Download Arrays

Arrays
Copyright © 2014 by John Wiley & Sons. All rights reserved.
1
Arrays
 An array collects a sequence of values of the same type
 Sequence of values are called elements
 The sequence of values are store under a single name.
 An array can be visualized as a series of boxes, each
capable of holding a single value belonging to this type:
 An array whose elements are arranged in a linear fashion is
said to be a one-dimensional array.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
2
Declaring arrays
 An array declaration contains [ and ] symbols (“square
brackets”):
int[] a;
 The elements of an array can be of any type. In particular,
array elements can be objects:
String[] b;
 It’s also legal to put the square brackets after the array
name:
int a[];
String b[];
Copyright © 2014 by John Wiley & Sons. All rights reserved.
3
Declaring Arrays
 Declaring an array variable doesn’t cause Java to allocate
any memory for the array’s elements.
 One way to allocate this memory is to use the new keyword:
a = new int[10];
 You can declare and reserve memory for array at the same
time:
int[] a = new int[10];
 The value inside the square brackets can be any expression
that evaluates to a single int value:
int n = 10;
int[] a = new int[n];
Copyright © 2014 by John Wiley & Sons. All rights reserved.
4
Initializing arrays
 When an array is created using new, the elements of
the array are given default values:
• Numbers are set to zero.
• boolean elements are set to false.
• Array and object elements are set to null.
 E.g int[] values = new int[10];
values
0
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
0
Copyright © 2014 by John Wiley & Sons. All rights reserved.
5
Arrays
 Array elements are indexed by position number
• Start index: 0
• End index: array length – 1
 Each index position uniquely identifies an element.
First index is always 0
a
0
1
2
3
4
Last index is always length-1
5
6
7
8
9
0 0 0 0 0 0 0 0 0 0
Copyright © 2014 by John Wiley & Sons. All rights reserved.
Index values
6
Initializing array element values
 Method 1: Initialize array at the time it’s declared
• specify the initial values when you create the array
double[] values ={32, 54, 67.5, 29, 35, 80, 115, 44.5, 100, 65 };
Do NOT use keyword new
You don’t need to specify number of elements.
0
1
2
3
4
5
6
7
8
9
32
54
67.5
29
35
80
115
44.5
100
65
 Method 2: use the [] operator
double[] values = new double[10];
• Specify index of element:
values[4] = 35;
values[1] = 20;
0
1
2
3
4
5
6
7
8
9
0
20
0
0
35
0
0
0
0
0
Copyright © 2014 by John Wiley & Sons. All rights reserved.
7
Accessing Array Elements
 Individual elements are accessed by an integer index i,
using the notation array[i] where array is the array
name
values[3]
 An array element can be used like any variable:
System.out.println(values[4]);
Result = 10.0 + values[2];
Copyright © 2014 by John Wiley & Sons. All rights reserved.
8
Arrays
Figure 1 An Array of Size 10
Copyright © 2014 by John Wiley & Sons. All rights reserved.
9
Syntax 7.1 Arrays
Copyright © 2014 by John Wiley & Sons. All rights reserved.
10
Programming Question
 Write a tester program called ArrayDemo. Class should declare an
int array called values of 5 elements. Assign each element the
value of its index. Print all elements along with the index.
 A sample run is shown:
values[0] = 0
values[1] = 1
values[2] = 2
values[3] = 3
values[4] = 4
Print the values variable. What happens?
Modify the program so that you have 1000 elements. As previous,
each element should store value of it’s index.
Try printing value at index=1000 . What happens?
Copyright © 2014 by John Wiley & Sons. All rights reserved.
11
Answer
ArrayDemo.java
public class ArrayDemo{
public static void main(String args[])
{
final int n = 10;
int values[] = new int[n];
for(int i=0; i<n;i++) {
values[i] = i;
}
for(int i=0; i<n;i++) {
System.out.printf("values[%d]=%d\n", i, values[i]);
}
System.out.println("values = "+values);
System.out.println("values[10]="+values[10]);
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
12
Copyright © 2014 by John Wiley & Sons. All rights reserved.
