Chapter 6: Arrays and ArrayLists
Highlights review:
- In Java, arrays are objects
- Arrays can be create with the “new” operator: int[] height = new int[11]
- Arrays can also be created in using this format: int[] scores = {76, 87, 99}
- The index operator [ ] performs automatic bounds checking
- Beware of the off-by-one errors in a program
- Either syntax int[] grades or int grades[] is correct
NOTE: The following programs and the pdf are stored in edmodo’s folder. Get familiar with all these programs and their algorithms to further develop assignments.
BasicArray
//********************************************************************
// BasicArray.java Author: Lewis/Loftus/Cocking
//
// Demonstrates basic array declaration and use.
//********************************************************************
public class BasicArray
{
final static int LIMIT = 15;
final static int MULTIPLE = 10;
//-----------------------------------------------------------------
// Creates an array, fills it with various integer values,
// modifies one value, then prints them out.
//-----------------------------------------------------------------
public static void main (String[] args)
{
int[] list = new int[LIMIT];
// Initialize the array values
for (int index = 0; index < LIMIT; index++)
list[index] = index * MULTIPLE;
list[5] = 999; // change one array value
for (int index = 0; index < LIMIT; index++)
System.out.print (list[index] + " ");
System.out.println ();
}
}
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Primes
//********************************************************************
// Primes.java Author: Lewis/Loftus/Cocking
//
// Demonstrates the use of an initializer list for an array.
//********************************************************************
public class Primes
{
//-----------------------------------------------------------------
// Stores some prime numbers in an array and prints them.
//-----------------------------------------------------------------
public static void main (String[] args)
{
int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19};
System.out.println ("Array length: " + primeNums.length);
System.out.println ("The first few prime numbers are:");
for (int scan = 0; scan < primeNums.length; scan++)
System.out.print (primeNums[scan] + " ");
System.out.println ();
}
}
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Printing an array of integers in reverse order:
ReverseOrder
//********************************************************************
// ReverseOrder.java Author: Lewis/Loftus/Cocking
//
// Demonstrates array index processing.
//********************************************************************
import java.util.Scanner;
public class ReverseOrder
{
//-----------------------------------------------------------------
// Reads a list of numbers from the user, storing them in an
// array, then prints them in the opposite order.
//-----------------------------------------------------------------
public static void main (String[] args)
{
double[] numbers = new double[10];
Scanner scan = new Scanner(System.in);
System.out.println ("The size of the array: " + numbers.length);
for (int index = 0; index < numbers.length; index++)
{
System.out.print ("Enter number " + (index+1) + ": ");
numbers[index] = scan.nextDouble();
}
System.out.println ("The numbers in reverse order:");
for (int index = numbers.length-1; index >= 0; index--)
System.out.print (numbers[index] + " ");
System.out.println ();
}
}
Output:
The size of the array: 10
Enter number 1: 18.36
Enter number 2: 48.9
Enter number 3: 53.3
Enter number 4: 21.9
Enter number 5: 34.7
Enter number 6: 12.5
Enter number 7: 78.4
Enter number 8: 98.1
Enter number 9: 99.0
Enter number 10: 37.9
The numbers in reverse order:
37.9 99.0 98.1 78.4 12.5 34.7 21.9 53.3 48.9 18.36
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Letter counting
LetterCount
//********************************************************************
// LetterCount.java Author: Lewis/Loftus/Cocking
//
// Demonstrates the relationship between arrays and strings.
//********************************************************************
import java.util.Scanner;
public class LetterCount
{
//-----------------------------------------------------------------
// Reads a sentence from the user and counts the number of
// uppercase and lowercase letters contained in it.
//-----------------------------------------------------------------
public static void main (String[] args)
{
final int NUMCHARS = 26;
Scanner scan = new Scanner(System.in);
int[] upper = new int[NUMCHARS];
int[] lower = new int[NUMCHARS];
char current; // the current character being processed
int other = 0; // counter for non-alphabetics
System.out.println ("Enter a sentence:");
String line = scan.nextLine();
// Count the number of each letter occurence
for (int ch = 0; ch < line.length(); ch++)
{
current = line.charAt(ch);
if (current >= 'A' && current <= 'Z')
upper[current-'A']++;
else
if (current >= 'a' && current <= 'z')
lower[current-'a']++;
else
other++;
}
// Print the results
System.out.println ();
for (int letter=0; letter < upper.length; letter++)
{
System.out.print ( (char) (letter + 'A') );
System.out.print (": " + upper[letter]);
System.out.print ("\t\t" + (char) (letter + 'a') );
System.out.println (": " + lower[letter]);
}
System.out.println ();
System.out.println ("Non-alphabetic characters: " + other);
}
}
Output:
Enter a sentence:
In Casablanca, Humprey Bogart never says "Play it again, Sam."
A: 0 a: 10 B: 1 b: 1 C: 1 c: 1 D: 0 d: 0 E: 0 e: 3 F: 0 f: 0 G: 0 g: 2 H: 1 h: 0 I: 1 i: 2 J: 0 j: 0 K: 0 k: 0 L: 0 l: 2 M: 0 m: 2 N: 0 n: 4 O: 0 o: 1 P: 1 p: 1 Q: 0 q: 0 R: 0 r: 3 S: 1 s: 3 T: 0 t: 2 U: 0 u: 1 V: 0 v: 1 W: 0 w: 0 X: 0 x: 0 Y: 0 y: 3 Z: 0 z: 0
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Non-alphabetic characters: 14
Array length: 8 The first few prime numbers are: 2 3 5 7 11 13 17 19
Helpful notes for testing purposes:
Instead of entering manually input, you could put the data in a file like input.data. Take a look at an easy way to read data from a file:
import java.util.Scanner;
public class ScannerInTerminal
{
public static void main(String [] args)
{
Scanner sc = new Scanner(System.in);
String fromTerm = sc.next();
System.out.println(fromTerm);
}
}
Then you can run it in Terminal
mrseliaTerminal%: java ScannerInTerminal < input.txt
3.14159…
Classwork:
Programming Projects 6.1, 6.2, 6.4 and 6.5
Homework:
Short Answers 6.1, 6.2 and 6.3
Read pages 298 through 311
