CS 112 Introduction to Programming

Transcription

1 CS 112 Introduction to Programming File as Input; Exceptions; while loops; Basic Arrays Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone:

2 Admin. q PS4 status q Kevin s Crash Review on Tuesday from 7 to 8 pm at AKW 400 q Puzzle Day from Friday to Monday 2

3 Recap q switch statement q String processing Access methods, e.g., length, substring, charat, indexof New string generator methods, e.g. tolowercase, touppercase, replace Output formatting: format string System.out.printf, String.format Boolean methods, e.g., equals, equalsignorecase, startswith, endswith, contains,

4 String charat method q The chars in a String can be accessed using the charat method. String food = "cookie"; char firstletter = food.charat(0); // 'c' System.out.println(firstLetter + " is for " + food); System.out.println("That's good enough for me!"); q You can use a for loop to print or examine each character. String major = "CS"; for (int i = 0; i < major.length(); i++) { char c = major.charat(i); System.out.println(c); } Output: C S

5 char vs. String q "h" is a String 'h' is a char (the two behave differently) q String is an object; it contains methods String s = "h"; s = s.touppercase(); // 'H' int len = s.length(); // 1 char first = s.charat(0); // 'H' q char is primitive; you can't call methods on it char c = 'h'; c = c.touppercase(); // ERROR

6 char vs. String: Comparison q You can compare char values with relational operators (ordering is alphabetical order): 'a' < 'b' and 'X' == 'X' and 'Q'!= 'q String word = console.next(); if (word.charat(word.length() - 1) == 's') { System.out.println(word + " is plural."); }

7 char vs. String: Comparison q Relational operators such as!= and == compares objects by references (seen later), so it often gives false even when two Strings have the same letters. Scanner console = new Scanner( System.in ); String name = console.next(); // user input Barney if (name == "Barney") { System.out.println("I love you, you love me,"); System.out.println("We're a happy family!"); }

8 char vs. int q Using char in arithmetic expressions will cause char to be converted to an int q To convert an int into the equivalent char, type-cast it. (char) ('a' + 2) is 'c' for (char c = 'a'; c <= 'z'; c = (char)(c+1) ) { System.out.print(c); } for (char c = 'a'; c <= 'z'; c ++ ) { System.out.print(c); }

9 Caesar Cipher If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others. Suetonius, Life of Julius Caesar 56

10 Caesar Cipher Plaintext attack said zeus προσβάλλω Ciphertext dwwdfn vdlg chxv στςφείξξό

11 Why Programming? 11

12 Caesar Cipher: Encoding Letter a b c d... w x y z Orig Shift (+3) Cipher (% 26) int cipherint = (origint + key) % 26

13 Caesar Cipher: Decoding Letter a b c d... w x y z Cipher Shift (-3) Text (+26 % 26) int origint = (cipherint key + 26) % 26

14 Unifying Design int cipherint = (origint + key) % 26 int origint = (cipherint key + 26) % 26 q A single encode method with parameter key To encode encode(key) To decode encode(26 key)

15 Extension: Encrypt a Text File q Goal: Instead of reading text from terminal, user specifies a file name, which gives the content to be encrypted. 15

16 Reading Specific Files in Program q To read a file, pass a File when constructing a Scanner. Example: File file = new File("mydata.txt"); Scanner input = new Scanner(file); or (shorter): Scanner input = new Scanner(new File("mydata.txt"));

17 Prompting and Checking Files q Recall: we can ask the user to tell us the file to read. // prompt for input file name Scanner console = new Scanner(System.in); System.out.print("Type a file name to use: "); String filename = console.nextline(); // next()? Scanner input = new Scanner(new File(filename)); q You can use File s exists method to test existence: File file = new File( data.txt"); if (!file.exists() ) { // try a second input file as a backup System.out.print( data file not found!"); file = new File( data2.txt"); }

18 Compiler Error w/ Scanning File import java.io.*; import java.util.*; // for File // for Scanner public class ReadFile { public static void main(string[] args) { File f = new File( msg.txt") Scanner input = new Scanner( f ); String text = input.next(); System.out.println(text); } } q The program fails to compile with the following error: ReadFile.java:9: unreported exception java.io.filenotfoundexception; must be caught or declared to be thrown Scanner input = new Scanner( f ); ^

19 Exceptions q Exception: representing a runtime error, e.g., dividing an integer by 0 calling substring on a String and passing too large an index trying to read the wrong type of value from a Scanner trying to read a file that does not exist q Java allows a method to "throw" an exception, when the method detects a run-time error q Consequence: if a method A invokes a method B that may throw an exception, method A must declare how it handles the exception

20 Handle Checked Exception q Declare a throw clause q Use try/catch 20

21 The throws Clause q throws clause: Keywords on a method's header to state that it may generate an exception (and will not handle it). q Syntax: public static <type> <name>(...) throws <type> { Example: public class ReadFile { public static void main(string[] args) throws FileNotFoundException { Like saying, "I hereby announce that this method might throw an exception, and I accept the consequences if this happens."

