EXERCISES SOFTWARE DEVELOPMENT I. 03 Control Flow & Branching Statements if/switch, while/do-while/for 2018W

Transcription

1 EXERCISES SOFTWARE DEVELOPMENT I 03 Control Flow & Branching Statements if/switch, while/do-while/for 2018W

2 1 st Test

3 I INFORMATION FOR THE 1ST TEST General information Date: Monday, November 12, 2018; 12:45 (- 13:05) Place: HS 1 & HS 16 HS 1: Exercise groups G1, G2, G3, G4 (instructors Haslgrübler / Jungwirth are present) HS 16: Exercise groups G5, G6 (instructors Grünberger / Trendafilov are present) Registration via KUSSS required and don t forget to bring your student id! Length: 20 minutes What/How? 1 short programming example (topic range: exercise lessons until Nov. 7/8, 2018) Allowed material/documents: 1 sheet of paper with notes, format A4, two-sided, e.g., loop constructs, type conversion table, switch/case construct) Not allowed: Books, Writing pad, Notebook, Smartphones, Calculators, or any other electronic equipment Software Development I // 2018W // 3

4 CHARACTER CONVERSION Complete the following program, which converts an upper case alphabetic character ("Buchstabe") to a lower case alphabetic character and vice versa, all other character types remain unchanged. e.g.: Input: 'b' => Output: 'B' Input: 'Y' => Output: 'y' Input: '9' => Output: '9' Input: '!' => Output: '! TODO Hints / Comments: You are not allowed to use any functions which operate on Strings or the Character class functions. Upper Case Characters have a lower decimal value in the ASCII table than lower case characters The distance between upper case and lower case characters is always the same, e.g. the distance between 'A' and 'a' is the same as between 'B' and 'b'. Software Development I // 2018W // 4

5 CHARACTER CONVERSION Complete the following program, which converts an upper case alphabetic character ("Buchstabe") to a lower case alphabetic character and vice versa, all other character types remain unchanged. e.g.: Input: 'b' => Output: 'B' Input: 'Y' => Output: 'y' Input: '9' => Output: '9' Input: '!' => Output: '! Hints / Comments: You are not allowed to use any functions which operate on Strings or the Character class functions. Upper Case Characters have a lower decimal value in the ASCII table than lower case characters The distance between upper case and lower case characters is always the same, e.g. the distance between 'A' and 'a' is the same as between 'B' and 'b'. Software Development I // 2018W // 5

6 DATA TYPES The figure on the right shows an electric circuit with two resistances. Resistance R1 is fixed with a value of 100 and resistance R2 can range from 5 to 15 and is entered by the user. Every correct entered value of R2 which is uneven should be decreased by 1. Afterwards, your program should calculate the total resistance of this circuit according to the following formula: Hints: Your program should calculate the result as floating-point value. Make sure that the user can only enter a correct number from 5 to 15. Use Input.readInt() to read in an integer value. TODO Software Development I // 2018W // 6

7 DATA TYPES The figure on the right shows an electric circuit with two resistances. Resistance R1 is fixed with a value of 100 and resistance R2 can range from 5 to 15 and is entered by the user. Every correct entered value of R2 which is uneven should be decreased by 1. Afterwards, your program should calculate the total resistance of this circuit according to the following formula: Hints: Your program should calculate the result as floating-point value. Make sure that the user can only enter a correct number from 5 to 15. Use Input.readInt() to read in an integer value. Software Development I // 2018W // 7

8 Control Flow & Branching Statements

9 CONTROL FLOW STATEMENTS :: DECISION MAKING Control flow statements break up the flow of execution by employing decision making (or looping and branching); this enables a program to conditionally execute particular statements or blocks of code. Java provides (as like most other modern programming languages) two types of decision-making statements for differentiation of cases: if statement (for binary branching) executes a certain section of code only if a particular test evaluates to true using the optional else statement, a secondary path of execution is provided when an if clause evaluates to false switch statement (for multiple branching) the switch statement can have any number of possible execution paths (if-else above is limited to 2) branching using switch works with the primitive data types byte, short, char, and int advanced: switch also works with enumerated types (enum types), the String-class, and a few special classes that wrap certain primitive types (Character, Byte, Short, Integer) Software Development I // 2018W // 9

