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6.9   Case Study: Random-Number Generation

6.9   Case Study: Random-Number Generation. Random-number ( 隨機數字 ) generation 有兩個方式： static method random from class Math Math.random( ) Returns double s in the range 0.0  x < 1.0 Thru object of class Random from package java.util

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6.9   Case Study: Random-Number Generation

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  1. 6.9  Case Study: Random-Number Generation • Random-number (隨機數字) generation 有兩個方式： • static method random from class Math • Math.random( ) • Returns doubles in the range 0.0 x < 1.0 • Thru object of class Random from package java.util • Can produce pseudorandom boolean, byte, float, double, int, long and Gaussian values • 如 nextInt()、nextDouble() 等方法(請參見範例) • Is seeded with the current time of day to generate different sequences of numbers each time the program executes，種子數用來決定第 1 個隨機數字的值，第 2個隨機數字的值由第 1個隨機數字的值決定，之後的值都由前一個值決定

  2. Outline Import class Random from the java.util package RandomIntegers .java (1 of 2) Create a Random object Generate a random die roll

  3. Outline Two different sets of results containing integers in the range 1-6 RandomIntegers .java (2 of 2)

  4. Outline Import class Random from the java.util package RollDie.java (1 of 2) Create a Random object Declare frequency counters

  5. Outline Iterate 6000 times RollDie.java (2 of 2) Generate a random die roll switch based on the die roll

  6. Outline Display die roll frequencies • RollDie.java • (3 of 3)

  7. 6.9.1 Generalized Scaling and Shifting of Random Numbers • To generate a random number in certain sequence or range (有規律的數字序列才可以) • Use the expression shiftingValue+ differenceBetweenValues*randomNumbers.nextInt(scalingFactor)where: • shiftingValue (初值) is the first number in the desired range of values • differenceBetweenValues (差值) represents the difference between consecutive numbers in the sequence • scalingFactor (個數) specifies how many numbers are in the range • 例如：介於 3 到 10 間的整數 • 3 + 1 * randomNumbers.nextInt(8) • 例如：由 7, 14, 21, 28, 35 間隨機產生 • 7 + 7 * randomNumbers.nextInt(5) • 那怎麼處理沒有規律的數字序列，例如 3, 5, 13, 15, 33, 37 用 switch

  8. 練習 • 以 random number 表示以下範圍的整數： • 1  n  2 1 + r1.nextInt(2) • 1  n  100 1 + r1.nextInt(100) • 0  n  9 r1.nextInt(10) • 1000  n  1112 1000 + r1.nextInt(13) • -1  n  1 -1 + r1.nextInt(3) • -3  n  11 -3 + r1.nextInt(15) • 2, 4, 6, 8, 10 2+2*r1.nextInt(5) • 3, 5, 7, 9, 11 3+2*r1.nextInt(5) • 6, 10, 14, 18, 22 6 + 4 * r1.nextInt(5) • 四捨五入到整數 • Math.round( x ) or Math.floor( x + 0.5) • 四捨五入到小數點後第 1 位，請類推四捨五入到小數點後第 n 位 • Math.floor( 10 * (x + 0.05) ) / 10.0

  9. 6.9.2 Random-Number Repeatability for Testing and Debugging • To get a Random object to generate the same sequence of random numbers every time the program executes, seed it with a certain value • When creating the Random object:RandomrandomNumbers= newRandom(seedValue); • Use the setSeed method:randomNumbers.setSeed(seedValue); • seedValue should be an argument of type long

  10. Error-Prevention Tip 6.2 • While a program is under development(還在開發中), create the Random object with a specific seed value to produce a repeatable (可重覆的) sequence of random numbers each time the program executes. • If a logic error occurs, fix the error and test the program again with the same seed value-this allows you to reconstruct the same sequence of random numbers that caused the error. • Once the logic errors have been removed, create the Random object without using a seed value, causing the Random object to generate a new sequence of random numbers each time the program executes.

  11. Outline Import class Random from the java.util package • Craps.java • (1 of 4) Create a Random object Declare an enumeration Declare constants

  12. Outline Call rollDice method • Craps.java • (2 of 4) Player wins with a roll of 7 or 11 Player loses with a roll of 2, 3 or 12 Set and display the point

  13. Outline Call rollDice method • Craps.java • (3 of 4) Player wins by making the point Player loses by rolling 7 Display outcome

  14. Outline Declare rollDice method • Craps.java • (4 of 4) Generate two dice rolls Display dice rolls and their sum

  15. Outline • CrapsTest.java • (1 of 2)

  16. 6.10  Case Study: A Game of Chance (Introducing Enumerations) • Enumerations • Programmer-declared types consisting of sets of constants • enum keyword • A type name (e.g. Status) • Enumeration constants (e.g. WON, LOST and CONTINUE) • cannot be compared against ints

  17. Good Programming Practice 6.3, 6.4 • Use only uppercase letters in the names of constants. This makes the constants stand out in a program and reminds the programmer that enumeration constants are not variables. Using enumeration constants (like Status.WON, Status.LOST and Status.CONTINUE) rather than literal integer values (such as 0, 1 and 2) can make programs easier to read and maintain.

