Monday, October 18, 2010

Workshop On Multi dimensional Arrays InJava


Multidimensional Arrays
The arrays you have been introduced to thus far in the hour all have one dimension; one line of numbers ranging from 0 to the largest element number is used to refer to an array. But some types of information require more dimensions to store adequately as arrays. An example would be the (x,y) coordinate system that's a staple of any math class. If you needed to store a list of x and y coordinates that have a point marked on them, you could use a two-dimensional array. One dimension of the array could store the x coordinate, and the other dimension could store the y coordinate.
To create an array that has two dimensions, you must use an additional set of square brackets when creating and using the array. Consider the following:
boolean[][] selectedPoint = new int[50][50];
selectedPoint[4][13] = true;
selectedPoint[7][6] = true;
selectedPoint[11][22] = true;
This example creates an array of Boolean values called selectedPoint. The array has 50 elements in its first dimension and 50 elements in its second dimension, so there are 2,500 individual array elements that can hold values (50 multiplied by 50). When the array is created, each element is given the default value of false. Three elements are then given the value of true: A point at the (x,y) position of (4,13), one at (7,6), and one at (11,22).
Arrays can have as many dimensions as you need, but keep in mind that they take up a lot of memory if they're extremely large. Creating the 50 by 50 selectedPoint array was equivalent to creating 2,500 individual variables.
Workshop: Array of Prizes Indeed
Watch the syndicated game show Wheel of Fortune for any length of time and you'll be surprised at how predictable the contestants are. In the years that this show has been one of the world's most successful television programs, fortune seekers have worked the puzzle-solving process into an exact science. Wheel contestants typically guess the same letters when they are starting out on a puzzle: R, S, T, L, and N. In the final round, when these letters and the vowel E are given to the players right away, they usually choose four other letters: C, D, M, and the vowel O. The reason for this predictablity is that these are the letters that appear most often in English words. The contestants are stifling their desire for spontaneity in order to better their chances to win a trip to Bermuda, cash, and a Yamaha Waverunner.
In case you're unfamiliar with the show, Wheel of Fortune is a game in which three contestants try to guess the letters of a phrase, name, quote, or other memorable item. If they get a letter right and it's a consonant, they win the amount of money they spun on a big wheel. To re-create the experience, play hangman with some of your friends in front of a studio audience, hand out random amounts of money when someone guesses a letter in the secret word or phrase, and give the winner a new Ford Explorer.
Your Java workshop during this hour will test the most-common-letter theory by looking at as many different phrases and expressions as you care to type. An array will be used to count the number of times that each letter appears. When you're done, the program will present each letter and the number of times it appears in the phrases you entered. It also will present some clues about which letters to avoid entirely unless you suspect that a puzzle's answer is the Aztec priest-ruler Quetzalcoatl or the fire god Xiuhtecuhtle.
Open up a new file in your word processor and call it Wheel.java. Enter Listing 9.1 and save the file when you're done.
Listing 9.1. The full source code of Wheel.java.

 1: class Wheel {
 2:    public static void main(String arguments[]) {
 3:        String phrase[] = {
 4:            "A STITCH IN TIME SAVES NINE",
 5:            "DON'T EAT YELLOW SNOW",
 6:            "JUST DO IT",
 7:            "EVERY GOOD BOY DOES FINE",
 8:            "I WANT MY MTV",
 9:            "HOW `BOUT THEM COWBOYS",
10:            "PLAY IT AGAIN, SAM",
11:            "FROSTY THE SNOWMAN",
12:            "ONE MORE FOR THE ROAD",
13:            "HOME FIELD ADVANTAGE",
14:            "VALENTINE'S DAY MASSACRE",
15:            "GROVER CLEVELAND OHIO",
16:            "WONDERFUL WORLD OF DISNEY",
17:            "COAL MINER'S DAUGHTER",
18:            "WILL IT PLAY IN PEORIA"
19:        };
20:        int[] letterCount = new int[26];
21:        for (int count = 0; count < phrase.length; count++) {
22:            String current = phrase[count];
23:            char[] letters = current.toCharArray();
24:            for (int count2 = 0;  count2 < letters.length; count2++) {
25:                char lett = letters[count2];
26:                if ( (lett >= `A') & (lett < `Z') ) {
27:                    letterCount[lett - `A']++;
28:                }
29:            }
30:         }
31:        for (char count = `A'; count <= `Z'; count++) {
32:            System.out.print(count + ": " +
33:                    letterCount[count - `A'] +
34:                    "\t");
35:        }
36:        System.out.println();
37:    }
38: }

After you compile the file and run it, the output should resemble Listing 9.2. 