10 CONTROL FLOW STATEMENTS :: BLOCKS Normally, every path of execution allows only a single statement; if you have more or less than one statement to execute, you have to group them using a block statement. A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed: EBNF: BlockStat = "" Stat Stat } "}". Example: public static void main (String[] arg) boolean condition = true; } if (condition) // begin block 1 System.out.println("Condition is true."); } // end block one else // begin block 2 ; } // end block 2 block with empty statement (";" can also be omitted) Software Development I // 2018W // 10

11 CONTROL FLOW STATEMENTS :: DECISION MAKING Binary branching EBNF: IfStat = "if" "(" BoolExpr ")" Stat1 [ "else" Stat2]. Interpretation when the expression BoolExpr evaluates to true, Stat1 will be executed, otherwise nothing or Stat2 BoolExpr needs to return a logical value (type boolean) by means of comparison operators: ==, <, >, <=, >=,!= using logical operations: &&,,! Conditional Operators: The && and operators perform conditional AND and conditional OR operations on two boolean expressions. These operators exhibit short-circuiting behavior, which means that the second operand is evaluated only if needed Software Development I // 2018W // 11

12 O CONTROL FLOW STATEMENTS :: EXAMPLES Calculation of postal charges Wanted: An algorithm that computes postal charges for packet delivery according to the following scheme: Cost table: first kilogram 13,00 2nd to 5th kilogram 06,00 (per kg) 6th to 10th kilogram 05,00 (per kg) from 11th kilogram 04,00 (per kg) Input: weight in kg Output: postal charges in Software Development I // 2018W // 12

13 CONTROL FLOW STATEMENTS :: EXAMPLES Calculation of postal charges Sample solution public class PostalCharges public static void main(string[] args) int weight = 0, costs = 0; System.out.print("Enter a weight (in kg): "); weight = Input.readInt(); if (weight > 10) costs = * * 5 + (weight - 10) * 4; } else if (weight > 5) costs = * 6 + (weight - 5) * 5; } else if (weight > 1) TODO costs = 13 + (weight - 1) * 6; } else costs = 13; // 1st kilogram } // end of if System.out.print("Postal charges (weight " + weight); System.out.println("kg): " + costs + " Euro"); } // end of main } // end of class UE03PostalCharges Software Development I // 2018W // 13

14 CONTROL FLOW STATEMENTS :: EXAMPLES Calculation of postal charges Sample solution public class PostalCharges public static void main(string[] args) int weight = 0, costs = 0; System.out.print("Enter a weight (in kg): "); weight = Input.readInt(); if (weight > 10) costs = * * 5 + (weight - 10) * 4; } else if (weight > 5) costs = * 6 + (weight - 5) * 5; } else if (weight > 1) costs = 13 + (weight - 1) * 6; } else costs = 13; // 1st kilogram } // end of if System.out.print("Postal charges (weight " + weight); System.out.println("kg): " + costs + " Euro"); } // end of main } // end of class UE03PostalCharges Software Development I // 2018W // 14

15 CONTROL FLOW STATEMENTS :: DECISION MAKING Multiple branches (Switchgroup) EBNF: SwitchStat = "switch" "(" Expr ")" "" SwitchGroup } "}". SwitchGroup = SwitchLabel SwitchLabel } Statement }. SwitchLabel = "case" ConstExpr ":" "default" ":". any number of statements; optionally closed by a break statement every constant expression (ConstExpr) only once! only one default label Interpretation Control expression Expr must be of data type byte, char, short or int (or enum, wrapper class) Data types of constant expressions SwitchLabel must be compliant with data type of control expression Expr Software Development I // 2018W // 15

