Re: Problem with how to assign an array

[Greg Wooledge]

On Thu, Feb 24, 2011 at 02:55:13PM -0500, Steven W. Orr wrote:
> I have three arrays
> 
> a1=(aaa bbb ccc ddd)
> a2=(qqq www eee rrr)
> a3=(fff ggg hhh)
> 
> I then set a_all
> 
> a_all=("${a1[*]}" "${a2[*]}" "${a3[*]}"

Missing ).  Also, a far more serious problem, you used * when you should
have used @.

a_all=("${a1[@]}" "${a2[@]}" "${a3[@]}")

This is absolutely critical.  Without this, you are no longer maintaining
the integrity of each element.  In fact, what you've got there will create
a_all with precisely 3 elements.  Each of these elements will be a
concatenation of the other arrays' elements with a space (or the first
char of IFS if you've set that) between them.

> Later on, I decide which element of a_all I really want. So with my new 
> index calculated, I say
> 
> real_a=( ${a_all[index]} )

This is also wrong.  This one does word-splitting on the element you're
indexing, and the resulting set of words becomes a new array.

In fact, I can only guess what you're trying to do here.

If you want to assign a single element to a scalar variable, you
should do:  element=${array[index]}

> And it worked really well until today. The problem is that I need an 
> element of the aNs to be allowed to be null values.

No problem.

> Like this:
> 
> a1=(aaa bbb ccc ddd '' jjj kkk lll)

No problem.

> such that if index is 0, I'd like real_a to end up with 8 elements instead 
> of 7.

Huh?  You mean during the concatenation?  (You changed the array name?)
Do it correctly:

imadev:~$ unset a1 a2 big; a1=(a b '' c) a2=(d e f '')
imadev:~$ big=("${a1[@]}" "${a2[@]}")
imadev:~$ printf "<%s> " "${big[@]}"; echo
<a> <b> <> <c> <d> <e> <f> <> 

> I could create a sentinel, I could use a case statement, I could create all 
> kinds of confabulations, but I'd like to know if there's a better way to do 
> it.

Huh?

> I literally tried everything I could think of.

You must learn the difference between "$*" and "$@".  (And the analogous
treatment of * and @ in an array indexing context.)

imadev:~$ wrong=("${a1[*]}" "${a2[*]}")
imadev:~$ printf "<%s> " "${wrong[@]}"; echo
<a b  c> <d e f > 

If you don't use the right syntax, you're going to have problems with
elements that contain whitespace (or IFS characters) as well as empty
elements as you already noted.