Class 10-13 Notes CS 107
Wednesday, October 13, 2010 1:00 PM
Announcements: Questions? Last Time: Description of program #4, Idea of arrays Today: Syntax for arrays, Arrays examples Syntax for arrays: Rather than having to store multiple values of the same type in different variables, we can store them in an array. This is appropriate when we have a group of values of the same type, such as a list of numbers, or a group of characters representing a playing board for a game. To declare an array, we use: type[ ] arrayName; where type can be any type (e.g. char, int, float), arrayName is an identifier you make up, and size indicates how many items will be stored in the array. We then must initialize the array using: arrayName = new type[ size]; For example int[ ] scores; scores = new int[ 10] ; declares and initializes an array called theScores that can hold 10 integers. In the array shown above the first element is the 0th element, and the last is the 9th element. This element number is known as the index (a.k.a. the subscript) , which starts at 0 and goes to n-1 for an array of n elements. Note that the default value of 0 is stored in each of the 10 array positions at this point. Array Usage 1. Store a value into an array by using the array name with the subscript on the left-hand side of an assignment statement. E.g.: scores[ 3] = 75; // store 75 into array element 3 (the 4th element) 2. Retrieve a value from an array by using the array name with the subscript anywhere a value is needed. E.g.: System.out.println( scores[ 1]); sum = sum + scores[ i]; // i is an integer between 0 and 10 3. You must make sure that you never read or write past the end of the array. The compiler will not stop you, but your program will generate an error and stop when you try to access an out-of-bounds element. 4. Arrays can be initialized element by element in a loop for (i=0; i<10; i++) { theScores[ i] = 0; } or by putting initial values in braces when the array is declared: int[ ] scores = {0,0,0,0,0,0,0,0,0,0}; In the above case the size is inferred from the number of initializing values.
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} or by putting initial values in braces when the array is declared: int[ ] scores = {0,0,0,0,0,0,0,0,0,0}; In the above case the size is inferred from the number of initializing values.
Pasted from <http://logos.cs.uic.edu/Examples%20And%20Notes/notes/Java/ArraysAverageGrades/Arrays.htm >
Sample programs using arrays (see code on web site)
import java.io.*; import java.util.Scanner; class UsingAnArray { public static void main(String[] args) throws IOException { int[] scores; scores = new int[3]; // declare the array // initialize the array // Scanner for keyboard input
Scanner keyboard = new Scanner( System.in);
System.out.println( "Enter 3 scores to be averaged:"); scores[0] = keyboard.nextInt(); scores[1] = keyboard.nextInt(); scores[2] = keyboard.nextInt(); System.out.print( "The scores are: " ); System.out.println( scores[0] + ", " + scores[1] + ", " + scores[2]); System.out.print( "The average is: " ); System.out.println( (float)(scores[0]+scores[1]+scores[2])/3); } } Pasted from <http://logos.cs.uic.edu/Examples%20And% 20Notes/notes/Java/ArraysAverageGrades/UsingAnArray/UsingAnArray.java >
And another version using a loop:
/** * Use an array to store scores. * Also use a constant to declare the size. * and a loop to calculate the average. * */ import java.io.*; import java.util.Scanner; class UsingAnArrayB { final int ARRAY_SIZE = 10; Then use a loop to read in the values
// declare a constant for the array size // note how easy it is to change this
public static void main(String[] args) { UsingAnArrayB instance; instance = new UsingAnArrayB(); // use the default constructor instance.doit(); // now call the method } /* Must use a separate method to do this due to the limitations * of main() being static. We can't use instance variables (e.g. ARRAY_SIZE) * inside a static function. */ public void doit()
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* of main() being static. We can't use instance variables (e.g. ARRAY_SIZE) * inside a static function. */ public void doit() { int[] scores; // declare the array scores = new int[ ARRAY_SIZE]; // initialize the array Scanner keyboard = new Scanner( System.in); // Scanner for keyboard input
// Prompt for scores and enter them. Use the built-in array length. System.out.println( "Enter " + ARRAY_SIZE + " scores to be averaged:"); for (int i=0; i<scores.length; i++) { scores[i] = keyboard.nextInt(); } // Display the scores, accumulating the sum as we go int sum = 0; System.out.print( "The scores are: " ); for (int i=0; i<scores.length; i++) { System.out.print( scores[i] + ", "); sum += scores[i]; } System.out.println(); // Display the average System.out.print( "The average is: " ); System.out.println( (float)sum/ARRAY_SIZE); } } Pasted from <http://logos.cs.uic.edu/Examples%20And% 20Notes/notes/Java/ArraysAverageGrades/UsingAnArrayB/UsingAnArrayB.java >
