lec14 - CSE 8A Lecture 14 2D arrays Arrays and the Java...

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CSE 8A UCSD LEC 14
CSE 8A: Lecture 14
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2D arrays
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Arrays and the Java type system
✔
Arrays and Vectors
(Reading: Savitch, Ch. 6)

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CSE 8A UCSD LEC 14
Midterm Exam #2
Midterm Exam #2 will be Thu Feb 28, during lecture time.
Place: Lecture room
Closed-book, closed-notes, no calculators. Bring picture ID!
Coverage: Chapters 1-6, Lectures 1-14, Assignments P1-P6
A practice midterm is available online (PDF format)
Lecture notes are available online
Midterm review: Lecture Tues Feb 26

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CSE 8A UCSD LEC 14
Multidimensional arrays in Java
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An array is a sequence of variables, all of the same type
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these reference type pointers or primitive values are the elements of the array
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these elements can be of any type (primitive types, classes... even arrays!), but they
have to be of the same type
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So, it is possible in Java to have an array whose elements are themselves pointers to
arrays
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this is called an array of arrays, or a multidimensional array
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It is possible to have arrays of arrays of arrays, and arrays of arrays of arrays of arrays,
etc., etc.
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we will concentrate on 2-dimensional arrays: arrays whose elements are arrays of
primitive type values or of non-array class type pointers

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CSE 8A UCSD LEC 14
Declaring 2-dimensional arrays in Java
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An 2-dimensional array is declared with a declaration statement of the form
<typename> <identifier>[][];
or
<typename>[][] <identifier>;
or
<typename>[] <identifier>[];
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This declares
<identifier>
to be the name of an array whose elements are arrays
that contain elements of type
<typename>
.
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All of these are legal, but the second form is probably clearest and most readable...
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Examples:
int arr[][]; // declares arr to be an array of arrays of ints
Button[][] button; // button is an array of arrays of Buttons
boolean[] vals[]; // vals is an array of arrays of booleans

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CSE 8A UCSD LEC 14
Creating 2-dimensional arrays in Java
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As with all objects in Java, a mere declaration statement never creates an array of arrays
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To create a 2-d array in Java, you use
new
, the type of elements in the array, and two
bracketed integer expressions that specify the dimensions of the array:
new <typename> [ <len1> ] [ <len 2> ]
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<len1>
is the number of arrays;
<len2>
is the number of elements in each array
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The elements of the arrays are automatically initialized to default values: zero for
numerical types;
false
for boolean;
null
for class types
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Examples:
new int[25][4] // creates an array of 25 arrys of 4 ints each,
// total 100 ints, all initially 0
new Button[10][10] // creates an array of 10 arrays of 10 Button
// pointers each, total 100 pointers, all
// null (this creates no Button objects!)
new boolean[40][50] // creates an array of 40 arrays of 50
// booleans each, total 2000 booleans,
// all initially false

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CSE 8A UCSD LEC 14
Indexing 2-dimensional arrays
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You refer to an an element of a 2-d array by using the array name and two bracketed int
indexing expressions
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