How to organize dynamic use of for?

Question

bash

It is necessary to go through all the elements of the array with a step of 2, but the number of elements is not known in advance.
more or less like this:

#!/bin/bash
arr=([1]=1 [2]=2 3 [5]=4 5);
for i in {0..${#arr[*]}..2};do
 printf "[$i]:${arr[$i]}\n";
done

But when processing throws out:

arr.sh: line 4: {0..5..2}: syntax error: expected operand (wrong label "{0..5..2}")

Answer 1

#!/bin/bash
arr=([1]=1 [2]=2 3 [5]=4 5);
eval 'for i in {0..'${#arr[*]}'..2};do
 printf "[$i]:${arr[$i]}\n";
done'

This brace expansion only works with literals. The above solution generates a loop body with literals and puts it in eval.