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Re: Issue with 'key' for associative arrays

From: Greg Wooledge
Subject: Re: Issue with 'key' for associative arrays
Date: Wed, 25 Nov 2020 09:56:19 -0500
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, Nov 25, 2020 at 02:48:44PM +0100, Frans de Boer wrote:
> Addressing associative arrays with an array statement - like echo
> ${k["${a[x]}"] does not work.

That seems to work for me, after I fix your quoting.  Using your example

unicorn:~$ declare -A  k
unicorn:~$ declare -a  a
unicorn:~$ sString="devel packager's guide"
unicorn:~$ i=2
unicorn:~$ k["$sString"]=$i
unicorn:~$ a[$i]=$sString
unicorn:~$ echo "${k[${a[i]}]}"

> Next a script to clarify my point:
> ! /bin/bash

This shebang is wrong.

> unset -v 'k["${a[$i]}"]' # does not work

So, it's actually "unset -v" that doesn't work, not echo.

> sTemp=${a[$i]}
> unset -v 'k["$sTemp"]'   # does work

Yeah, I can easily believe that there are bugs or misfeatures hiding
in unset -v.  Personally, I'd avoid it entirely.

If you're using unset -v so that you can do existence testing ("make a
decision based on whether my associative array contains this key or not"),
then you can change that to a simple length test on the value.  Use a
non-empty string as the value.  Then your existence tests becomes
something like:

if [[ ${hash[$key]} ]]; then
  echo "key exists"

If you want to remove a key, just assign an empty string as the value,
rather than trying to use unset -v.

Yes, that's just a workaround to something that might very well be a bug.
You can either wait for a bug fix, and then realize that you still can't
actually USE unset -v in any of your scripts for the next 5-10 years, until
everyone in your target audience is on the fixed version of bash... or
you can use the workaround.

Yes, this doesn't work if you use set -u.  So don't use set -u.

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