And yet another example, passing the array as a parameter to a method. /** * Use an array to store scores. * Also use a constant to declare the size. Then use a loop to read in the values * and a loop to calculate the average. * */ import java.io.*; import java.util.Scanner; class UsingAnArrayWithFunction { final int ARRAY_SIZE = 3; // declare a constant for the array size // note how easy it is to change this Scanner keyboard; // used for console IO public static void main(String[] args) { UsingAnArrayWithFunction instance; instance = new UsingAnArrayWithFunction(); // use the default constructor instance.doit(); // now call the method } /* Must use a separate method to do this * of main() being static. We can't use * inside a static function. */ public void doit() { int[] scores; // scores = new int[ ARRAY_SIZE]; // due to the limitations instance variables (e.g. ARRAY_SIZE)
declare the array initialize the array
keyboard = new Scanner( System.in); // Scanner for keyboard input getScores( scores); // call the method to get the scores // changes ARE reflected back!
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keyboard = new Scanner( System.in); // Scanner for keyboard input getScores( scores); // call the method to get the scores // changes ARE reflected back!
// Display the scores, accumulating the sum as we go int sum = 0; System.out.print( "The scores are: " ); for (int i=0; i<scores.length; i++) { System.out.print( scores[i] + ", "); sum += scores[i]; } System.out.println(); Display // the average System.out.print( "The average is: " ); System.out.println( (float)sum/ARRAY_SIZE); } public void getScores(int[] values) { // Prompt for scores and enter them. Use the built-in array length. System.out.println( "Enter " + ARRAY_SIZE + " scores to be averaged:"); for (int i=0; i<values.length; i++) { values[i] = keyboard.nextInt(); } } } Pasted from <http://logos.cs.uic.edu/Examples%20And% 20Notes/notes/Java/ArraysAverageGrades/UsingAnArrayWithFunction/UsingAnArrayWithFunction.java >
Let's review what we know about sending parameters to methods and having them change, in light of the above example.
See also other examples using arrays to count characters from an input line. (see code on web site)
/** * Use an array to count the number of each character found on an inputline. * Running the program gives: * Enter some characters to be counted: aardvark A A R D V A R K The non-zero counts of alphabetic characters found are: A:3; D:1; K:1; R:2; V:1; * */ import java.util.Scanner; class CountCharacters { final int ARRAY_SIZE = 26; // for reading from input line
// declare a constant for the array size
public static void main(String[] args) { CountCharacters instance; instance = new CountCharacters(); instance.doit(); }
// use the default constructor // now call the method
/* Must use a separate method to do this due to the limitations * of main() being static. We can't use instance variables (e.g. ARRAY_SIZE) * inside a static function, like main is above. */
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*/ public void doit() { int[] lettersArray; lettersArray = new int[ ARRAY_SIZE]; int i;
// declare the array // initialize the array // loop counter
Scanner keyboard = new Scanner( System.in); // initialize lettersArray count values for (i=0; i<lettersArray.length; i++) { lettersArray[ i] = 0; } // Prompt for and store a line of input. System.out.println("Enter some characters to be counted: "); String inputLine; inputLine = keyboard.nextLine(); // convert the string to all upper case inputLine = inputLine.toUpperCase(); // process the input line (from the array) one character at a time char c; // used to store an input character for analysis for ( i=0; i < inputLine.length(); i++) { c = inputLine.charAt( i); if ( Character.isLetter( c)) { // only count alphabetic characters lettersArray[ c - 'A']++; // update letter count, using // the character itself as the index System.out.print(c + " "); // echo input to the screen } } System.out.println(); // newline
// display select portions of the table System.out.print("The non-zero counts of alphabetic characters found are: \n"); for (i=0; i<lettersArray.length; i++) { // display the array element and the number of occurences if( lettersArray[ i]>0) { System.out.print( (char)(i + 'A') + ":" + lettersArray[ i] + "; "); } } System.out.println(); // newline
} }
Pasted from <http://logos.cs.uic.edu/Examples%20And% 20Notes/notes/Java/ArraysAverageGrades/CountCharacters/CountCharacters.java >
This can be modified to count characters from a file instead (e.g. all the characters in Shakespeare's Macbeth) Note where the data file should be stored!
