Programmierpraktikum

Transcription

1 Programmierpraktikum Claudius Gros, SS2012 Institut für theoretische Physik Goethe-University Frankfurt a.m. 1 of 37 11/02/ :24 PM

2 Java - Control Flow 2 of 37 11/02/ :24 PM

3 controlling the flow do different things depending on circumstances Lamp doesn't work Do B While (A = TRUE) End While Lamp plugged in? No Plug in lamp B Yes Bulb burned out? Yes Replace bulb TRUE A No FALSE Repair lamp 3 of 37 11/02/ :24 PM

4 logical operators comparison and logics Symbol Description Examples < <= > >= less/-equal, greater/-equal if (i<=8) { ==!= equal, unequal if (i==3) { &&! logical AND, OR, NOT if ((a==0)&&(b!=3)) { instanceof object type comparison if (s instanceof String) { 4 of 37 11/02/ :24 PM

5 conditional if conditional if then else the String[] array args contains the input parameters when launching the program with java IfThenElse par1 par public class IfThenElse { public static void main(string[] args) { if (args.length == 1) { System.out.println("one: " + args[0]); if (args.length == 2) { System.out.println("two: " + args[0] + " " + args[1]); if (args.length == 1) { System.out.println("one"); 5 of 37 11/02/ :24 PM

6 else { System.out.println("not one"); if (args.length > 2) System.out.println("more than two parameters"); of 37 11/02/ :24 PM

7 nested if conditional nested if 1. public class NestedIf { public static void main(string[] args) { if (args.length == 0) System.out.println("zero"); else if (args.length == 1) System.out.println("one"); else if (args.length == 2) System.out.println("two"); else if (args.length == 3) System.out.println("three"); else System.out.println("more"); of 37 11/02/ :24 PM

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9 conditional switch conditional switch 1. public class Switch { public static void main(string[] args) { char operator = args[0].charat(0); // get first letter of first parameter in command line int operand1 = 12; int operand2 = 4; switch (operator) { case '+': System.out.println(operand1 + operand2); break; case '-': System.out.println(operand1 - operand2); break; case '*': 9 of 37 11/02/ :24 PM

10 System.out.println(operand1 * operand2); break; case '/': System.out.println(operand1 / operand2); break; default: System.out.println("unknown operator"); break; of 37 11/02/ :24 PM

11 control Flow: loop while loop while public class While { public static void main(string[] args) { int i = 0; while(i <= 100){ System.out.println(i); i += 10; of 37 11/02/ :24 PM

12 do while - loop loop do while 1. import java.util.scanner; public class DoWhile { public static void main(string[] args) { int i = 0; do { System.out.println(i); i += 10; while (i <= 100); Scanner scanner = new Scanner(System.in); System.out.println(scanner.delimiter()); String input; do { input = scanner.next(); 12 of 37 11/02/ :24 PM

13 18. while (!input.equalsignorecase("quit")); of 37 11/02/ :24 PM

14 for - loop loop for loops are everywhere public class For { public static void main(string[] args) { for (int i = 0; i < 10; i++) { System.out.println(i); of 37 11/02/ :24 PM

15 break & continue loop break & continue break out of the loop: break; re-entry the loop: continue; public class BreakContinue { public static void main(string[] args) { while(true){ // infinite loop // label α int i = (int) Math.round(Math.random() * 100); // random number [0, 99] if(i > 50) continue; // re-entry the loop (go to label α) System.out.print(i + ", "); if(i == 0) break; // break out of the loop (go to label β) // label β 15 of 37 11/02/ :24 PM

16 14. System.out.println("done"); of 37 11/02/ :24 PM

17 throwing and catching exceptions exception try catch finally exceptions are non regular program events exceptions are thrown whenever something goes wrong, like division by zero (runtime) trying to access a non-existing array elemnt (out of bound) trying to open a non existent file (IO)... a program will never crash if all exceptions are handled correctly for some operations, like opening files for reading or writing, exception handling is mandatory exception handling is optional for RuntimeException and subclasses exceptions are handled by wrapping the dangerous operation with a try {... catch () { bracket 17 of 37 11/02/ :24 PM

