Arrays

1. Arrays

2. Memory organization • Table at right shows 16 bytes, each consisting of 8 bits • Each byte has an address, shown in the column to the left

3. Arrays • A collection of variables, all of the same type. • Each variable in an array is accessed by an index. • An array of size n has indices from 0 to n-1. • An array occupies a contiguous block of memory. • The size of an array is fixed, specified at creation.

4. Accessing an array member • t type of elements in array • s size (in bytes) of an element of type t • b base address of array • address of element i is b + i * s

5. Array access example In Java, an int occupies 4 bytes: int [] a = new int[5]; The base address of ‘a’ is 21380002. Indices are 0, 1, 2, 3 and 4. Total size needed for array is 20 bytes (5 cells times 4 bytes per cell)

6. Array access example a[0] Where is a[0]? Address of a[0] is: b + s * i where b = 21380002, s = 4 and i = 0: 21380002 + 4 * 0 = 21380002 a[0] occupies bytes 21380002, 21380003, 21380004 and 21380005.

7. Array access example Where is a[3]? Address of a[3] is: b + s * i where b = 21380002, s = 4 and i = 3: 21380002 + 4 * 3 = 21380014 a[3] occupies bytes 21380014, 21380015, 21380016 and 21380017. a[3]

8. Array access example • Where is a[7]? There is no such array cell, so what happens if we try to access it? • In some languages (e.g. C and C++) the access is permitted, with unpredictable results. • Java throws an ArrayIndexOutOfBoundsException