I have an array which I would like to iterate through in reverse order from start index to end index. This is what I tried
limits=( 10 20 26 39 48 )
echo "forward"
for i in ${limits[@]:0:4}; do
echo "$i"
done
echo ""
echo "reverse"
for i in ${limits[@]:4:0}; do
echo "$i"
done
In bash, the indices of an array do not have to be contiguous:
$ limits=( 10 20 26 39 48 )
$ declare -p limits
declare -a limits=([0]="10" [1]="20" [2]="26" [3]="39" [4]="48")
but:
$ a[10]=x
$ a[20]=y
$ b=(p q r s t)
$ unset b[3]
$ declare -p a b
declare -a a=([10]="x" [20]="y")
declare -a b=([0]="p" [1]="q" [2]="r" [4]="t")
The naïve approach of counting the number of elements and looping from max to min should work on simple arrays but may fail on arrays that have been created piece-meal or had elements removed.
However, you can convert the indices to a contiguous range by defining a function and assigning the elements of the array as its parameters. Here's an example that also passes in the name of a command/function to run:
$ applyInReverse(){
local func=$1
shift
local i
for ((i=$#; i>0; --i)); do
"$func" "${@:i:1}"
done
}
then invoking with echo gives:
$ applyInReverse echo "${limits[@]}"
48
39
26
20
10
$ applyInReverse echo "${b[@]}"
t
r
q
p
$