/** * Use an array to count the number of each character found on an inputline. * Running the program gives: * Now counting the characters stored in Macbeth.txt The counts of alphabetic characters found are: A:6585; B:1555; C:2300; D:3485; E:11898; F:1801; G:1590; H:5450; I:5254; J:44; K:788; L:3717; M:2615; N:5590; O:6875; P:1289; Q:131; R:5160; S:5545; T:7965; U:3081; V:425; W:2058; X:208; Y:1990; Z:30; * */ import java.util.Scanner; import java.io.FileInputStream; import java.io.FileNotFoundException; // for reading from a file // for file errors
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class CountCharsFromFileUsingScanner { final int ARRAY_SIZE = 26; final String FILENAME = "Macbeth.txt";
// declare a constant for the array size // declare filename to use
public static void main(String[] args) { CountCharsFromFileUsingScanner instance; instance = new CountCharsFromFileUsingScanner(); // use the default constructor instance.doit(); // now call the method } /* Must use a separate method to do this due to the limitations * of main() being static. We can't use instance variables (e.g. ARRAY_SIZE) * inside a static function, like main is above. */ public void doit() { int[] lettersArray; // declare the array lettersArray = new int[ ARRAY_SIZE]; // initialize the array int i; // loop counter // declare the Scanner to read from a file Scanner inputStream = null; // use a "try - catch" block to gracefully handle errors if file is not found try { inputStream = new Scanner( new FileInputStream( FILENAME)); } catch( FileNotFoundException e) { // handle the "exception" (the error) if the file was not found System.out.println("*** Sorry, the input file " + FILENAME + " was not found. " + " Exiting program...\n"); System.exit( -1); // terminate program } // initialize lettersArray count values for (i=0; i<lettersArray.length; i++) { lettersArray[ i] = 0; } System.out.println("Now counting the characters stored in " + FILENAME); // now read from the file one line at at time, verifying that the read succeeded. String inputLine; while ( inputStream.hasNextLine()) { inputLine = inputStream.nextLine(); // convert the string to all upper case inputLine = inputLine.toUpperCase(); // process the input line (from the array) one character at a time. // We don't echo input characters to the screen because that would take too long. char c; // used to store an input character for analysis for ( i=0; i < inputLine.length(); i++) { c = inputLine.charAt( i); if ( Character.isLetter( c)) { // only count alphabetic characters lettersArray[ c - 'A']++; // update letter count, using the // character as the index } }
}// end while System.out.println();
// newline
// display select portions of the table System.out.print("The counts of alphabetic characters found are: \n"); for (i=0; i<lettersArray.length; i++) { // display the array element and the number of occurences System.out.print( (char)(i + 'A') + ":" + lettersArray[ i] + "; "); } System.out.println(); // newline
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System.out.println(); } }
// newline
Pasted from <http://logos.cs.uic.edu/Examples%20And% 20Notes/notes/Java/ArraysAverageGrades/CountCharactersFromFile/CountCharsFromFileUsingScanner.ja va>
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