22 The try/catch Structure q try/catch: if an exception happens, program execution jumps to the catch statement, skipping the rest in the try block. import java.io.*; import java.util.*; // for File // for Scanner public class ReadFile { public static void main(string[] args) { try { Scanner input = new Scanner(new File( msg.txt")); String text = input.next(); System.out.println(text); } catch (FileNotFoundException e) { // print an error message System.out.println( File not found exception ); } // end of catch } } // end of main 22

23 Remaining Issue q CaesarFile needs to read a file containing an arbitrary number of lines of text. Hence, it will need an indefinite loop. q Indefinite loops are common in programming design, e.g., Read from user until input is a non-negative number. Repeat until the user has typed "q" to quit. Print random numbers until a prime number is printed. Search for a solution in a set. q A key to define an indefinite loop is to identify the loop s test condition.

24 Scanner Test for Input Status Method hasnext() hasnextint() hasnextdouble() hasnextline() Description returns true if there is a next token returns true if there is a next token and it can be read as an int returns true if there is a next token and it can be read as a double returns true if there is a next line. q These methods of the Scanner do not consume input; they just give information about what the next input will be. 24

25 CaesarFile using for Loop public static void encode(scanner scan, int key) { for ( ; scan.hasnextline(); ) String text = scan.nextline(); reads ugly } } String result = Caesar.encode(text, key); System.out.println( result ); 25

26 Java Repetition /Loop Statements q Java has three kinds of loop statements: the for loop the while loop, the do loop, and q They have the same expressive power q Which loop statements to use will depend on the context to be more intuitive 26

27 The while loop q while loop: Repeatedly executes its body as long as a logical test is true. while (<test>) { <statement(s)>; } q Example: while is a keyword int num = 1; while (num <= 10) { System.out.print(num + " "); num = num * 2; } // output: // initialization // test // update

28 The do/while loop q do/while loop: Similar to while, except move test at the end: do { <statement(s)>; } while (<test>); q Example: int num = 1; // initialization do { System.out.print(num + " "); num = num * 2; // update } while (num <= 10); // test // output:

29 CaesarFile using a while Loop public static void encode(scanner scan, int key) { while ( scan.hasnextline() ) { String text = scan.nextline(); } } String result = Caesar.encode(text, key); System.out.println( result ); 29

30 Additional Usage of Caesar Cipher q Augustus Right shift 1 without rotate, with AA for Z q Mezuzah Shift of one to encrypt the names of God q Vigenere (a variant) was used by the Confederacy during Civil War q Mafia boss Provenzano (2006): A (4), 30

31 Exercise: Breaking Caesar Encryption q Caesar encryption (or permutation ciphers in general) is easy to break because it does not change the frequency of letters.

32 Frequency Analysis Code (Attempt 1) int na = 0, nb = 0, nc = 0, nd = 0, ne = 0, nf = 0, ng = 0, nz = 0; while ( scan.hasnextline() ) { String line = scan.nextline(); for (int i = 0; i < line.length(); i++) { char ch = line.charat( i ); if ( ch == a ) na ++; else if ( ch == b ) nb ++; else if ( ch == c ) nc ++; } // find the max of na, nb, nc, 32

33 Improving Attempt 1 q We need a way to declare many variables in one step. q We need an effective scheme to refer to the variables q The objective of array is to provide these

34 Arrays q array: object that stores many values of the same type. element: one value in an array. index: array element referred to by position number starting from 0 index value element 0 element 4 element 9

35 Array Declaration After declaration, all elements of an int array are initialized to 0. <type>[] <name> = new <type>[<length>]; Example: int[] numbers = new int[10]; index value o The length in declaration can be any non-negative integer expression, e.g., int x = 2 * 3 + 1; double[] data = new double[x + 2]; // arr w/ 9 elem

36 Accessing Array Elements <name>[<index>] // access <name>[<index>] = <value>; // modify numbers[ 0 ] 85 numbers[ 1 ] 60 int[] numbers = new int[10]; numbers[ 2 ] numbers[ 3 ] numbers[ 4 ] 54 position number (index or subscript) of the element within array numbers numbers numbers[ 5 ] numbers[ 6 ] numbers[ 7 ] numbers[ 8] numbers[ 9 ] 81 36

37 Accessing Elements: Example int[] numbers = new int[10]; numbers[1] = 3; numbers[4] = 99; numbers[6] = 2; System.out.println(numbers[2-1]); if (numbers[4] > 10) { System.out.println( numbers[4] > 10."); } numbers index value BasicArrays.java

38 Accessing Elements: Example q What does this code segment do? int[] numbers = new int[8]; numbers[0] = 1; numbers[1] = 1; for (int i = 2; i < 8; i++) { numbers[i] = numbers[i-1] + numbers[i-2]; } index value

39 Backup Slides 39

40 File Input/output (I/O) import java.io.*; q Create a File object to get info about a file on your drive. (This doesn't actually create a new file on the hard disk.) File f = new File("example.txt"); if (f.exists() && f.length() > 1000) { f.delete(); } Method name canread() delete() exists() getname() length() Description returns whether file is able to be read removes file from disk whether this file exists on disk returns file's name returns number of bytes in file

41 File Paths q Absolute path: specifies a drive or a top "/" folder C:/Documents/smith/prob6/input/data.csv Windows can also use backslashes to separate folders. q Relative path: does not specify any top-level folder names.dat input/kinglear.txt Assumed to be relative to the current directory: Scanner input = new Scanner(new File("data/readme.txt")); If our program is in C:/prob6, File will look for C:/prob6/data/readme.txt ReadFile.java