Re: Problem with how to assign an array

[Steven W. Orr]

Ok.  Thanks for reading and responding. I thought I expressed myself but I see 
that I left too much out. I'll amplify below.


On 2/24/2011 3:42 PM, Greg Wooledge wrote:
> 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[*]}"

Yes, I forgot the closing paren.

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

I intentionally used the star instead of the atsign because I'm taking 
advantage of the fact that these array elements all have no whitespace. So, 
after the assignment to a_all, the value of a[0] is equal to the single string 
represented by "${a1[*]}" (with quotes).

If I were to use the atsign then I lose the boundaries of what was contributed 
by each of a1, a2 and a3. By selecting *which* index to use into a_all and 
assigning that value to real_a and deliberately not using quotes

real_a=( ${a_all[index]} )

I get all of the values I want in real_a.

I apologize for not making it clear, but the whole thing does depend on the 
values not containing any whitespace.

Now the wrinkle comes into play:

I need the values of a1 a2 and a3 to be allowed to contain possible null 
values (which I coded up as '').

The problem is that when I make my assignment to a_all, my null value 
disappears.

Instead of assigning

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

is it possible instead to say something like this:

a_all=(a1 a2 a3)

and then later when I calculate my index, use a fancy eval to get me what I 
want?

For example, I can say:

513 > a1=(aa1 aa2 aa3)
514 > a2=(aa21 aa22 aa23)
515 > a3=(aa31 aa32 aa33 aa34)
516 > a_all=(a1 a2 a3)
517 > r=($(eval echo "\${${a_all[1]}[@]}"))
518 > echo "${r[@]}"
aa21 aa22 aa23
*519 > echo ${#r[@]}
3

which is correct. But if I change a2 to contain a null value, I lose it:

520 > a2=(aa21 aa22 '' aa23)
521 > r=($(eval echo "\${${a_all[1]}[@]}"))
522 > echo ${#r[@]}
3
*523 > echo "${r[@]}"
aa21 aa22 aa23

instead of the 4 values that I want.

(many hours pass...)

I came up with a solution that works. I offer it up for posterity and as an 
apology for not being clear before. :-(

#! /bin/bash

a1=(
    SLES10      sles-10         sles10  0       '' wahoo
    RHEL5       rhel-5          rhel5   1
                        'stuffwith|areserved|delim|forlater' foobar
)
a2=(
    SLES11      sles-11         sles11  0       '')
a3=(
    SLES11SP1   sles-11-sp1     sles11sp1       0       ''
    RHEL6       rhel-6          rhel6           1       ''
    RHEL6q      rhel-6q         rhel6q          3       '')
a_all=(
    "${a1[@]}"
    "${a2[@]}"
    "${a3[@]}" )
ddsizes=( "${#a1[@]}"
    "${#a2[@]}"
    "${#a3[@]}" )
echo "ddsizes:${#ddsizes[@]}:${ddsizes[@]}"
ddstarts=(0)
curr=0
for (( ii=0; ii < ${#ddsizes[@]}; ii++ ))
do
    ddstarts+=( $(( ${ddsizes[ii]} + curr )) )
    (( curr+=ddsizes[ii] ))
done
echo "ddstarts:${ddstarts[@]}"

for ((ii=0; ii<2; ii++
do
    real_a=("${a_all[@]:${ddstarts[ii]}:${ddstarts[ii+1]}-${ddstarts[ii]}}")
    echo "real_a:${#real_a[@]}:${real_a[@]}"
done






> 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.


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