Listing 9.2. The output of the Wheel program.

 A: 22  B: 3   C: 5   D: 13  E: 28  F: 6   G: 5   H: 8   I: 18  J: 1
 K: 0   L: 13  M: 10  N: 19  O: 27  P: 3   Q: 0   R: 13  S: 15  T: 19
U: 4 V: 7 W: 9 X: 0 Y: 10 Z: 0

The following things are taking place in the Wheel program:
·         Lines 1 and 2: The Wheel program and the main() block of the program begin.
·         Lines 3-19: Phrases are stored in a String array called phrase. Every phrase between the { on Line 3 and the } on Line 19 will be stored in its own element of the array, beginning with A STITCH IN TIME SAVES NINE in phrase[0].
·         Line 20: An integer array called letterCount is created with 26 elements. This array will be used to store the number of times each letter appears. The order of the elements is from A to Z. letterCount[0] will store the count for letter A, letterCount[1] will store the count for B, and so on up to letterCount[25] for Z.
·         Line 21: A for loop is begun that cycles through the phrases stored in the phrase array. The phrase.length variable is used in the for statement to end the loop after the last phrase is reached.
·         Line 22: A String variable named current is created and set with the value of the current element of the phrase array.
·         Line 23: A character array is created that stores all of the characters in the current phrase.
·         Line 24: A for loop is begun that cycles through the letters of the current phrase. The letters.length variable is used to end the loop after the last letter is reached.
·         Line 25: A character variable called lett is created with the value of the current letter. In addition to their text value, characters have a numeric value. Because elements of an array are numbered, the numeric value of each character will be used to determine its element number.
·         Lines 26-28: An if statement is used to weed out all characters that are not part of the alphabet, such as punctuation and spaces. An element of the letterCount array is increased by 1 depending on the numeric value of the current character, which is stored in lett. The numeric values of the alphabet range from 65 for `A' to 90 for `Z'. Because the letterCount array begins at 0 and ends at 25, `A' (65) is subtracted from lett to determine which array element to increase.
·         Line 29: One pass through the inner for loop ends, and the loop returns to Line 24 to get the next letter in the current phrase, if there is one.
·         Line 30: One pass through the outer for loop ends, and the loop returns to Line 21 to get the next phrase, if there is one.
·         Line 31: A for loop is used to cycle through the alphabet from `A' to `Z'.
·         Lines 32-34: The current letter is displayed followed by a semicolon and the number of times the letter appeared in the phrases stored in the phrase array. The \t inserts a Tab character.
·         Line 35: One pass through the for loop ends, and the loop returns to Line 31 to get the next character in the alphabet, unless `Z' has been reached.
·         Lines 36-38: A blank line is displayed, followed by the end of the program's main() block, and the end of the program.
This workshop project shows how two nested for loops can be used to cycle through a group of phrases one letter at a time. Java attaches a numeric value to each character; this value is easier to use than the character inside arrays. Using the length variable makes it possible for you to add as many phrases as desired within the { and } marks. The letters in each of the new phrases that you add will be analyzed, and you can build up a better idea of what it takes to make a small fortune in 30 minutes on television. 

Summary
Arrays make it possible to store complicated types of information in a program and manipulate that information. They're ideal for anything that can be arranged in a list and can be accessed easily using the loop statements that you learned about during Hour 8, "Repeating an Action with Loops."
The information processing needs of Santa Claus possibly have outgrown arrays. There are more children being manufactured each year, and the gifts they want are increasing in complexity and expense. Tickle Me Elmo alone created a logistical nightmare for him in Christmas 1996. Your programs are likely to use arrays to store information that is unwieldy to work with through variables, even if you're not making any lists or checking them twice.

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