16 CONTROL FLOW STATEMENTS :: DECISION MAKING Multiple branches (Switchgroup) switch (...) case...: stat1a; case...: stat1b; stat2b; stat3b; break; case...:... default:.. } // end of switch - arbitrary number of SwitchLabels - any number of statements per SwitchGroup - last statement in a SwitchGroup should be the break statement - default can be used to cover unexpected failures (if possible at all) Software Development I // 2018W // 16

17 I CONTROL FLOW STATEMENTS :: EXAMPLES Determination of the following date Wanted: An algorithm that determines based on a given date (day, month) the date of the next day (leap years not considered) Example Input: day (int), month (int) Output: date of the next day (format day. month.) Software Development I // 2018W // 17

18 CONTROL FLOW STATEMENTS :: EXAMPLES Determination of the following date Sample solution public class FollowDate public static void main(string[] args) int day; int month; int nrofdays = 0; day = Input.readInt(); month = Input.readInt(); System.out.print("Given date: " + day + "." + month + "., "); switch(month) case 1: case 3: case 5: case 7: case 8: case 10: case 12: nrofdays = 31; break; case 2: TODO nrofdays = 28; break; case 4: case 6: case 9: case 11: nrofdays = 30; break; } // end of switch // Correct number of days for the given month is now stored in nrofdays Software Development I // 2018W // 18

19 CONTROL FLOW STATEMENTS :: EXAMPLES Determination of the following date Sample solution public class FollowDate public static void main(string[] args) int day; int month; int nrofdays = 0; day = Input.readInt(); month = Input.readInt(); System.out.print("Given date: " + day + "." + month + "., "); switch(month) case 1: case 3: case 5: case 7: case 8: case 10: case 12: nrofdays = 31; break; case 2: nrofdays = 28; break; case 4: case 6: case 9: case 11: nrofdays = 30; break; } // end of switch //continued on next page Software Development I // 2018W // 19

20 CONTROL FLOW STATEMENTS :: EXAMPLES Determination of the following date Sample solution if (day < nrofdays) day = day + 1; else day = 1; month = month + 1; if (month == 13) month = 1; } System.out.println("date of the next day: " + day + "." + month + "."); } // end of main } // end of class UE03FollowDate I/O dialog 28 2 Given date: 28.2., date of the next day: Given date: , date of the next day: 1.1. Software Development I // 2018W // 20

21 I CONTROL FLOW STATEMENTS :: WHILE LOOP The while statement continually executes a block of statements (Stat) while a particular condition (Expr) is true Evaluation of the expression before first execution of Stat Stat will possibly not be executed at all EBNF: "while" "(" Expr ")" Stat. Example: public static void main (String[] arg) int count = 1; } while (count < 11) // repeated block System.out.println("Count is: " + count); count ++; } Expr==true Stat Yes No Software Development I // 2018W // 21

22 I CONTROL FLOW STATEMENTS :: DO-WHILE LOOP The do-while statement is similar to the while loop, however, do-while evaluates its expression (Expr) at the bottom of the loop instead of the top. Therefore, the statements (Stat) within the block are always executed at least once Evaluation of the expression after first execution of Stat Stat will be executed at least once EBNF: "do" Stat "while" "(" Expr ")". Stmt Example: public static void main (String[] arg) int count = 1; } do // repeated block System.out.println("Count is: " + count); count ++; } while (count < 11); Expr==true No Yes Software Development I // 2018W // 22

23 I CONTROL FLOW STATEMENTS :: FOR LOOP The for statement provides a compact way to iterate over a range of values. The general form is given in the following EBNF representation EBNF: "for" "(" Expr1 ";" Expr2 ";" Expr3 ")" Stat. Please keep in mind that The initialization expression (Expr1) initializes the loop; it is executed once at the start of the loop When the termination expression (Expr2) evaluates to false, the loop terminates The increment expression (Expr3) is invoked after each iteration through the loop Example: public static void main (String[] arg) for (int i=1; i<11; i++) // looping over i System.out.println("Count is: " + i); } } Expr1 Expr2==true Stat Expr3 Yes No Software Development I // 2018W // 23