13
Arrays – Bounds Error
 A error you just saw is called a bounds error
 Bounds error occurs if you supply an invalid array index.
 Causes your program to terminate with a run-time error.
 Example:
double[] values = new double[10];
values[10] = value; // Error, non-existing index
Copyright © 2014 by John Wiley & Sons. All rights reserved.
14
Array Length
 values.length yields the length of the values array.
 There are no parentheses following length.
 When iterating through elements in an already
initialized array, we use this:
for(int i=0; i<values.length;i++)
{
values[i] = i;
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
15
Declaring Arrays
Copyright © 2014 by John Wiley & Sons. All rights reserved.
16
Array References
 Copying an array reference yields a second reference to
the same array. (like objects)
 When you copy an array variable into another, both
variables refer to the same array
int[] scores = { 10, 9, 7, 4, 5 };
int[] values = scores; // Copying array reference
 You can modify the array through either of the variables:
scores[3] = 10;
System.out.println(values[3]); // Prints 10
Copyright © 2014 by John Wiley & Sons. All rights reserved.
17
Array References
int[] scores = { 10, 9, 7, 4, 5 };
int[] values = scores; // Copying array reference
Figure 2 Two Array Variables Referencing the Same Array
Copyright © 2014 by John Wiley & Sons. All rights reserved.
18
Using Arrays with Methods
 Arrays can occur as method arguments and return
values.
 An array as a method argument
public void addScores(int[] values)
{
for (int i = 0; i < values.length; i++)
{
totalScore = totalScore + values[i];
}
}
• To call this method
int[] scores = { 10, 9, 7, 10 };
fred.addScores(scores);
 A method with an array return value
public int[] getScores()
Copyright © 2014 by John Wiley & Sons. All rights reserved.
19
Partially Filled Arrays
 Array length = maximum number of elements in array.
 Usually, array is partially filled
 Define an array larger than you will need
final int LENGTH = 100;
double[] values = new double[LENGTH];
 How do you find out how many elements are actually in
the array?
Copyright © 2014 by John Wiley & Sons. All rights reserved.
20
Partially Filled Arrays
 How do you find out how many elements are actually
in the array?
• Use companion variable to keep track of current size:
• call it currentSize
Copyright © 2014 by John Wiley & Sons. All rights reserved.
21
Partially Filled Arrays
 A loop to fill the array
int currentSize = 0;
Scanner in = new Scanner(System.in);
while (in.hasNextDouble())
{
if (currentSize < values.length)
{
values[currentSize] = in.nextDouble();
currentSize++;
}
}
 At the end of the loop, currentSize contains the actual
number of elements in the array.
 Note: Stop accepting inputs when currentSize reaches the
array length.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
22
Partially Filled Arrays
Copyright © 2014 by John Wiley & Sons. All rights reserved.
23
Partially Filled Arrays
 To process the gathered array elements, use the
companion variable, not the array length:
for (int i = 0; i < currentSize; i++)
{
System.out.println(values[i]);
}
 With a partially filled array, you need to remember how
many elements are filled.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
24
Question
Given an array of integers containing the first five prime numbers,
int[] primes = { 2, 3, 5, 7, 11 };
what does it contain after executing the following loop?
for (int i = 0; i < 2; i++)
{
primes[4 - i] = primes[i];
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
25
Answer
Given an array of integers containing the first five prime numbers,
int[] primes = { 2, 3, 5, 7, 11 };
what does it contain after executing the following loop?
for (int i = 0; i < 2; i++)
{
primes[4 - i] = primes[i];
}
Answer: 2, 3, 5, 3, 2
Copyright © 2014 by John Wiley & Sons. All rights reserved.
26
Programming Question
 Write a class ArrayDemo2 to do the following:
• Define a double array called rainfall of 100 elements.
• Initialize each element by user keyboard input.
• User can any time denote that he/she is done entering
numbers by inputting non-number value. i.e. user is not
required to enter values for all 100 elements.
 Calculate and print average rainfall.