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19 example: exception handling Integer.parseInt(String)) member function of the class Integer converting a string into integers, if possible public class TryCatchFinally { /** This function throws an artimetic runtime exception * whenever a division by zero is perfomed. */ public static int devide (int dividend, int divisor) throws ArithmeticException{ if (divisor == 0) { throw new ArithmeticException("division by zero"); else return dividend / divisor; // end of TryCatchFinally.divide() /** Entry point. */ public static void main(string[] args) { 19 of 37 11/02/ :24 PM

20 try{ int result = TryCatchFinally.devide(Integer.parseInt(args[0]), Integer.parseInt(args[1])); System.out.println(result); catch(arithmeticexception exception) { // thrown by TryCatchFinally.devide() System.out.println("Division by zero!"); catch(arrayindexoutofboundsexception exception) { // thrown by calling args[] System.out.println("Provide two parameters!"); System.out.println(exception.toString()); catch(numberformatexception exception) { // thrown by Integer.parseInt() System.out.println("Provide only integer numbers!"); finally { // always executed System.out.println("Done with or without error."); // end of TryCatchFinally.main() // end of class TryCatchFinally() 20 of 37 11/02/ :24 PM

21 exercise: number game The computer chooses a random integer number of a given interval. The user tries to guess the number while the computer gives the feedbacks "too high" or "too low". get random number: 1. int random = (int) Math.round(Math.random() * 100); get input: Scanner scanner = new Scanner(System.in).useDelimiter("\n"); String input = scanner.next(); parse integer: 1. int number = Integer.parseInt(input); 21 of 37 11/02/ :24 PM

22 solution: number game 1. import java.util.scanner; 2. public class NumberGame { public static void main(string[] args) { Scanner scanner = new Scanner(System.in).useDelimiter("\n"); int min = 0, max = 100, input = 0, numberofguesses = 0; int random = (int) Math.round(Math.random() * (max - min)) + min; System.out.println("Guess the random integer number between " + min + " and " + max + "!") do { numberofguesses++; while (true) { System.out.print("guess " + numberofguesses + ": "); try { input = Integer.parseInt(scanner.next()); break; catch (NumberFormatException exception) { System.out.println("Integer number requested!"); 22 of 37 11/02/ :24 PM

23 if (input < random) { System.out.println(input + " is too low."); else if (input > random) { System.out.println(input + " is too high."); while (input!= random); System.out.println(input + " is correct!"); of 37 11/02/ :24 PM

24 exercise: factorial (iterative) The computer has to calculate the factorial of a non-negative integer number given by the user using iteration. 0! 1 n N \ {0: n! k [1, n] k = 1 2 n create a large number: 1. BigInteger number = BigInteger.valueOf(15); multiply numbers: 1. BigInteger product = number1.multiply(number2); 24 of 37 11/02/ :24 PM

25 factorials via iteration BigInteger: integers with arbitrary length 1. import java.math.biginteger; // arbitrary length integers public class IterativeFactorial { /** Factorial using BigIntegers. */ public static BigInteger factorial(int arg) { BigInteger result = BigInteger.valueOf(1); for (int i = 2; i <= arg; i++) { result = result.multiply(biginteger.valueof(i)); // Z return result; // only correct if arg /** Factorial using Long integers. */ public static long factorial(byte arg) { long result = 1; 25 of 37 11/02/ :24 PM

26 for (byte i = 2; i <= arg; i++) { result *= i; // [ 9,223,372,036,854,775,808, 9,223,372,036,854,775,807] return result; // only correct if arg [0, 20] public static void main(string[] args) { try { int intnumber = Integer.parseInt(args[0]); // [ 2,147,483,648, 2,147,483,647] BigInteger bigintegerfactorial = IterativeFactorial.factorial(intNumber); System.out.println(intNumber + "! = " + bigintegerfactorial); byte bytenumber = Byte.parseByte(args[0]); // [ 128, 127] long longfactorial = IterativeFactorial.factorial(byteNumber); System.out.println(byteNumber + "! = " + longfactorial); catch (Exception exception) { System.out.println("An error occured!"); System.out.println("usage: java IterativeFactorial "); of 37 11/02/ :24 PM

