In bash, how do I get the index of the last element of an array without a loop

Question

In bash, is it possible to get the index of the last element of an array (that might be sparse) without looping through the entire array like so:

a=( e0 e1 ... )
i=0
while [ "$i" -lt $(( ${#a[@]} - 1 )) ]
do
  let 'i=i+1'
done
echo "$i"

Since at least bash v 4.2, I can get the value of the last element in an array using

e="${array[-1]}"

but that will not get me the positive index since other elements may have the same value.

Answer 1

In case of an array which is not sparse, last index is number of elements - 1:

i=$(( ${#a[@]} - 1 ))

To include the case of a sparse array, you can create the array of indexes and get the last one:

a=( [0]=a [1]=b [9]=c )

indexes=( "${!a[@]}" )
i="${indexes[-1]}"

echo "$i"
9

Answer 2

lst=( [0]=1 [1]=2 [9]=3 )
echo ${lst[@]@A}           # show elements and indexes
echo ${lst[-1]}            # last  element
echo ${!lst[@]}            # list of indexes
: ${!lst[@]} ; echo $_     # last index