer $, although I have no particular objection to
using some other symbol or syntax.  There was an earlier
suggestion to use %(VARIABLE) instead of $VARIABLE.

Even so I agree that *if* we allow string functions, then
some equivalents to printf()  and sprintf() need to be built in. 
I never got as far as a solid implementation of string-valued
functions in my earlier attempts, however.

-- 
Ethan A Merritt       merritt@u.washington.edu
Biomolecular Structure Center
Mailstop 357742
University of Washington, Seattle, WA 98195