Lecture Set 15 Two Dimensional Arrays 2 dimensional arrays 1 1 2 Ragged Arrays Rectangular Arrays CMSC 131 Spring 2008 Jan Plane adapted from Bonnie Dorr Recall Arrays Arrays sequences of elements from the same base type int a Date d Base type may be array of ints array of references to Dates Primitive i e int Reference i e Date other objects Arrays are also objects Notice the similarities Arrays created using new Array elements stored on heap Array variables store references to space on the heap CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 1 Allocation of Space Syntax for allocating space for the 1st level array char a a new char 3 Array of char arrays Create array of 3 arrays Syntax for allocating space for the 2nd level of arrays a 0 new char 4 a 1 new char 6 a 2 new char 3 Create array of 4 char Create array of 6 char Create array of 3 char CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 2 1 Example char a a new char 3 a 0 new char 4 a 1 new char 6 a 2 new char 3 a 1 3 a Stack Heap a This array has two dimensions rows columns This kind of array is called ragged because the rows are of unequal length CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr a 3 Questions char a a new char 3 a 0 new char 4 a 1 new char 6 a 2 new char 3 What does a 1 2 x do Set element in row 2 column 3 to x What does a length return 3 What does a 1 length return 6 What type is a a reference to an array of array references What Type is a 0 a reference to an array of characters What type is a 0 0 a character CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 4 Initializers In one dimension char a a new char 3 a 0 a b c d a 1 x y z a 2 m n In two dimensions char a a b c d x y z m n CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 5 2 Rectangular Arrays Often we want 2 dimensional arrays in which rows have the same length Tables Matrices Java has a special short hand syntax for creating rectangular arrays int a new int 2 4 2 rows 4 cols Equivalent to int a new int 2 a 0 new int 4 a 1 new int 4 The short hand takes care of allocating each row initializing each cell in each row CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 6 Example int a new int 2 4 Stack Heap a Note each cell is initialized to default value 0 Each row is a 1 dim array 0 0 0 0 0 0 0 0 CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 7 2 D Arrays of Objects Also Possible Of Strings String s new String 4 2 s 0 0 Fred s 1 1 Jane Of Cats Cat c new Cat 4 2 c 0 0 new Cat Fred c 1 1 new Cat Jane CMSC 131 Spring 2008 Jan Plane adapted by Bonnie Dorr 8 3
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