 A sample run is shown:
>Enter rainfall: 20 30 45 65.8 34 A
> The average rainfall is 38.96
Copyright © 2014 by John Wiley & Sons. All rights reserved.
27
Answer
ArrayDemo2.java
import java.util.Scanner;
public class ArrayDemo2
{
public static void main(String args[])
{
double[] rainfall = new double[100];
Scanner in = new Scanner(System.in);
System.out.println("Enter rainfall: ");
int currentSize=0;
do
{
if(currentSize < rainfall.length)
{
rainfall[currentSize] = in.nextDouble();
currentSize++;
}
}while(in.hasNextDouble());
double average = 0.0;
double total = 0.0;
for(int i=0;i<currentSize;i++)
{
total += rainfall[i];
}
average = total/currentSize;
System.out.printf("Average rainfall=%3.2f", average);
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
28
The Enhanced for Loop
 You can use the enhanced for loop to visit all elements of an
array.
 E.g. Totaling the elements in an array with the enhanced for
loop
double[] values = {10.5, 3.2, 6.7};
double total = 0;
for (double element : values) //for each element in array
{
total = total + element;
}
Element type Element name Array name
 The loop body is executed for each element in the array
values.
 Indices are not accessible
Copyright © 2014 by John Wiley & Sons. All rights reserved.
29
The Enhanced for Loop
 Traditional alternative:
for (int i = 0; i < values.length; i++)
{
double element = values[i];
total = total + element;
}
Indices are accessible
Copyright © 2014 by John Wiley & Sons. All rights reserved.
30
The Enhanced for Loop
 Not suitable for all array algorithms.
• Does not allow you to modify the contents of an array.
 E.g. The following loop does not fill an array with
zeros:
for (double element : values)
{
element = 0;
// ERROR: this assignment does not modify
// array elements
}
 Use a basic for loop instead:
for (int i = 0; i < values.length; i++)
{
values[i] = 0; // OK
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
31
The Enhanced for Loop
 Use the enhanced for loop if you do not need the index
values in the loop body.
 The enhanced for loop is a convenient mechanism for
traversing all elements in a collection.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
32
Syntax 7.2 The Enhanced for Loop
Copyright © 2014 by John Wiley & Sons. All rights reserved.
33
Programming Question
 Given the following array:
int[] a =
{2, 78, 6 ,67, 23, 2 , 7 , 8 , 0 , 23}
 Write ArrayDemo3 class to write a method to find and
print the maximum of array elements:
public static int maximum(int values[])
• Modify main method to call this method with array a as
parameter
Copyright © 2014 by John Wiley & Sons. All rights reserved.
34
Answer
ArrayDemo3.java
import java.util.Scanner;
public class ArrayDemo3{
public static void main(String args[]) {
int[] a = {2, 78, 6 ,67, 23, 2 , 7 , 8 , 0 , 23};
System.out.println(“maximum= "+ maximum(a));
}
public static int maximum(int values[]) {
int largest = values[0];
for (int i = 1; i < values.length; i++)
if (values[i] > largest)
{
largest= values[i];
}
}
return largest;
}
{
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
35
Question
Consider the algorithm to find the largest element in an array.
Why don’t we initialize largest and i with zero, like this?
double largest = 0;
for (int i = 0; i < values.length; i++)
{
if (values[i] > largest) {
largest = values[i];
}
}
Original code:
double largest = values[0];
for (int i = 1; i < currentSize; i++){
if (values[i] > largest){
largest = values[i];
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
36
Answer
If all elements of values are negative, then the result
is incorrectly computed as 0.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
37
Programming Question
 Linear Search:
 Given the following array:
int[] a =
{2, 78, 6 ,67, 23, 2 , 7 , 8 , 0 , 23}
 Write ArrayDemo4 class to add a method to search for a
value and print index of element in array if found.
Otherwise print “not found!”
public static
void search(int searhedValue, int values[])
• Modify main method to call this method with search value 23 and
array a as parameter
public static
int[] a =
search(78,
search(28,
}
void main(String args[]) {
{2, 78, 6 ,67, 23, 2 , 7 , 8 , 0 , 23};
a);
a);
Copyright © 2014 by John Wiley & Sons. All rights reserved.
