Midterm Review Session Programming Problems

1. Midterm Review Session Programming Problems For more practice: http://webster.cs.washington.edu:8080/practiceit/

2. Midterm Logistics • Bring your UW Student ID card!! • Will check your ID while you’re leaving • Bring your stuff with you while you exit (don’t go back to seat) • 60 minute exam • -10 if you start early or try to write after time • Any paper notes allowed, but NO ELECTRONICS • Book OK, section handouts OK, past exams + solutions OK, etc. • NO CALCULATORS! NO LAPTOPS! • Aiming for a median of 80 • Will be a curve if the exam is too hard, will not have a hurtful curve if exam is too easy

3. About the Exam • 100 pts, 20% of course grade • Non-programming – 61 pts • Expressions (10), parameter mystery (12), if/else simulation (12), while loop simulation (12), assertions (15) • Easy Programming – 15 pts • Medium Programming – 15 pts • Really Hard Programming – 9 pts • Topics covered: Chapters 1-5 • Topics not covered: see website! • Includes Graphics, printf, do/while, break

4. More Exam Info • Non-programming questions • You do not need to show your work • You are not allowed to use calculators (e.g. on expressions) • Programming questions • Are not graded on style, only external correctness • Must write valid Java code that would compile • Will not grade comments or pseudo-code • Do NOT use abbreviations • Do not have to write import statements • Substantial partial credit is given for partially working solutions • Write as much as you can: even a method header and the beginnings of a while loop could earn you partial points! • More info on course website!

5. General Tips • BE AFRAID. Success on HW is not an indication of being prepared for the exam. • Watch the clock! • The hardest part about this exam will be the time limit. • PRACTICE Q1-Q5 so that you can go through them fast and accurately! You want as much time as possible for the programming questions. • Write as much as you can! • A partial answer is better than none at all. • Writing the correct method header will earn at least 1 point. • Writing a partial loop, if statements, returns, etc. will points too

6. General Tips • Read the instructions now and learn the format so you won't be surprised tomorrow • The format of the midterm will be the same as the samples • For more practice, use Marty Stepp's Practice-it tool: http://webster.cs.washington.edu:8080/practiceit/

7. Concepts Review Some essential topics from Ch1-5

8. For-loops • Use to repeat stuff when you know exactly* how much you want to repeat • * Could also have a variable or expression that tells you exactly how much you want to repeat • Nested for-loops: loops within a loop • When you want to do task multiple times, and that task has repetition to it • E.g. multiplication table • Cumulative sum: variable that keeps a sum in progress and is updated repeatedly until summing is finished • In general: think about bounds, possible fencepost issues

9. Conditionals and Tests • If/else statement: Decide between several logical choices • While loop: Repeat unknown number of times • Sentinel loops: repeat until a certain signal is seen (e.g. prompt until you see a -1) • boolean: primitive type with a true or false value • When to return?? • Boolean flag is helpful

10. Strings • length() to get length of a String • indexOf/containsto search for substring within String • equals/equalsIgnoreCase to test equality • startsWith/endsWith to test beginning/end of word • substring(index1, index2)to get a piece of a String • index1 inclusive, index2 exclusive • Index 0 for first letter, index length() -1 for last letter • + to concatenate Strings together

11. Other miscellaneous stuff • Scanner to prompt for input • hasNext, hasNextInt, hasNextDouble, hasNextLine • Random to get a random value • To generate random number between [min, max]:r.nextInt(range) + min Where range: max – min + 1 • r.nextInt(51) + 50; // produces random value 50 - 100 • Expressions stuff: • % 10 and / 10 to get digits • % 2 == 0 to test for even (!= for odd)

12. Programming Practice Practice problems from previous exams

13. Programming Tips • Read the problems carefully: • Does it want you to print a result, or return it? • What values does the method use for computation? Are these values parameters, are they read from a Scanner, etc.? • What type of value (if any) does the method return? • Have you handled all special cases? • What if the integer is 0, or negative? • What if the string has no letters? • What if there is only one word in the string? Many words?

