CS1001

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CS1001CS1001

Lecture 14Lecture 14

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OverviewOverview

Java ProgrammingJava Programming ArraysArrays

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GoalsGoals

Understand the basics of Java Understand the basics of Java programmingprogramming

Control Statements and ArraysControl Statements and Arrays

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AssignmentsAssignments

Brookshear: Ch 4, Ch 5 (Read)Brookshear: Ch 4, Ch 5 (Read) Read linked documents on these Read linked documents on these

slides (slides will be posted in slides (slides will be posted in courseworks)courseworks)

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ArithmeticArithmetic Operators: Operators: ++, , --, , //, , ** , , %%

The precedence of operators and The precedence of operators and parentheses work the same way as in parentheses work the same way as in algebra.algebra.

m % nm % n means the remainder when means the remainder when mm is divided by is divided by nn (e.g. 17 % 5 is 2). (e.g. 17 % 5 is 2).

%% has the same rank as has the same rank as // and and **

Same-rank binary operators are Same-rank binary operators are performed in order from left to right.performed in order from left to right.

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Arithmetic (cont’d)Arithmetic (cont’d)

The type of the result is determined by The type of the result is determined by the the typestypes of the operands, not their of the operands, not their values; this rule applies to values; this rule applies to all all intermediate resultsintermediate results in expressions. in expressions.

If one operand is an If one operand is an intint and another is a and another is a doubledouble, the result is a , the result is a doubledouble; if both ; if both operands are operands are intints, the result is an s, the result is an intint..

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Caution:Caution: if if aa and and bb are are intints, then s, then a a // b b is is truncatedtruncated to an to an intint……

1717 // 5 gives 5 gives 33 33 // 4 gives 4 gives 00

……even if you assign the result to a even if you assign the result to a doubledouble::

double ratio = 2double ratio = 2 // 3; 3;The double type of the result doesn’t help: ratio still gets the value 0.0.

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Arithmetic (cont’d)Arithmetic (cont’d) To get the correct To get the correct doubledouble result, use result, use doubledouble

constants or the constants or the castcast operator: operator:

double ratio = double ratio = 2.02.0 // 3; 3;

double ratio = 2double ratio = 2 // 3.03.0;;

double factor = double factor = (double)(double) m m // (double)(double) n; n;

double factor = mdouble factor = m // (double)(double) n; n;

double r2 = kdouble r2 = k // 2.02.0;;

double r2 = double r2 = (double)(double) k / 2; k / 2;Casts

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Arithmetic (cont’d)Arithmetic (cont’d) Caution: Caution: the range for the range for intints is from s is from

--223131 to 2 to 23131--1 (about 1 (about --2·102·1099 to to 2·102·1099))

Overflow is Overflow is notnot detected by the Java detected by the Java compiler or interpretercompiler or interpretern = 8 10^n = 100000000 n! = 40320n = 9 10^n = 1000000000 n! = 362880n = 10 10^n = 1410065408 n! = 3628800n = 11 10^n = 1215752192 n! = 39916800n = 12 10^n = -727379968 n! = 479001600n = 13 10^n = 1316134912 n! = 1932053504n = 14 10^n = 276447232 n! = 1278945280

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Use compound Use compound assignment assignment operators:operators:

a a == a a ++ b; b; a a +=+= b; b;

a a == a a -- b; b; a a -=-= b; b;

a a == a a ** b; b; a a *=*= b; b;

a a == a a // b; b; a a /=/= b; b;

a a == a a %% b; b; a a %%== b; b;

Use increment Use increment and decrement and decrement operators:operators:

a a == a a ++ 1; 1; a a++++;;

a a == a a -- 1; 1; a a----;;

Do not use these in larger expressions

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Review:Review: What is a variable? What is a variable? What is the type of variable that holds What is the type of variable that holds

an object?an object?

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Review (cont’d):Review (cont’d): What is the range for What is the range for intints?s? When is a cast to When is a cast to doubledouble used? used? GivenGiven

double dF = 68.0;double dF = 68.0;double dC = 5 double dC = 5 / / 9 9 ** (dF (dF -- 32); 32);

what is the value of what is the value of dCdC?? When is a cast to When is a cast to intint used? used? Should compound assignment Should compound assignment

operators be avoided?operators be avoided?

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Objectives:Objectives:

Learn about arrays and when to use Learn about arrays and when to use themthem

Learn the syntax for declaring and Learn the syntax for declaring and initializing arraysinitializing arrays

Learn how to access array’s size Learn how to access array’s size and elementsand elements

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What is an ArrayWhat is an Array

An array is a block of consecutive memory An array is a block of consecutive memory locations of the same data type.locations of the same data type.

Individual locations are called array’s Individual locations are called array’s elementselements..

Sometimes when we say “array’s element” we Sometimes when we say “array’s element” we mean the mean the valuevalue stored in that element. stored in that element.

1.39

1.69

1.74

0.0

An array of doubles

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What is an Array What is an Array (cont’d)(cont’d)

Rather than treating each element as Rather than treating each element as a separate named variable, the whole a separate named variable, the whole array gets array gets oneone name. name.

Specific array elements are referred to Specific array elements are referred to by using array’s name and the by using array’s name and the element’s number, called element’s number, called indexindex or or subscriptsubscript..

c[0] c[1] c[2] c[3]

1.39

1.69

1.74

0.0 o is array’s

name

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Indices Indices (Subscripts)(Subscripts)

In Java, an index is written within square In Java, an index is written within square brackets following array’s name (e.g., brackets following array’s name (e.g., a[k]a[k]).).

