Case Study: Savings Account Interest

Transcription

1 ecture 8 Loops: recap + example. Files: abstracting from a specific devise. Streams and Tokens. Examples. Material from the second half of Holmes Chapter 4. 1 w do you add up a sequence of numbers? = 1; do { for ( i=0; i<k; i++) { ile (n > 0) { input next n; input next n; input next n; sum = sum + n; sum = sum + n; sum = sum + n; while (n > 0); but notice the subtle differences between the three. The next is a slightly tier way of using a for loop to simulate a while. for ( n=1; n>0; ) { input next n; sum = sum + n; 2 4 Before the interest for the next quarter is computed must be redefined as. The length of the investment is given as input as an integer. Analysis. The interest for one quarter of a year, given a lump sum of pounds and an interest rate is 3 Problem. You open a savings account with a specified amount of money; the bank will pay 1.5% interest each quarter if your deposit is below 10,000 pounds and 2% otherwise. You plan to make no deposit or withdrawals, but you want to see what each quarterly balance, including interest, will be over a given period of time. Case Study: Savings Account Interest

2 eudo-code input balance and term (in number of years) output headings calculate the number of quarters for every quarter in the term calculate interest calculate balance output quarter year, interest and balance 5 ata dictionary. nal float QUARTERLY_RATE_LOW = 0.015f; nal float QUARTERLY_RATE_HIGH = 0.02f; oat balance; oat interest; t term; t lastquarter; yout. We want to print a number of lines each containing the newly oduced interest and the current balance. 6 // chap_4\ex_9.java // program to calculate the accumulated interest payable on a savings // account import java.io.*; class Ex_9 { static BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); final float QUARTERLY_RATE_LOW = 0.015f; final float QUARTERLY_RATE_HIGH = 0.02f; float balance, interest; int term, lastquarter; boolean high = false; System.out.print("Input initial balance "); balance = new Float(keyboard.readLine()).floatValue(); System.out.print("Input length of investment in years "); term = new Integer(keyboard.readLine()).intValue(); System.out.println("\nquarter\tinterest\tbalance\n"); lastquarter = 4 * term; 6-1 for (int q = 1; q <= lastquarter; q++) { if (balance > 10000) { interest = balance * QUARTERLY_RATE_HIGH; if (!high) { System.out.println("Start of high interest regime!"); high = true; else interest = balance * QUARTERLY_RATE_LOW; balance = balance + interest; System.out.println(q + "\t" + interest + "\t" + balance); 6-2

3 JAVA]\$ java Ex_9 ut initial balance 9000 ut length of investment in years 4 rter interest balance rt of high interest regime! ore General Input and Output so far: keyboard and (Holmes) screen. atic BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); atic PrintWriter screen = new PrintWriter(System.out, true); might be from different places : the program might receive data from ext file or save data to a text file. How is this handled in Java? and (FIRST) What is a file? 7 File [A. Tanenbaum] A file consists of a sequence of bytes written to an I/O device such as a disk, a CD, a printer. Example. Your Java programs are stored as text files. Getting into the jargon: variables are declared, files are opened! FileReader Physical File Your Program FileReader file = new FileReader("/chap 4/data.txt"); But that s not all!! 8 Buffering FileReader BufferedReader Physical File Your Program BufferReader inputfile = new BufferReader(file); By doing this we abstract further from the physical details and provide a mechanism for treating, say, keyboard input in the same way as input coming from a file. In practice this gives us more things (or methods) that we can do with a file. How to close a file (buffered reader). inputfile.close(); After this method the relationship between inputfile and file is lost. 9

