Re: associative array questions

[Ray Andrews <rayandrews@xxxxxxxxxxx>]

On 2022-12-11 11:53, Bart Schaefer wrote:
> There are no pointers to data structures in zsh. 
I know, I'm trying to fake it.
> Parameter reference
> is always by value.  Even ${(P)other} returns a value, it's just the
> value of the parameter whose name is in turn the value of $other.
> Anything you do with the result of ${(P)other} acts on that value, not
> on the indirectly-named parameter itself.
So it seems.  I thought the (P) might force the issue.
> There is one sort-of exception:  The ${NAME=WORD} and related
> assign-and-substitute operations.  E.g., if you use
> ${(P)other::=text}, then the parameter named by $other is assigned the
> value "text".  I say this is only sort-of an exception because if
> $other names an array, you can only assign to the entire array, not to
> individual subscripted fields.
>
> If you want to assign indirectly to a particular field (whether a
> position in an ordinary array or a named element in an associative)
> you have to store the entire subscript expression in $other, such as
>    other="aa[second]"
>    : ${(P)other::='tea for two'}
That works.  I can intuit that parsing could be easier given the two 
stages, 'aa[second]' can only be interpreted one way.
> Assignments are only parsed as such when there is an identifier
> (string of nothing but alphanumerics or underscore, plus optional
> subscript) to the left of the equal sign BEFORE any substitutions are
> performed.
Before?  Nuts I thought the substitutions always happened first. 
Obviously not.  So leave expansion complexities on the right hand side.
>   So this doesn't "look like" an assignment to the parser.

set -x reports it as looking like nothing.

> Lacking the outer ${...} enclosing the subscript, ${(P)aarray} expands
> to all the values of the pointed-to associative array, and you get the
> equivalent of
>
>    "1" "two" "three blind mice"[second]
Exactly.  I was trying to force it to accept it as an identifier.

> You could also do (another thing that doesn't work in older zsh)
>    eval "${aarray}+=([second]='tea for two')"
Of course I'm trying to avoid the eval entirely.  You've always advised 
against it.

> Nope. It expands to a value, not a variable reference. Zsh doesn't
> (yet) have what ksh calls "namerefs" ... block out everything you know
> about pointers in C.
Well, at least I knew I was sinning.  Purely curiosity tho, I have no 
real issue to solve.  So we still have functionality to steal from ksh, 
that's interesting.
> Associative arrays are unordered hash tables.  It might print "upside
> down" or "inside out" depending on what keys are present.

Ok, so no order at all.  That's quite understandable, it would be extra 
overhead for the shell to even think about it and there's no need.

Thank you Sensei.