Index operator allows constant time access to the data

The index operator ([]) is used to access an element from a sequential data in constant time. This fast random access is possible because data are managed in a contiguous memory layout.

A string for example, the index operator can be used to access specific characters.

str = "hello"
str[0] # => "h"

str[0] = 'H'
str # => "Hello"

Another example would be an array. When an array is declared, it allocates a block of contiguous memory allowing you to access, retrieve, and/or modify data in constant time.

int arr = {1,1,2,3,4}; // block of memory allocated for 5 ints
arr[0] = 0;
arr // => [0, 1, 2, 3, 4]

If data are not stored contiguously in memory, constant-time access using the index operator is not possible. When data are laid out adjacent to each other, the system can locate any element by calculating an offset from the first element.