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28 factorials via recursion 1. public class Recursion { public static void down(int i) { System.out.println(i); if (i > 0) Recursion.down(i - 1); public static int sum(int i){ if(i > 0){ return i + sum(i - 1); else{ return i; public static void main(string[] args) { Recursion.down(10); System.out.println("100*101/2 = " + Recursion.sum(100)); 28 of 37 11/02/ :24 PM

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30 recursion: Fibonacci series training suggestion write a code for evaluating the n-th Fibonacci number f 0 0, f 1 1 n 2: f n f n 1 + f n + 2 possible solution 1. public class Fibonacci { public static long fibonacci(int arg) { switch (arg) { case 0: return 0; // f_0 := 0 case 1: return 1; // f_1 := 1 default: return Fibonacci.fibonacci(arg - 1) + Fibonacci.fibonacci(arg - 2); 30 of 37 11/02/ :24 PM

31 11. // f_n := f_(n-1) + f_(n-2) public static void main(string[] args) { try { int number = Integer.parseInt(args[0]); long fibonacci = Fibonacci.fibonacci(number); System.out.println("f_" + number + " = " + fibonacci); catch (Exception exception) { System.out.println("An error occured!"); System.out.println("usage: java Fibonacci "); of 37 11/02/ :24 PM

32 loops and sums training suggestion calculate results first and store them in vectors, then one loop for the output what happens? 1. public class TwoSum { public static void main(string args[]) { double sum, smallnumber; double bignumber = 1.0; int n = 100; // *** // *** with not too small number first // *** for (int exponent=4;exponent<19;exponent=exponent+2) { System.out.printf("exponent: %d\n",exponent); smallnumber = Math.pow(10.0,-exponent); 32 of 37 11/02/ :24 PM

33 sum = n*smallnumber + bignumber; // sum of smallnumber first System.out.printf("adding sum of small numbers to big number: %20.18f\n", sum); 17. // sum = bignumber; for (int i=0;i<n;i++) sum += smallnumber; System.out.printf(" sum); System.out.println(" "); adding small numbers to big number: %20.18f\n", 24. // // end of loop over exponents // end of TwoSum.main() // end of TwoSum 33 of 37 11/02/ :24 PM

34 numerics: summation large number of small numbers small terms first! 1. import java.lang.math; // standard import, always useful /** Series summation may depend on the order. * Summing 1/k^2 upwards and downwards. */ public class SeriesSummation { public static void main(string[] args) { double exact = Math.PI*Math.PI/6.0; // infinite series double resultup = 0.0; double resultdown = 0.0; int maxn = 0; for (int ii=4; ii<21; ii+=4) { 34 of 37 11/02/ :24 PM

35 14. maxn = ii* ; 15. resultup = SeriesSummation.summationUp(maxN); // both calls resultdown = summationdown(maxn); // possible System.out.printf("%20.16f %20.16f %20.16f\n", exact,resultup,resultdown); System.out.println(" "); System.out.printf("exact result : %20.16f\n",exact); System.out.printf("summing down : %20.16f\n",resultDown); System.out.printf(" summing up : %20.16f\n",resultUp); System.out.printf(" error : ^^\n"); System.out.printf(" maxn : %d\n",maxn); /** Summing upwards. */ public static double summationup(int maxn) { double result = 0.0; for (int i=1; i<=maxn; i++) result += 1.0/(1.0*i*i); return result; // end of summationup of 37 11/02/ :24 PM

36 37. /** Summing downwards */ public static double summationdown(int maxn) { double result = 0.0; for (int i=maxn; i>0; i--) result += 1.0/(1.0*i*i); return result; // end of summationup // end of class seriessummation error: why? exact result : summing down : summing up : of 37 11/02/ :24 PM

37 error : ^^ maxn : of 37 11/02/ :24 PM