14. Practice Problem 1: Print Multiples • Write a static method named printMultiples • Takes two integers n and m as parameters and prints the first m multiples of n • Assume m >= 1 • Multiples are separated by commas printMultiples(3, 5); The first 5 multiples of 3 are 3, 6, 9, 12, 15 printMultiples(7, 3); The first 3 multiples of 7 are 7, 14, 21

15. Print Multiples Solution public static void printMultiples(int n, int times) { System.out.print("The first " + times + " multiples of " + n + " are " + n); for (int i = 2; i <= times; i++) { System.out.print(", " + i * n); } System.out.println(); }

16. Practice Problem 2: Count Even Digits • Write a static method named countEvenDigits • Accepts an integer as its parameter and returns the number of even-valued digits in that number. • Even digits: 0, 2, 4, 6, or 8 • Assume value passed to your method is non-negative // 4 even digits: two 8s, the 4, and 6 int x = countEvenDigits(8346387) ; System.out.println("x is " + x); // x is 4

17. Count Even Digits Solution public static int countEvenDigits(int n) { int count = 0; while (n != 0) { int digit = n % 10; n = n / 10; if (digit % 2 == 0) { count++; } } return count; }

18. Practice Problem 3: Cheerleader • Write a static method named cheerleader • Takes two params: • number of lines of output • number of "cheers" per line • Cheer structure: • 1 cheer: Go • 2 cheers: Go Team Go • 3 cheers: Go Team Go Team Go • Each line indented by 3 spaces; first line at 0 spaces cheerleader(2, 1); Go Go cheerleader(4, 3); Go Team Go Team Go Go Team Go Team Go Go Team Go Team Go Go Team Go Team Go cheerleader(2, 4); Go Team Go Team Go Team Go Go Team Go Team Go Team Go

19. Cheerleader Solution public static void cheerleader(int lines, int cheers) { for (int line = 1; line <= lines; line++) { for (int space = 1; space <= line - 1; space++) { System.out.print(" "); } for (int cheer = 1; cheer <= cheers - 1; cheer++) { System.out.print("Go Team "); } System.out.println("Go"); } }

20. Practice Problem 4: Favorite Letter • Write a static method named faveLetter • Accepts two parameters: • Scanner for the console • Favorite letter represented as a one-letter String • Repeatedly prompt the user until two consecutive words are entered that start with that letter. • Case sensitive • Print a message showing the last word typed. (assume a Scanner named console was made earlier in code) faveLetter(console, "y"); Looking for two "y" words. Type a word: hi Type a word: bye Type a word: yes Type a word: what? Type a word: yellow Type a word: yippee "y" is for "yippee"

21. Favorite Letter Solution public static void faveLetter(Scanner console, String letter) { System.out.println("Looking for two \"" + letter + "\" words."); int count = 0; String word = ""; while (count < 2) { System.out.print("Type a word: "); word = console.next(); if (word.startsWith(letter)) { count++; } else { count = 0; } } System.out.println("\"" + letter + "\" is for \"" + word + "\""); }

22. Practice Problem 5: Random Rects • Write a static method named randomRects • Calculates and displays the area of randomly-generated rectangles. • Each rectangle has random width and height btwn 1 and 10 inclusive • Keep generating rectangles until an increasing sequence of four areas is printed. • Stop when the last four rectangles generated have areas of a1, a2, a3 and a4 such that a1 < a2 < a3 < a4 randomRects(); w: 5, h: 6, area: 30 w: 10, h: 5, area: 50 w: 2, h: 8, area: 16 w: 4, h: 4, area: 16 w: 2, h: 9, area: 18 w: 8, h: 3, area: 24 w: 7, h: 2, area: 14 w: 3, h: 10, area: 30 w: 7, h: 9, area: 63 w: 9, h: 8, area: 72 End random rectangles.

