Re: math operations with base#prefix

[Robert Elz]

    Date:        Tue, 19 Sep 2023 09:52:21 +0100
    From:        "Kerin Millar" <kfm@plushkava.net>
    Message-ID:  <4c2e3d39-0392-41ae-b73c-3e17296a949c@app.fastmail.com>

  | On Tue, 19 Sep 2023, at 8:40 AM, Victor Pasko wrote:
  | > Thanks for your response.
  | > In my opinion, in let "<>" and $((<>)) constructs all variables should be
  | > evaluated, so that $-sign for them is to be  just optional
  |
  | You haven't thought this through.

You didn't think that through.

  | It would amount to an egregious break of backward compatibility,

That's correct, but

  | How would you expect for digits above 9 to be expressed for bases above 10?

that's completely missing the point Victor was making.

But Victor, they are all evaluated (in many shells, just once), if you
have

        a=10

then
        $(( a + 5 )) and $(( $a + 5 ))

achieve exactly the same thing.    To be postable however if we also
have

        b=a

then
        $(( b + 5 )) is quite likely to fail, whereas $(( $b + 5 )) will
work fine (after expansions it becomes $(( a + 5 )) which is evaluated as
above.

However, where the expansions are really needed is with

        o1=+
        o2=*

used in

        $(( 10 $o1 5 $o2 6 ))

If we were to write it instead as

        $(( 10 o1 5 o2 6 ))

then the expression parser would have no idea at all how to
interpret that, as var names are not evaluated until needed in
the expression - what's there is just meaningless nonsense.  But
when the variables have the $ ahead of them, the expression parser
sees
        $(( 10 + 5 * 6 ))

and can build the correct evaluation tree to generate 40 as the answer.
(The "10" could have been any of 'a' '$a' or '$b' given the values assigned
earlier, that would make no difference, the ' chars there are just for
clarity in this e-mail, not to be included).

Much more complex expressions, which rely upon vars being expanded before
the expression is parsed, can also be created, but not really needed here.

kre