Transcript Arrays of Primitives

Introduction to Programming
with Java, for Beginners
Arrays
What if our Frog (from lab) could say 10
different things?
public class Frog{
private boolean formerPrince;
private String phrase1;
private String phrase2;
private String phrase3;
private String phrase4;
private String phrase5;
private String phrase6;
private String phrase7;
private String phrase8;
private String phrase9;
private String phrase10;
...
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What a Person could adopt lots of Pets ?
public class Person{
private String name;
private
private
private
private
private
private
private
private
private
private
private
private
private
private
private
private
. . .
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
Pet
pet1;
pet2;
pet3;
pet4;
pet5;
pet5;
pet6;
pet6;
pet8;
pet9;
pet10;
pet11;
pet12;
pet13;
pet14;
pet15;
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What if we wanted to store 500 Itoons?
public class IToonList{
private IToon toon1;
private IToon toon2;
private IToon toon3;
private IToon toon4;
private IToon toon5;
private IToon toon6;
private Ttoon toon7;
private IToon toon8;
private IToon toon9;
private Ttoon toon10;
private IToon toon11;
private IToon toon12;
private IToon toon13;
. . .
private IToon toon500;
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What if we want to store lots of things…

But we don’t want to declare a separate
variable for each one?
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That’s what arrays are good for
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What is an Array?
It’s an easy way to declare lots of variables
that all have the same type.
int[] data;
data = new int[5];
// array of 5 ints
data[0] = 6;
data[1]= 10;
data[2] = 12;
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Array Elements and Indices
 The “data” array has 5 elements. The value of
 data[0] is 6
 data[1] is 10
 data[2] is 12
 data[3] is 0
 data[4] is 0
 The number within square brackets is called an index
 The valid indices are 0 thru (array length - 1)
 NOTE: Whenever you see square brackets [ ] in a Java
program, it means you’re dealing with an array.
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An Array is an Object
int[] data;
data is a reference variable
whose type is int[], meaning
“array of ints”. At this point its
value is null.
data = new int[5];
The new operator causes a
chunk of memory big enough for
5 ints to be allocated on the
heap. Here, data is a assigned a
reference to the heap address.
Initially, all five ints are 0. Here,
three of them are assigned other
values.
data[0] = 6;
data[1]= 10;
data[2] = 12;
int[] info = {6, 10, 12, 0, 0};
info = new int[]{6, 10, 12, 0, 0};
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Using Array Elements in Expressions
 An element of an array of ints can be used virtually
anywhere an expression of type int is valid.
 Likewise for arrays of other types
int[] data = new int[] {6, 10, 12, 0, 0};
int x = data[0];
data[3] = data[2];
data[4] = data[3] + data[2] * 2;
System.out.println(“data[0] is “ + data[0]);
// note order of operations below:
data[4] = Math.pow(2, data[4]);
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Accessing an Array’s Length

ArrayName.length gives size of the array
int[] data;
data = new int[5];
data[0] = 6;
data[1]= 10;
data[2] = 12;
// data.length is 5
int result = 0;
for (int i =0; i < data.length; i++){
result = result + data[i];
}
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Complete the sum(..) method
public class ArrayTool{
/**
* Takes an array of ints as an argument.
* returns the sum of all the integers in the array.
*/
public static int sum ( ?? ) {
}
}
Welcome to DrJava
> int[] data = new int[] {6, 10, 12, 0, 0};
> ArrayTool.sum(data)
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Array Out of Bounds Exceptions
public class ArrayTool{
public static int sum(int[] data){
int sum = 0;
for (int i = 0; i < data.length; i++){
sum = sum + data[i]; // sum += data[i];
}
return sum;
}
public static int sum2(int[] data){
int sum = 0;
for (int i = 0; i <= data.length; i++){ // error!
sum = sum + data[i]; // sum += data[i];
}
return sum;
}
}
> ArrayTool tool = new ArrayTool();
> int[] data = new int[] {6, 10, 12, 0, 0};
> tool.sum(data)
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> tool.sum2(data)
ArrayIndexOutOfBoundsException:
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Examples of Arrays of Other Primitive Types
double[] temps;
temps = new double[24];
temps[0] = 78.5;
temps[1] = 84.2;
boolean[] answers = new boolean[6];
. . .
if (answers[0]){ //do something }
char[] buffer = new char[500];
open a file for reading
while (more chars in file & buffer not full)
buffer[i++] = char from file
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Declaring & Initializing Arrays of Primitives
int[ ] info1 = { 2000, 100, 40, 60};
int[ ] info2 = new int[ ]{ 2000, 100, 40, 60};
char[ ] choices1 = { 'p', 's', 'q'};
char[ ] choices2 = new char[ ]{ 'p', 's', 'q'};
double[ ] temps1 = {75.6, 99.4, 86.7};
double[ ] temps2 = new double[ ] {75.6, 99.4, 86.7};
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Complete this method
public class ArrayTool{
/* Returns true if all integers in the
data array
* are positive, false otherwise.
*/
public static boolean allPositive(int[] data){
}
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One Solution
public class ArrayTool{
/* Returns true if all integers in the
data array
* are positive, false otherwise.
*/
public static boolean allPositive(int[]
data){
for (int i = 0; i < data.length; i++){
if (data[i] <= 0) {
return false;
}
}
return true;
}
}
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Another Solution
public class ArrayTool{
/* Returns true if all integers in the
data array
* are positive, false otherwise.
*/
public static boolean allPositive(int[]
data){
boolean result = true;
for (int i = 0; i < data.length; i++){
if (data[i] <= 0) {
result = false;
break; //break is java keyword
}
}
return result;
}
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break statement