38
Answer
import java.util.Scanner;
public class ArrayDemo4{
public static void main(String args[]) {
int[] a = {2, 78, 6 ,67, 23, 2 , 7 , 8 , 0 , 23};
search(78, a);
search(28, a);
}
public static void search (int searchedValue, int values[]) {
int index = 0;
boolean found = false;
while (index < values.length && !found)
{
if (values[index] == searchedValue) { found = true; }
else { index++; }
}
if (found) { System.out.println("Found at position: " + index); }
else { System.out.println("Not found"); }
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
39
Common Array Algorithm: Linear Search
 This algorithm is called a linear search.
 A linear search inspects elements in sequence until a
match is found.
 To search for a specific element, visit the elements and
stop when you encounter the match.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
40
Common Array Algorithm: Removing an
Element
Problem:
-array: values
- current number of elements : currentSize
-remove the element at index: pos
 How to do this in a:
• Unordered array? (order of elements does not matter)
• Ordered array? (order of elements must be maintained)
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41
Common Array Algorithm: Removing an
Element
 Unordered
1.
Overwrite the element to be removed with the last element of the array.
values[pos] = values[currentSize - 1];
2.
Decrement the currentSize variable.
currentSize--;
Figure 6 Removing an Element in an Unordered Array
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42
Common Array Algorithm: Removing an
Element

Ordered array
1.
Move all elements following the element to be removed to a lower index.
for (int i = pos + 1; i < currentSize; i++){
values[i - 1] = values[i];
}
2.
Decrement the variable holding the size of the array.
currentSize--;
Figure 7 Removing an Element in an Ordered Array
Copyright © 2014 by John Wiley & Sons. All rights reserved.
43
Common Array Algorithm: Inserting an
Element
 If order does not matter
1. Insert the new element at the end of the array.
2. Increment the variable tracking the size of the array.
if (currentSize < values.length){
currentSize++;
values[currentSize -1 ] = newElement;
}
Figure 8 Inserting an Element in an Unordered Array
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44
Common Array Algorithm: Inserting an
Element

If order matters : Increment the variable tracking the size of the array.
1. Move all elements after the insertion location to a higher index.
2. Insert the element.
if (currentSize < values.length){
currentSize++;
for (int i = currentSize - 1; i > pos; i--)
values[i] = values[i - 1];
}
values[pos] = newElement;
}
{
Figure 9 Inserting an Element in an Ordered Array
Copyright © 2014 by John Wiley & Sons. All rights reserved.
45
Question
When inserting an element into an array, we moved the
elements with larger index values, starting at the end of
the array. Why is it wrong to start at the insertion
location, like this?
for (int i = pos; i < currentSize - 1; i++)
{
values[i + 1] = values[i];
}
Original code:
for (int i = pos + 1; i < currentSize; i++)
{
values[i - 1] = values[i];
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
46
Answer
This loop sets all elements to values[pos].
Copyright © 2014 by John Wiley & Sons. All rights reserved.
47
Programming Question
 Write a program ArrayDemo5 that reverse array in-place.
• Hint swap elements at same distance to array middle
 Use following template to get started:
public class ArrayDemo5{
public static void main(String args[])
int[] a = {2, 78, 6 ,67, 23};
printArray("before reverse:", a);
reverse(a);
printArray("after reverse:", a);
}
public static void reverse(int vals[])
//TODO
}
{
{
public static void printArray(String message, int ar[])
System.out.println(message);
for(int i=0; i<ar.length;i++)
{
System.out.println("ar["+i+"]="+ar[i]);
}
}
{
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
48
 Sample output:
> java ArrayDemo5
before reverse:
ar[0]=2
ar[1]=78
ar[2]=6
ar[3]=67
ar[4]=23
after reverse:
ar[0]=23
ar[1]=67
ar[2]=6
ar[3]=78
ar[4]=2
>
Copyright © 2014 by John Wiley & Sons. All rights reserved.
