Re: "${assoc[@]@k}" doesn't get expanded to separate words within compound assignment syntax

[Greg Wooledge]

On Sun, Mar 24, 2024 at 03:54:10PM -0500, Dennis Williamson wrote:
> The @K transform outputs key value pairs for indexed arrays as well as
> associative arrays (you used the @k transform which does word splitting and
> loses the k-v sequence).

The @K (capital) transformation gives you quoted strings which need to
be eval'ed.  Very Bourne-shell-ish.

The @k (lowercase) transformation gives you a list of alternating raw
key/value strings, like what you'd expect from a Tcl command.

hobbit:~$ unset -v hash kvlist
hobbit:~$ declare -A hash=([key 1]='value 1' [key 2]='value 2')
hobbit:~$ kvlist=( "${hash[@]@k}" )
hobbit:~$ declare -p kvlist
declare -a kvlist=([0]="key 2" [1]="value 2" [2]="key 1" [3]="value 1")
hobbit:~$ kvlist2=( "${hash[@]@K}" )
hobbit:~$ declare -p kvlist2
declare -a kvlist2=([0]="\"key 2\" \"value 2\" \"key 1\" \"value 1\" ")
hobbit:~$ eval kvlist3=\("${hash[@]@K}"\)
hobbit:~$ declare -p kvlist3
declare -a kvlist3=([0]="key 2" [1]="value 2" [2]="key 1" [3]="value 1")

kvlist2 is an undesired result.  Don't do that.  kvlist and kvlist3 are
both usable.

> Thus the @K allows preserving indices in a sparse
> indexed array.

Both of them do that:

hobbit:~$ sparse=(a b [42]=z)
hobbit:~$ echo "${sparse[@]@K}"
0 "a" 1 "b" 42 "z"
hobbit:~$ echo "${sparse[@]@k}"
0 a 1 b 42 z