24 I CONTROL FLOW STATEMENTS :: INFINITE LOOPS Using these control flow statements, it is very easy to implement an infinite loop Examples while (true) // any code can go here } i = 5; do // any code can go here } while (true); i = 5; Java recognizes such primitive endless loops and signals this with - "Unreachable Code" or - "Statement not reachable" (displayed in the line i = 5;) Eclipse screenshot for ( ; ; ) // any code can go here } i = 5; Software Development I // 2018W // 24

25 CONTROL FLOW STATEMENTS :: UNINTENDED INFINITE LOOPS Avoid to use floating point variables/expression for exact comparison! Examples double d = 1; while (d!= 0.0) d = d * 2 / 3; System.out.println("value: " + d); } Console output: value: value: value: value: value: value: value: value: 4.9E-324 value: 4.9E-324 value: 4.9E-324 Final result: prints 4.9E-324 forever (and never 0!!), i.e., results in an infinite loop - problem caused by imprecise representation of floating point variables (in termination condition) - even tough there is an exact representation of 0.0f/d in Java This problem cannot happen when using integer data types: int i = 4; while (i!= 0.0) i = i * 2 / 3; System.out.println("value: " + i); } Console output: value: 2 value: 1 value: 0 Software Development I // 2018W // 25

26 I CONTROL FLOW STATEMENTS :: WHICH LOOPS TERMINATES? int i, j, k; // declaration used for all code snippets Example 1 yes Example 3 j = 5; do j = j 1; } while (j!= 0); yes j = 5; do j = j 2; } while (j >= 0); Example 2 no Example 4 yes j = 5; do j = j 2; } while (j!= 0); j = -5; do j = j / 2; } while (j!= 0); Software Development I // 2018W // 26

27 I CONTROL FLOW STATEMENTS :: WHICH LOOPS TERMINATES? int i, j, k; // declaration used for all code snippets Example 5 no Example 7 i = 1; j = 1; while (i!= 100) k = i; i = j + i; j = k; } Example 6 yes i = 1; j = 1; while (i <= 100) k = i; i = j + i; j = k; } no i = 1; j = 1; while (i < 100) j = j + i; } Software Development I // 2018W // 27

28 I EXAMPLE :: FIBONACCI NUMBERS We are looking for a program that expects a number n (n > 1) as input and checks, whether the provided input is a Fibonacci number or not. If n is not a Fibonacci number, the program returns the next smaller Fibonacci number If n is a Fibonacci number, the number itself is returned Test your program by simulating it with a dry run test for the values n = 2, 5, and 6 The sequence F n of Fibonacci numbers is defined as follows: Fib(0) = 0 Fib(1) = 1 Fib(n) = Fib(n - 1) + Fib(n - 2) Software Development I // 2018W // 28

29 SAMPLE SOLUTION :: FIBONACCI NUMBERS public class Fibonacci public static void main(string[] arg) int n, fm0 = 0, fm1 = 1, f = 1; System.out.print("Enter a number: "); n = Input.readInt(); while (n <= 1) System.out.println("Number needs to be > 1!"); n = Input.readInt(); } while (n > f) f = fm0 + fm1; fm1 = fm0; fm0 = f; } if (n == f) System.out.println(n + " is a Fibonacci number!"); else System.out.println(n + " is not a Fibonacci number!"); System.out.println("Next smaller Fibonacci number: " + fm1); } } // end of main } // end of class UE03Fibonacci Software Development I // 2018W // 29

30 printf (for your Reference)

31 FORMATTED OUTPUT :: PRINTF Signature: printf(string format, Object... args) format is a text with placeholders Syntax see args is a comma separated list of objects to be used for the placeholders Example Placeholder Format for Numbers: %[argument_index$][flags][width][.precision]conversion int a = 65; //print an integer as a character and an integer System.out.printf("%c %1$d\n", a); //alternative way System.out.printf("%c %d\n", a, a); //print a float with 2 decimal places and without restriction System.out.printf("%.2f %1$f\n", (float)a); //Left Padding System.out.printf("%3d \n%3d ", a+a, a); Program Output: A 65 A Software Development I // 2018W // 31