23. Random Rectangles Solution public static void randomRects() { Random r = new Random(); int last = 0; int count = 0; while (count != 4) { int w = r.nextInt(10) + 1; int h = r.nextInt(10) + 1; int area = w * h; if (area <= last) { count = 1; // need to count first rect in sequence } else { count++; } System.out.println("w: " + w + ", h: " + h + ", area: " + area); last = area; } System.out.println("End random rectangles."); }

24. Non-Programming Practice Tips on Questions 1-4 (For assertions help, see lecture 14)

25. Expressions • integer division and mod: quotient and remainder 14 / 3 is 4, 14 % 3 is 2 7 / 10 is 0, 7 % 10 is 7 • precedence: ( ) before * / % before + - 5 + 2 * 6 - (1 + 7) % 3 5 + 2 * 6 - 8 % 3 5 + 12 - 8 % 3 5 + 12 - 2 17 - 2 15

26. Expressions 2 • String concatenation: same precedence as integer + -, evaluated left-to-right with other + - operations 1 + 2 + "3" + 4 + 5 3 + "3" + 4 + 5 "33" + 4 + 5 "334" + 5 "3345" • type promotion: done as needed when int and double are mixed 50 / 6 / 5.0 8 / 5.0 1.6

27. Expression questions • Evaluate the following expressions: 16 / 3 + 3.2 * 2 8 / 7 + "5 / 4" + 8 / 3 88 % 10 % 3 * 16 / 10 29 / 3 / 2 / 4.0 + 3.6 * 2 1.4 + (3 + 2 * 6) / (8 - 14 / 3) * 2.2

28. Expression answers • Correct answers: 16 / 3 + 3.2 * 2 11.4 8 / 7 + "5 / 4" + 8 / 3 "15 / 42" 88 % 10 % 3 * 16 / 10 3 29 / 3 / 2 / 4.0 + 3.6 * 2 8.2 1.4 + (3 + 2 * 6) / (8 - 14 / 3) * 2.2 8.0

29. Parameter mystery question • What is the output of the following program? public class Mystery { public static void main(String[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); speak(george, work, john); speak(fun, "george", "work"); speak(george, mary, john); speak(george, "john", "dance"); } public static void speak(String mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); } }

30. Parameter mystery tips • Try making a table of the parameters passed to each call: public class Mystery { public static void main(String[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); speak(george, work, john); speak(fun, "george", "work"); speak(george, mary, john); speak(george, "john", "dance"); } public static void speak(String mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); } }

31. Parameter mystery answer • What is the output of the following program? public class Mystery { public static void main(String[] args) { String john = "skip"; String mary = "george"; String george = "mary"; String fun = "drive"; String work = "john"; speak(mary, john, fun); skip likes to drive with george speak(george, work, john); john likes to skip with mary speak(fun, "george", "work"); george likes to work with drive speak(george, mary, john); george likes to skip with mary speak(george, "john", "dance"); john likes to dance with mary } public static void speak(String mary, String john, String fun) { System.out.println(john + " likes to " + fun + " with " + mary); } }

32. While loop mystery question • Given the following program, public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; } System.out.println(x + " " + z); } • What is the output of the following sequence of calls? mystery(0); mystery(8); mystery(32); mystery(72); mystery(184); mystery(8239);

33. While loop mystery tips • Keep track of each variable's value as it changes: public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; } System.out.println(x + " " + z); } • What is the output of the following call? mystery(184); • Sometimes, these problems are performingreal computations in disguise. • What is this problem really doing?

34. While loop mystery answers • Given the following program, public static void mystery(int y) { int x = 0; int z = 0; while (y > 0) { System.out.print(x + " " + z + " "); x++; z = z + y % 10; y = y / 10; } System.out.println(x + " " + z); } • What is the output of the following sequence of calls? mystery(0); 0 0 mystery(8); 0 0 1 8 mystery(32); 0 0 1 2 2 5 mystery(72); 0 0 1 2 2 9 mystery(184); 0 0 1 4 2 12 3 13 mystery(8239); 0 0 1 9 2 12 3 14 4 22