49
Answer
public class ArrayDemo5
{
public static void main(String args[])
{
int[] a = {2, 78, 6 ,67, 23};
printArray("before swap:", a);
swap(a);
printArray("after swap:", a);
}
public static void swap (int vals[])
{
for(int i=0;i<vals.length/2;i++)
{
int temp = vals[i];
vals[i] = vals[vals.length-i-1];
vals[vals.length-i-1] = temp;
}
ArrayDemo5.java
Array elements at indices i and (length-1)-I are
swapped
Should go only half the length.
}
public static void printArray(String message, int ar[])
{
System.out.println(message);
for(int i=0; i<ar.length;i++)
{
System.out.println("ar["+i+"]="+ar[i]);
}
}
}
Copyright
© 2014 by John Wiley & Sons. All rights reserved.
50
Common Array Algorithm: Swapping Elements
 To swap two elements, you need a temporary variable.
 We need to save the first value in the temporary variable
before replacing it.
double temp = values[i];
values[i] = values[j];
 Now we can set values[j] to the saved value.
values[j] = temp;
Copyright © 2014 by John Wiley & Sons. All rights reserved.
51
Common Array Algorithm: Swapping
Elements
Figure 10 Swapping Array Elements
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52
Common Array Algorithm: Copying an
Array
 Copying an array variable yields a second reference to the
same array:
double[] values = {1, 3, 45, 5, 20}
double[] prices = values;
 To make a true copy of an array, call the Arrays.copyOf
method:
double[] prices = Arrays.copyOf(values, values.length);
Explore javadoc API page for Arrays
Copyright © 2014 by John Wiley & Sons. All rights reserved.
53
Copyright © 2014 by John Wiley & Sons. All rights reserved.
54
Copyright © 2014 by John Wiley & Sons. All rights reserved.
55
Copyright © 2014 by John Wiley & Sons. All rights reserved.
56
Common Array Algorithm: Copying an
Array
Figure 11 Copying an Array Reference versus Copying an
Array
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57
Common Array Algorithm: Growing an
Array
 To grow an array that has run out of space, use the
Arrays.copyOf method to double the length of an array
double[] newValues = Arrays.copyOf(values, 2 * values.length);
values = newValues;
Copyright © 2014 by John Wiley & Sons. All rights reserved.
58
Common Array Algorithm: Growing an
Array
Figure 12 Growing an Array
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59
Reading Input
 To read a sequence of arbitrary length:
• Add the inputs to an array until the end of the input has been reached.
• Grow when needed.
double[] inputs = new double[INITIAL_SIZE];
int currentSize = 0;
while (in.hasNextDouble())
{
// Grow the array if it has been completely filled
if (currentSize >= inputs.length)
{
inputs = Arrays.copyOf(inputs, 2 * inputs.length); // Grow the inputs array
}
inputs[currentSize] = in.nextDouble();
currentSize++;
}
• Discard unfilled elements.
inputs = Arrays.copyOf(inputs, currentSize);
Copyright © 2014 by John Wiley & Sons. All rights reserved.
60
Two-Dimensional Arrays
 An arrangement consisting of rows and columns of values
• Also called a matrix.
 Example:
• medal counts of the figure skating competitions at the 2010 Winter
Olympics.
Figure 13 Figure Skating Medal counts
Copyright © 2014 by John Wiley & Sons. All rights reserved.
61
Two-Dimensional Arrays
 Use a two-dimensional (2-D)array to store tabular data.
 When constructing a two-dimensional array, specify how
many rows and columns are needed:
final int COUNTRIES = 7; //number of rows (dimension 1)
final int MEDALS = 3; //number of columns (dimension 2)
int[][] counts = new int[COUNTRIES][MEDALS];
rows
columns
 2-d array of 7 rows and 3 columns OR
 7 arrays each having 3 elements (array of arrays)
Copyright © 2014 by John Wiley & Sons. All rights reserved.
62
Two-Dimensional Arrays
 int[][] counts = new int[COUNTRIES][MEDALS];
memory
counts
Each row is a 1-d array
Copyright © 2014 by John Wiley & Sons. All rights reserved.
