Java Foundations: Arrays

Java Foundations
Arrays in Java

Your Course
Instructors
Svetlin Nakov
George Georgiev

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Arrays
Fixed-Size Sequences
of Numbered Elements

Table of Contents
1. Arrays
2. Array Operations
3. Reading Arrays from the Console
4. For-each Loop
7

Arrays in Java
Working with Arrays of Elements

 In programming, an array is a sequence of elements
 Arrays have fixed size (array.length)
cannot be resized
 Elements are of the same type (e.g. integers)
 Elements are numbered from 0 to length-1
What are Arrays?
9
Array of 5
elements
Element’s index
Element of an array
… … … … …
0 1 2 3 4

 Allocating an array of 10 integers:
 Assigning values to the array elements:
 Accessing array elements by index:
Working with Arrays
10
int[] numbers = new int[10];
for (int i = 0; i < numbers.length; i++)
numbers[i] = 1;
numbers[5] = numbers[2] + numbers[7];
numbers[10] = 1; // ArrayIndexOutOfBoundsException
All elements are
initially == 0
The length holds
the number of
array elements
The [] operator
accesses
elements by index

 The days of a week can be stored in an array of strings:
Days of Week – Example
11
String[] days = {
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
"Sunday"
};
Operator Value
days[0] Monday
days[1] Tuesday
days[2] Wednesday
days[3] Thursday
days[4] Friday
days[5] Saturday
days[6] Sunday

 Enter a day number [1…7] and print
the day name (in English) or "Invalid day!"
Problem: Day of Week
12
String[] days = { "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday", "Sunday" };
int day = Integer.parseInt(sc.nextLine());
if (day >= 1 && day <= 7)
System.out.println(days[day - 1]);
else
System.out.println("Invalid day!");
The first day in our array
is on index 0, not 1.

Reading Array
Using a for Loop or String.split()

 First, read the array length from the console :
 Next, create an array of given size n and read its elements:
Reading Arrays From the Console
14
int n = Integer.parseInt(sc.nextLine());
int[] arr = new int[n];
for (int i = 0; i < n; i++) {
arr[i] = Integer.parseInt(sc.nextLine());
}

 Arrays can be read from a single line of separated values
Reading Array Values from a Single Line
15
String values = sc.nextLine();
String[] items = values.split(" ");
int[] arr = new int[items.length];
for (int i = 0; i < items.length; i++)
arr[i] = Integer.parseInt(items[i]);
2 8 30 25 40 72 -2 44 56

 Read an array of integers using functional programming:
Shorter: Reading Array from a Single Line
16
int[] arr = Arrays
.stream(sc.nextLine().split(" "))
.mapToInt(e -> Integer.parseInt(e)).toArray();
String inputLine = sc.nextLine();
String[] items = inputLine.split(" ");
int[] arr = Arrays.stream(items)
You can chain
methods
import
java.util.Arrays;

 To print all array elements, a for-loop can be used
 Separate elements with white space or a new line
Printing Arrays on the Console
17
String[] arr = {"one", "two"};
// == new String [] {"one", "two"};
// Process all array elements
for (int i = 0; i < arr.length; i++) {
System.out.printf("arr[%d] = %s%n", i, arr[i]);
}

 Read an array of integers (n lines of integers), reverse it and
print its elements on a single line, space-separated:
Problem: Reverse an Array of Integers
18
3
10
20
30
30 20 10
4
-1
20
99
5
5 99 20 -1

Solution: Reverse an Array of Integers
19
// Read the array (n lines of integers)
int n = Integer.parseInt(sc.nextLine());
int[] arr = new int[n];
for (int i = 0; i < n; i++)
arr[i] = Integer.parseInt(sc.nextLine());
// Print the elements from the last to the first
for (int i = n - 1; i >= 0; i--)
System.out.print(arr[i] + " ");
System.out.println();

 Use for-loop:
 Use String.join(separator, array):
Printing Arrays with for / String.join(…)
20
String[] strings = { "one", "two" };
System.out.println(String.join(" ", strings)); // one two
int[] arr = { 1, 2, 3 };
System.out.println(String.join(" ", arr)); // Compile error
String[] arr = {"one", "two"};
for (int i = 0; i < arr.length; i++)
System.out.println(arr[i]);
Works only
with strings

 Read an array of strings (space separated values), reverse it and
print its elements:
 Reversing array elements:
Problem: Reverse Array of Strings
21
a b c d e e d c b a -1 hi ho w w ho hi -1
a b c d e
exchange

Solution: Reverse Array of Strings
22
String[] elements = sc.nextLine().split(" ");
for (int i = 0; i < elements.length / 2; i++) {
String oldElement = elements[i];
elements[i] = elements[elements.length - 1 - i];
elements[elements.length - 1 - i] = oldElement;
}
System.out.println(String.join(" ", elements));

For-each Loop
Iterate through Collections

 Iterates through all elements in a collection
 Cannot access the current index
 Read-only
For-each Loop
for (var item : collection) {
// Process the value here
}

int[] numbers = { 1, 2, 3, 4, 5 };
for (int number : numbers) {
System.out.println(number + " ");
}
Print an Array with Foreach
25
1 2 3 4 5

 Read an array of integers
 Sum all even and odd numbers
 Find the difference
 Examples:
Problem: Even and Odd Subtraction
26
1 2 3 4 5 6 3
3 5 7 9 11 -35
2 4 6 8 10 30
2 2 2 2 2 2 12

int[] arr = Arrays.stream(sc.nextLine().split(" "))
int evenSum = 0;
int oddSum = 0;
for (int num : arr) {
if (num % 2 == 0) evenSum += num;
else oddSum += num;
}
// TODO: Find the difference and print it
Solution: Even and Odd Subtraction
27

 …
 …
 …
Summary
29
 Arrays hold a sequence of elements
 Elements are numbered
from 0 to length – 1
 Creating (allocating) an array
 Accessing array elements by index
 Printing array elements

 …
 …
 …
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Java Foundations: Arrays

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