32 Modulo Operation in Java (for your Reference)

33 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Calculates the remainder of a division; operator is in Java (as compared to other programming languages such as C#, C++) also defined as floating point operator Operation mode Result is negative if the numerator is negative, otherwise positive (a % b) is always <= b (float), < b (int) ArithmeticException thrown if denominator equal to zero "Dividend" a... Numerator b... Denominator "Divisor" a mod I b = a (a / b) * b (Integer) a mod F b = a ((int)(a / b)) * b (Float) Software Development I // 2018W // 33

34 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Definition of the Integer type int a, b; a mod I b (a % b) a - (a / b) * b; (a / b) * b + (a % b) = a Example 1: 5 % 3 ==? [ 5 / 3 = 1 ] (a / b) * b + (a % b) == a (5 / 3) * 3 + (5 % 3) == 5 (1) * 3 + (5 % 3) == 5-3 (5 % 3) == 2 Example 2: 5 % -3 ==? [ 5 / -3 = -1 ] Please note the higher precedence of unary operators! 5 % -3 5 % (-3) (5 / -3) * -3 + (5 % -3) == 5 (-1) * -3 + (5 % -3) == (5 % -3) == 5-3 (5 % -3) == 2 Software Development I // 2018W // 34

35 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Definition of the Integer type int a, b; a mod I b (a % b) a - (a / b) * b; (a / b) * b + (a % b) = a Example 3: -5 % 3 ==? [ -5 / 3 = -1 ] (-5 / 3) * 3 + (-5 % 3) == -5 (-1) * 3 + (-5 % 3) == (-5 % 3) == -2 Example 4: -5 % -3 ==? [ -5 / -3 = 1 ] (-5 / -3) * -3 + (-5 % -3) == -5 (1) * -3 + (-5 % -3) == (-5 % -3) == -2 Software Development I // 2018W // 35

36 I JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Floating point type float a, b a modf b Operation mode If one operand is NaN, the result is also NaN (not a number) If the result is not NaN it has the same sign as the numerator If the numerator is infinite or the denominator == 0, the result is NaN If the numerator is finite and the denominator infinite, the result equals the value of the numerator If the numerator == 0 and the denominator is finite, the result equals the value of the numerator Else (regardless of NaN, and 0): No exceptions will be thrown and there is no overflow, underflow or loss of precision Software Development I // 2018W // 36

37 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Definition of the Floating point type float a, b a mod F b a - b * (int)(a / b) == a mod b Example 1: 5.0 % 3.0 ==? * (int)(5.0 / 3.0) == 5.0 % * 1 == 5.0 % == 5.0 % 3.0 Example 2: 5.0 % -3.0 ==? * (int)(5.0 / -3.0) == 5.0 % * -1 == 5.0 % == 5.0 % == 5.0 % -3.0 Software Development I // 2018W // 37

38 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Definition of the Floating point type float a, b a mod F b a - b * (int)(a / b) == a mod b Example 3: -5.0 % 3.0 ==? * (int)(-5.0 / 3.0) == -5.0 % * -1 == -5.0 % == -5.0 % == -5.0 % 3.0 Example 4: -5.0 % -3.0 ==? * (int)(-5.0 / -3.0) == -5.0 % * 1 == -5.0 % == -5.0 % == -5.0 % 3.0 Software Development I // 2018W // 38

39 JAVA OPERATORS :: MODULO (REMAINDER) OPERATOR Definition of the Floating point type float a, b a mod F b a - b * (int)(a / b) == a mod b Example 5: -5.2 % 2.67 ==? * (int)(-5.2 / 2.67) == -5.2 % * (int)(-1.94) == -5.2 % == -5.2 % == -5.2 % 2.67 Software Development I // 2018W // 39

40 EXERCISES SOFTWARE DEVELOPMENT I 03 Control Flow & Branching Statements if/switch, while/do-while/for 2018W