Bash - arithmetic in array index

Question

Say I have an array arr and an index x. How do I assign something to the array at index x+1? I'm getting bugs by trying the following, if one of them is correct I'd love to know which one and if not what am I doing wrong?

arr[$x+1]="hi"      # Doesn't work
arr[$((x+1))]="hi"  # Nope

Answer 1

Almost there.

arr[(($x+1))]="hi"