Re: [bug-gawk] eval in gawk as a regular command

[Andrew J. Schorr]

On Fri, Nov 09, 2018 at 10:48:51PM -0600, Peng Yu wrote:
> > On Wed, Dec 30, 2015 at 07:06:29AM -0600, Peng Yu wrote:
> > > Why not, considering that many other languages have eval? This can
> > > dramatically increase the expression power of a language.
> >
> > In addition to Arnold's points, I think it's worth mentioning that
> > gawk already has the ability to call functions indirectly and the
> > ability to access variables indirectly through the SYMTAB array. So
> > there are already some safer language facilities that allow you
> > to execute code that is a function of the input.
> 
> Could you show me some examples of how to mimic some feature of eval()
> using the features that you mentioned? Thanks.

Wow -- I don't think I have ever before received a reply to a message
I sent almost three years ago.

You can consult the manual for documentation and examples. Indirect
function calls are discussed here:

https://www.gnu.org/software/gawk/manual/html_node/Indirect-Calls.html

And there's a cute SYMTAB example here:

https://www.gnu.org/software/gawk/manual/html_node/Auto_002dset.html

   The SYMTAB array is more interesting than it looks. Andrew Schorr points out 
that it effectively gives awk data pointers. Consider his example:

   # Indirect multiply of any variable by amount, return result

   function multiply(variable, amount)
   {
       return SYMTAB[variable] *= amount
   }
   You would use it like this:

   BEGIN {
       answer = 10.5
       multiply("answer", 4)
       print "The answer is", answer
   }
   When run, this produces:

   $ gawk -f answer.awk
   -| The answer is 42

With these two capabilities, you have a fair bit of flexibility if you put your
mind to it.

Regards,
Andy