Array iteration comes in two flavors, foreach and the classic for-loop:

a=(1 2 3 4)
# foreach loop
for y in "${a[@]}"; do
    # act on $y
    echo "$y"
done
# classic for-loop
for ((idx=0; idx < ${#a[@]}; ++idx)); do
    # act on ${a[$idx]}
    echo "${a[$idx]}"
done

You can also iterate over the output of a command:

a=($(tr ',' ' ' <<<"a,b,c,d")) # tr can transform one character to another
for y in "${a[@]}"; do
    echo "$y"
done