Using:

set -o nounset

1. Having an indexed array like:

   myArray=( "red" "black" "blue" )
   

   What is the shortest way to check if element 1 is set?
   I sometimes use the following:

   test "${#myArray[@]}" -gt "1" && echo "1 exists" || echo "1 doesn't exist"
   

   I would like to know if there's a preferred one.

2. How to deal with non-consecutive indexes?

   myArray=()
   myArray[12]="red"
   myArray[51]="black"
   myArray[129]="blue"
   

   How to quick check that 51 is already set for example?

3. How to deal with associative arrays?

   declare -A myArray
   myArray["key1"]="red"
   myArray["key2"]="black"
   myArray["key3"]="blue"
   

   How to quick check that key2 is already used for example?