The problem: when reading algorithms, beginners don't immediately recognize key[5] means the key value at array position 5. A naive reader gets flooded with too many symbols, variable, arrays, functions.
It's good to stick to the notation "[in1]", [in2] as far as possible. "in" means integer index. When there's no ambiguity, use [i], [j].
In your own writing, It's good to reserve the square bracket [] for array index only, if possible. Book authors don't follow this but don't worry.
next[x], prev[] refer to nodes in a linked list. Sometimes [x] doesn't refer to integer indices in an array -- unnecessary confusion.