Indices start from 0; the first element of Indices start from 0; the first element of an array an array aa is referred to as is referred to as a[0]a[0] and the and the nn--th element as th element as a[n-1]a[n-1]..

An index can have any An index can have any intint value from 0 to value from 0 to array’s length array’s length -- 1. 1.

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Indices Indices (cont’d)(cont’d)

We can use an We can use an intint variable or any variable or any expression that evaluates to an expression that evaluates to an intint value as an index:value as an index:

a [3] a [k] a [k - 2] a [ (int) (6 * Math.random()) ]

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Indices Indices (cont’d)(cont’d)

In Java, an array is declared with In Java, an array is declared with fixed length that cannot be changed.fixed length that cannot be changed.

Java interpreter checks the values of Java interpreter checks the values of indices at run time and throws indices at run time and throws IndexOutOfBoundsExceptionIndexOutOfBoundsException if an if an index is negative or if it is greater index is negative or if it is greater than the length of the array than the length of the array -- 1. 1.

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Why Do We Need Why Do We Need Arrays?Arrays? The power of arrays comes from the fact The power of arrays comes from the fact

that the value of a subscript can be that the value of a subscript can be computed and updated at run time.computed and updated at run time.

int sum = 0;sum += score0;sum += score1;…sum += score999;

Before (no arrays):

int n = 1000;int sum = 0, k;

for (k = 0; k < n; k++) sum += scores[k];

After (with arrays):

1000times!

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Why Arrays? (cont’d)Why Arrays? (cont’d) Arrays give Arrays give direct accessdirect access to any to any

element — no need to scan the array.element — no need to scan the array.

if (k == 0) display (score0);else if (k == 1) display (score1);else … // etc.

Before (no arrays):

display (scores[k]);

After (with arrays):

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Arrays as ObjectsArrays as Objects In Java, an array is an object. If the In Java, an array is an object. If the

type of its elements is type of its elements is anyTypeanyType, the , the type of the array object is type of the array object is anyType[ ]anyType[ ]..

There are two ways to declare an array:There are two ways to declare an array:

anyType [ ] arrName; The difference becomes significant only when several variables are declared in one statement:

int [ ] a, b; // both a, b are arrays

int a [ ], b; // a is an array, b is notanyType arrName [ ];

or

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Arrays as Objects Arrays as Objects (cont’d)(cont’d) As with other objects, the declaration As with other objects, the declaration

creates only a reference, initially set to creates only a reference, initially set to nullnull. . An array must be created before it can be An array must be created before it can be used.used.

There are two ways to create an array:There are two ways to create an array:

arrName = new anyType [ length] ; Brackets, not parens!

or

arrName = new anyType [ ] { val1, val2, …, valN };

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Declaration and Declaration and InitializationInitialization When an array is created, space is When an array is created, space is

allocated to hold its elements. If a list allocated to hold its elements. If a list of values is not given, the elements get of values is not given, the elements get the default values.the default values.

scores = new int [10] ; // length 10, all values set to 0

words = new String [10000]; // length 10000, all values set to null

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Initialization (cont’d)Initialization (cont’d) An array can be declared an initialized in An array can be declared an initialized in

one statement:one statement:

int scores [ ] = new int [10] ; // length 10

private double gasPrices [ ] = { 1.49, 1.69, 1.74 };

String words [ ] = new String [10000];

String cities [ ] = {"Atlanta", "Boston", "Cincinnati" };

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Initialization (cont’d)Initialization (cont’d) Otherwise, initialization can be postponed Otherwise, initialization can be postponed

until later:until later:

String words [ ]; // not yet initialized ... words = new String [ console.readInt() ];

private double gasPrices [ ]; // not yet initialized … gasPrices = new double [ ] { 1.52, 1.69, 1.75 };

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Array’s LengthArray’s Length The length of an array is determined when The length of an array is determined when

that array is created.that array is created.

The length is either given explicitly or The length is either given explicitly or comes from the length of the comes from the length of the {…}{…} initialization list.initialization list.

The length of an array The length of an array arrNamearrName is referred is referred to in the code as to in the code as arrName.lengtharrName.length..

lengthlength appears like a public appears like a public fieldfield (not a (not a method) in an array object.method) in an array object.

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Review:Review: Why are arrays useful?Why are arrays useful? What types of elements can an array What types of elements can an array

have?have? How do we refer to an array’s How do we refer to an array’s

element in Java?element in Java? What happens if an index has an What happens if an index has an

invalid value?invalid value? How do we refer to the length of an How do we refer to the length of an

array?array?

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SortingSorting

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Alternate Sorting Alternate Sorting AlgorithmAlgorithm(Selection Sort)(Selection Sort)main(String[] args) {main(String[] args) {

int inputSize = args.length;int inputSize = args.length;int[] sortedArray = new int[inputSize];int[] sortedArray = new int[inputSize];for (i=0; i < inputSize; i++) {for (i=0; i < inputSize; i++) {

(1) Loop through args, find largest element (1) Loop through args, find largest element and and remember its index (ie for (int j = 0; …))remember its index (ie for (int j = 0; …))(2) Put the largest element into the ith location (2) Put the largest element into the ith location of sortedArray;of sortedArray;(3) Delete (set to -1) the ith element of args(3) Delete (set to -1) the ith element of args

}}

http://http://www.ee.unb.ca/brp/lib/java/selectionsortwww.ee.unb.ca/brp/lib/java/selectionsort//

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