For example on the previous slide:

Terminates the for loop if the value found is
negative
Used only with loop structures
 Rule: Terminates the innermost for, while

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2D Arrays
 Array
can have 2, 3, or more dimensions
 When declaring a variable for such an array, use a pair of
square brackets for each dimension
 For 2D arrays, the elements are indexed [row][column]
 Remember “RC” [row][column]
char[][] board;
board = new char[3][3]; // 3
columns
board[0][0] = ‘O’; // row 0,
board[1][1] = ‘X’; // row 1,
board[0][1] = ‘X’; // row 0,
rows, 3
column 0
column 2
column 1
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Nested Loops
 A “nested
loop” occurs when one loop is inside another
Examples
// doubly nested loop
for (expr; condition; expr){
..
for (expr; condition; expr){
..
}
}
// triply nested loop
while(condition){
for(expr; condition; expr){
while(condition){
…
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Nested Loops
 “Doubly
nested”: one loop inside another
 “Triply nested”: a loop within a loop within a loop
 Java does not place a limit on nesting levels
 Deeply nested loops can be hard to maintain/debug
 Fix: create a “helper method” to handle inner loops and
call that method in an outer loop
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Processing a 2D Array
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
Use a doubly-nested for-loop to process a 2D array
In this example we know the number of rows (3) and columns (5)
In general, it’s better not to use “magic numbers” (here the 3 and 5) in the
loop. It’s better to write code that handles the general case (see next slides)
char[][] data = new char[3][5];
for (int row = 0; row < 3; row++){
for (int col = 0; col < 5; col++){
System.out.print(data[row][col] + ‘\t’);
}
System.out.println();
}
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Processing a 2D Array (cont.)
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
Most 2D arrays have a uniform number of rows and colums
An “n x m” array (an “n by m array”) has n rows and m columns
// prints an array with dimensions “nWeeks x nDays”
public static void print(double[][] temps,int nWeeks, int
nDays){
for (int row = 0; row < nWeeks; row++){
for (int col = 0; col < nDays; col++){
System.out.print(temps[row][col] + “\t”);
}
System.out.println();
}
}
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Processing a 2D Array (cont.)


This illustrates a general purpose way to print a 2D array
It works even for “ragged” arrays, whose row lengths vary.
public static void printArray(int[][] data){
if (data == null || data.length == 0)
return;
for (int row = 0; row < data.length; row++){
for (int col = 0; col < data[row].length; col++){
System.out.print(data[row][col] + “\t”);
}
System.out.println();
}
}
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Ragged Array
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
Row lengths vary.
Motivation: save space
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3
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6
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2
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6
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int[] one = {1,2,3}
int[] two = {1,2,3,4,5,6}
int[] three = {1,2};
int[][] data = {one, two, three}
data[0].length
data[1].length
data[2].length
data[0] = three;
data[1] = two;
data[2] = one;
data[0].length
data[1].length
data[2].length
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