4 chap_4\ex_6 program to read data from a file and display it on the screen ort java.io.*; ss Ex_6 { static BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in)); FileReader file = new FileReader("data.txt"); BufferedReader inputfile = new BufferedReader(file); String name; float price; while (price!= 0) { name = inputfile.readline(); System.out.println(price + "\t"+ name); inputfile.close(); 9-1 od for Thought.. Write a program that copies the content of a text file into another. (Assume that both files exist in your current directory). (Hints: see example further on).. Write a program that merges the contents of two text files into a third one, interleaving lines from one file with lines from the other. (Assume that all files exist in your current directory). e following example shows how the content of a file can be read, dified and copied in another file. Note that the output file is rewritten ery time the program is started! 10 // chap_4\ex_7 // program to read data from a file, modify the information // and write the information back to a different text file import java.io.*; class Ex_7 { final float RATE_OF_INFLATION = 0.025f; FileReader file1 = new FileReader("data.txt"); BufferedReader inputfile = new BufferedReader(file1); FileWriter file2 = new FileWriter("results.txt"); PrintWriter outputfile = new PrintWriter(file2); String name; float price; while (price!= 0) { price = price + (price * RATE_OF_INFLATION); name = inputfile.readline(); outputfile.println(price + " " + name); inputfile.close(); outputfile.close(); 10-1 Tokenizing Java offers other features for dealing with text files. We need to make a little diversion first! Strings of text handled by a program my have some structure. String inputdata = "First 124 Second 9012\n"; An object in the StringTokenizer class is an entity that is capable of breaking strings into tokens. In our Factory metaphor, it is like a worker that gets a string from a moving belt and chops it into pieces, giving the pieces on request, one after the other. Once the pieces have been used once they are lost. 11

5 lass Definition: StringTokenizer Constructors (used in declarations) blic StringTokenizer (String str); Constructs a string tokenizer for the specified string. Delimiters are "\t\n\r\f". blic StringTokenizer (String str, String delim); Constructs a string tokenizer for the specified string. User defined delimiters. blic StringTokenizer (String str, String delim, boolean rettok); Constructs a string tokenizer for the specified string. User defined delimiters. If the returntokens flag is true, then the delimiters are also returned as tokens (each delimiter returned as a string of length one. If the flag is false, the delimiter characters are skipped. 12 ethods Methods (services provided by variables/objects in the class) blic boolean hasmoretokens (); Tests if there are more tokens available from this tokenizer s string. blic String nexttoken (); blic String nexttoken (String delim); Methods for reading the next token to a string. The specification of the delimiter overrides any previous setting. blic int counttokens (); Calculates the number of times that this tokenizer s nexttoken method can be called before it generates an error. 13 // Simple program to demostrate the use of a StringTokenizer // What is this program actually doing?? import java.io.*; import java.util.*; class StringTok { static BufferedReader keyboard = new BufferedReader ( new InputStreamReader(System.in) ); public static void main (String[] args) throws IOException { String s; StringTokenizer data; System.out.println("Waiting for your input... "); s = keyboard.readline(); data = new StringTokenizer ( s ); System.out.println( new Integer ( data.nexttoken() ).intvalue() + new Integer ( data.nexttoken() ).intvalue() ); System.out.println( data.counttokens() + "Tokens still available to the Tokenizer!" ); 13-1 Now then... Text files are just a sequence of lines, each line being a string! String tokenizers can be applied to these strings! Here comes an example! 14

6 chap_4\ex_8.java program to demonstrate how a string of text can be tokenized into single words that are separated by white space characters ort java.io.*; import java.util.*; ss Ex_8 { FileReader file = new FileReader("passage.txt"); BufferedReader inputfile = new BufferedReader(file); StringTokenizer data; int numberoftokens, numberofwords = 0, words; data = new StringTokenizer(inputFile.readLine()); numberoftokens = data.counttokens(); while (numberoftokens!=0) { for ( words=1; words <= numberoftokens; words++ ) System.out.print(data.nextToken() + " "); System.out.println(); numberofwords = numberofwords + numberoftokens; data = new StringTokenizer(inputFile.readLine()); numberoftokens = data.counttokens(); System.out.println("There are "+numberofwords+" words in the text."); 14-1 torial. Write a program using while loops to output the arithmetic mean of a sequence of positive integers. (For instance if the input is 12, 7, 12, 10, 9, then the result should be 10.00).. Rewrite the program above using for loops.. (A real mind-cracker ) Write a program for sorting the lines of a text file. Comment: for this exercise it is very important that you first think on paper to the algorithm you want to use. (Hints: you may assume that the file contains one number (as a sequence of characters) per line and that the numbers are integers; alternatively (harder) you can order the lines of the file in lexicographic order using the compareto method in the String class. the program will need to use additional files for copying purposes.) 15