How to both get the original and reverse order of an array?

Question

So, Let's say i have an array arr, with two element in it:

read -a arr <<< "$@"

where i would then either use it in a function or script and input two string or element like so:

read_me() {
read -a arr <<< "$@"
}
read_me "first test"

Now i already know how to get through all the elements of an array:

for i in "${arr[@]}"
do
    echo "$i" # where i do something with the respective element of said array.
done

But that only do it using the normal/original order in which the element were added to the previously mentioned array... Of course, i also know how to get the elements of an array in reverse order:

indices=( ${!arr[@]} )
for ((i=${#indices[@]} - 1; i >= 0; i--)) ; do
    echo "${arr[indices[i]]}"
done

Both of these ways work as intended. Problem though, is that i need both the normal and the reverse order on the same loop. Mostly so i wouldn't need to do this:

echo "${arr[0]}" "${arr[1]}"
echo "${arr[1]}" "${arr[0]}"

How could i do this in a single loop?

Answer 1

array=( 1 2 3 a b c )

for i in "${!array[@]}"; do
    j=$(( ${#array[@]} - i - 1 ))
    printf '%s\t%s\n' "${array[i]}" "${array[j]}"
done

Output:

1       c
2       b
3       a
a       3
b       2
c       1

In short, there is nothing stopping you from traversing the array in any order and at the same time calculate a new index from the current index and use that too.

---

In comments to the question, there is a suggestion for the following arithmetic loop:

for (( i = 0, j = ${#array[@]} - 1; i < ${#array[@]}; ++i, --j ))
do
    printf '%s\t%s\n' "${array[i]}" "${array[j]}"
done

This uses the fact that the comma operator can be used in the initialization and update parts of the loop header to maintain two separate loop variables.

---

Depending on what you want to achieve and depending on what your actual array values are, you may also get away with using tac:

$ paste <( printf '%s\n' "${array[@]}" ) <( printf '%s\n' "${array[@]}" | tac )
1       c
2       b
3       a
a       3
b       2
c       1