63
Two-Dimensional Arrays
 Method1:
 You can declare and initialize the array by grouping each row:
int[][]
{
{ 1,
{ 1,
{ 0,
{ 1,
{ 0,
{ 0,
{ 1,
};
counts =
0,
1,
0,
0,
1,
1,
1,
1
0
1
0
1
1
0
},
},
},
},
},
},
}
 You cannot change the size of a two-dimensional array
once it has been declared.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
64
Two-Dimensional Arrays
 Method2:
 Assign values to individual elements:
 E.g.
[0]
counts[0][2]=1
row=0, column-2
Copyright © 2014 by John Wiley & Sons. All rights reserved.
[1]
[2]
[0]
[1]
[2]
[3]
[4]
[5]
[6]
65
Syntax 7.3 Two-Dimensional Array Declaration
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66
Accessing Elements
 Access by using two index values, array[i][j]
int medalCount = counts[3][1];
Copyright © 2014 by John Wiley & Sons. All rights reserved.
67
Question
 Given
int[][] rating= new int[3][4];
 What is the value of rating.length ?
 What is the value of rating[0].length ?
 What is the value of rating[0]?
Use DrJava interaction pane to find out!
Copyright © 2014 by John Wiley & Sons. All rights reserved.
68
Answer
 Given
int[][] rating= new int[3][4];
 What is the value of rating.length ?
• Answer: 3, the number of rows (first dimension)
 What is the value of rating[0].length ?
• Answer: 4, the number of columns (second dimension)
 What is the value of rating[0]?
• Answer:{0,0,0,0}
Use nested loops to access all elements in a two-dimensional array.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
69
Programming Question
 Write ArrayDemo6 class to declare and initialize the
counts array and then print the values in the array in a
tabular format.
• Hint:
• for printing the row and column headers, define two separate arrays
• Number of rows: counts.length
• Number of columns: counts[0].length
 A sample program run is shown below:
Program
template
in next
slide
Copyright © 2014 by John Wiley & Sons. All rights reserved.
70

Start by copying the following code to ArrayDemo6.java main method:
int[][] counts =
{
{ 1, 0, 1 },
{ 1, 1, 0 },
{ 0, 0, 1 },
{ 1, 0, 0 },
{ 0, 1, 1 },
{ 0, 1, 1 },
{ 1, 1, 0 }
};
//headers
String[] columnHeader = {"Country", "Gold", "Silver", "Bronze"};
String[] rowHeader = {"Canada", "China", "Germany", "Korea", "Japan",
"Russia", "United States"};
//TODO:print column headers
//TODO:print array in tabular format
Hint: use System.out.printf statements
Copyright © 2014 by John Wiley & Sons. All rights reserved.
71
Answer
ArrayDemo6.java
public class ArrayDemo6{
public static void main(String args[]){
int[][] counts = {{ 1, 0, 1 }, { 1, 1, 0 }, { 0, 0, 1 }, { 1, 0, 0 },
{ 0, 1, 1 }, { 0, 1, 1 }, { 1, 1, 0 } };
//iniialize headers
String[] columnHeader = {"Country", "Gold", "Silver", "Bronze"};
String[] rowHeader = {"Canada","China","Germany","Korea","Japan","Russia","United States"};
//print column headers
for(int i = 0; i < columnHeader.length; i++)//for each column
{
System.out.printf("%15s",columnHeader[i]); //print column header
}
System.out.println();
//print array in tabular format
for (int i = 0; i < counts.length; i++) //for each row
{
//print row header
System.out.printf("%15s",rowHeader[i]);
for (int j = 0; j < counts[0].length; j++)//for each element in row i
{
System.out.printf("%15d", counts[i][j]); //print element
}
System.out.println();
}
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
72
Programming Question
 Modify ArraysDemo6.java to print a new column with
the total number of medals won by each country
• Hint: This is same as sum of each row
 A sample run is shown below:
Copyright © 2014 by John Wiley & Sons. All rights reserved.
