Array Review
Posted on: July 26, 2006 at 12:00 AM
Posted on: July 26, 2006 at 12:00 AM
Subscripts, Declaration, Allocation
Array Review
Subscripts, Declaration, Allocation
- Array subscripts start at 0.
- Array subscription checks bounds. May throw
ArrayIndexOutOfBoundsException. - Array declaration doesn't create an array.
int[] a; // Declares an array reference. a = new int[200]; // Allocates an array.
- Arrays are allocated on the heap, not on the stack.
- Copying an array reference doesn't copy the array.
int[] a = int[100]; int[] b;
- The declared size of an array is accessed with the
lengthfield.for (int i=0; i<a.length; i++) { ...
Array Initialization
- Array elements are initialized to 0 / null / false by default.
- Arrays may be allocated and initialized on declaration.
int a[] = {31, 28, 31, 30, 31};
Array Algorithms
- The
java.util.Arraysclass has many algorithms for working with arrays.equals(...)fill(...)sort(...)binarySearch(...)asList(...)- Do not confuse this with the
java.lang.reflect.Arrayclass orjava.sql.Arrayinterface.
System.arrayCopy(...)is another useful utility.- More algorithms are available for the Collections data structures, but not
arrays directly (see
asListto convert
Multidimensional Arrays
- This discussion is about two-dimensional arrays, but there's no limit on the number of dimensions.
- There are two kinds of two-dimensional array implementations
- Compiler maps two subscripts into one linear space. (C, C++)
- Array of arrays (C, C++, Java).
- The Array-of-array approach allows ragged arrays.
- Initilization is supported with nested braces.
- Example
int[][] a2 = { {1,2,4}, {5,5,7,8}, {9}}; . . . for (int row=0; row<a2.length; row++) { for (int col=0; col<a2[row].length; col++) { if (a2[row][col] == specialValue) { . . .
Beyond Arrays
- Arrays are very good for fixed-length data.
- They are bad for variable length data, leading to buffer overflow vulnerabilities in C and C++, or ArrayIndexOutOfBoundsException in Java.
- Insertions and deletions may give poor O() performance. LinkedList might be a good choice in this case.
- A better choice is often one of the Collections data structures: eg, ArrayList (or the older Vector), etc.