73
Accessing Rows and Columns
 Hint
•
To find the number of medals won by a country
– Find the sum of the elements in a row
 To find the sum of the ith row
• compute the sum of counts[i][j], where j ranges from 0 to
MEDALS - 1.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
74
Answer
public class ArrayDemo6{
public static void main(String args[]){
int[][] counts = {{ 1, 0, 1 }, { 1, 1, 0 }, { 0, 0, 1 },
{ 1, 0, 0 },{ 0, 1, 1 }, { 0, 1, 1 }, { 1, 1, 0 }
ArrayDemo6.java
};
//iniialize headers
String[] columnHeader = {"Country", "Gold", "Silver", "Bronze","Total"};
String[] rowHeader = {"Canada","China","Germany","Korea","Japan","Russia","United States"};
//print column headers
for(int i = 0; i < columnHeader.length; i++)//for each column
{
System.out.printf("%15s",columnHeader[i]); //print column header
}
System.out.println();
//print array in tabular format
for (int i = 0; i < counts.length; i++) //for each row
{
//print row header
System.out.printf("%15s",rowHeader[i]);
int total = 0;
for (int j = 0; j < counts[0].length; j++)//for each element in row i
{
System.out.printf("%15d", counts[i][j]); //print element
total += counts[i][j];
}
System.out.printf("%15d", total); //print total
System.out.println();
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
}
75
Accessing Rows and Columns
 To find the sum of the jth column
• Form the sum of counts[i][j], where i ranges from 0 to
COUNTRIES – 1
int total = 0;
for (int i = 0; i < COUNTRIES; i++
{
total = total + counts[i][j];
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
76
Ragged Arrays
 Does every row in a 2d array need to be the
same size?
 No!
 An array with different length rows is said to be
ragged.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
77
Initializing Ragged Arrays
 E.g.:
int[][] x=
{
{ 710, 1000},
{ 790, 1830, 1090},
{2190, 3020, 2050, 1540}
};
Copyright © 2014 by John Wiley & Sons. All rights reserved.
78
Processing Ragged Arrays
 Ragged arrays can be useful, but have a big pitfall
• It is very easy to get ArrayIndexOutOfBounds error
for(row = 0; row < matrix.length; row++) {
for(col = 0; col < matrix[0].length; col++) {
System.out.printf("%7d", matrix[row][col]);
}
System.out.println();
}
How do we correct this?
Copyright © 2014 by John Wiley & Sons. All rights reserved.
79
Programming Question
 Write a class ArrayDemo7 with a method countOcc that counts the
number of times each digit between 0 and 9 occurs in a 2d array:
void countOcc(int[][] a, int[] count)
 Use following main to test the countOcc method:
public static void main(String args[]){
int[] counts = new int[10];
int[][] myArray = {{23, 0,9},{2,56,7,-2},{5 ,5,7},{8,45}};
countOcc(myArray, counts);
for(int i=0;i<counts.length;i++)
System.out.println("counts["+i+"]="+counts[i]);
}
Hint: use a 1-d array counts to keep track of counts of each number 0-9

For example, in above array, the output should be:
counts[0]=1
counts[1]=0
counts[2]=1
counts[3]=0
counts[4]=0
counts[5]=2
counts[6]=0
counts[7]=2
counts[8]=1
counts[9]=1
Copyright © 2014 by John Wiley & Sons. All rights reserved.
80
Answer
ArrayDemo7.java
public class ArrayDemo7
{
public static void main(String args[])
{
int[] counts = new int[10];
int[][] myArray = {{23, 0,9},{2,56,7,-2},{5 ,5,7},{8,45}};
countOcc(myArray, counts);
for(int i=0;i<counts.length;i++)
System.out.println("counts["+i+"]="+counts[i]);
}
public static void countOcc(int[][] a, int[] count)
{
for(int i=0;i<a.length;i++)
{
for(int j=0;j<a[i].length;j++)
{
if(a[i][j]>9 || a[i][j]<0)
continue;
count[a[i][j]]++;
}
}
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
81