  18. 6.11  Scope of Declarations (宣告的有效範圍) • Basic scope rules • Scope of a parameter declaration (方法參數) is the body of the method in which appears • Scope of a local-variable (區域變數) declaration is from the point of declaration to the end of that block (區塊) • Scope of a local-variable declaration in the initialization section of a for header is the rest of the for header and the body of the for statement • Scope of a method or field of a class is the entire body of the class

  19. 6.11  Scope of Declarations (Cont.) • Shadowing (覆蓋) • A field is shadowed (or hidden，不能用) if a local variable or parameter has the same name as the field • This lasts until the local variable or parameter goes out of scope

  20. Common Programming Error 6.10 • A compilation error occurs when a local variable is declared more than once in a method. Use different names for fields and local variables to help prevent subtle logic errors that occur when a method is called and a local variable of the method shadows a field of the same name in the class. (範例 6-11)

  21. Outline • Scope.java • (1 of 2) Shadows field x Display value of local variable x

  22. Outline Shadows field x • Scope.java • (2 of 2) Display value of local variable x Display value of field x

  23. Outline • ScopeTest.java

  24. 練習 • 請問 fig06_09_10 中的 Craps.java程式裡，以下變數或方法之有效範圍(結果請以 “第 x 列 –第 y 列”表示)： • 變數 randomNumbers • 變數 die1 • 方法 rollDice • 方法 play • 變數 sumOfDice • 請再以 isPrime( ) 方法，參數為一正整數，寫一遍第 6 次小考的程式

  25. 6.12  Method Overloading • Method overloading (多載) • Multiple methods with the same name, but different types, number or order of parameters in their parameter lists • Compiler decides which method is being called by matching the method call’s argument list to one of the overloaded methods’ parameter lists • A method’s name and number, type and order of its parameters form its signature • Differences in return type are irrelevant (不相干) in method overloading • Overloaded methods can have different return types • Methods with different return types but the same signature cause a compilation error

  26. Outline Correctly calls the “square of int” method • MethodOverload.java Correctly calls the “square of double” method Declaring the “square of int” method Declaring the “square of double” method

  27. Outline • MethodOverloadTest.java

  28. Outline • MethodOverload • Error.java Same method signature Compilation error

  29. Common Programming Error 6.11 • Declaring overloaded methods with identical parameter lists is a compilation error regardless of whether the return types are different.

  30. 練習 • Ex6_17: 請寫出以下方法的內容： • fillSquare( int size, char fillCharacter ) • 這方法會畫出以 fillCharacter填滿之 size * size的正方形，不需回傳任何值。並請使用者輸入 size及 fillCharacter後，呼叫此方法加以測試。 • Ex6_22: 一個數字被稱為 完美數字 (perfect number )如果其因數加總的和正好等於該數，例如：6 的因數有 1、2、3 ，而 6 = 1 + 2 +3，所以 6 是一個 perfect number。請寫以下方法的內容： • String perfect (int n) • 這方法會檢查 n是否為完美數字，如果是則傳回 n 的所有因數的字串，否則傳回 null 物件。再請寫一應用程式列印出 2 至 1000 中的完美數字，列印時請將其因數一併印出，以利檢驗。

  31. 練習 • Ex6_30: 猜數字遊戲 (Guess) • 請寫一程式來玩猜數字遊戲 ，每次遊戲程式會產生一個介於 1 到 100 的正整數，請使用者持續猜到那個數字為止。 • 如果使用者猜的答案太大，則回應 “太大”；並請使用者 • 繼續猜。 • 2. 如果使用者猜的答案太小，則回應 “太小”；並請使用者繼續猜。 • 3. 如果使用者猜對了，則回應 “Bingo”，並問使用者 • ”要不要再玩？”。 • 3.1 使用者回答 ”要”的話，就開始下一新回合， • 3.2 使用者回答 ”不要”的話，就結束程式。

  32. 期末重點提示 • if (含 logical operators) • while • for • switch • break, continue • 物件導向 (方法、實體變數、static) • Math, Random • enum • overload • argument promotion, type casting • precedence and associativity of operators

  33. 練習 Ex6_30 • (1) Guess2 類別中需含一個 正整數 實體變數 answer • (2) Guess2 類別中要使用 enum 設計一個使用者自訂的資料型態 • RESULT 用來設定使用者猜的數字跟答案比較的結果 • 太大 (TOO_BIG)、 • 太小 (TOO_SMALL)、 • 答對 (BINGO) • (3) Guess2 類別的方法： • 1. RESULT check(int n)： • 比較使用者猜的數字跟答案，可能傳回 • RESULT.TOO_BIG, RESULT.TOO_SMALL, 或 RESULT.BINGO • 2. void newGame( )： 代表使用者的新一回合，須完成以下動作 • 先將 answer 設定為一個隨機變數， • 請使用者輸入一個數字 userGuess， • 呼叫 check(userGuess)， • 檢查結果直到 BINGO 為止，如果不是 BINGO，持續 • 請使用者輸入一個數字 userGuess， • 呼叫 check(userGuess) • 問使用者要不要再玩， • 要的話，呼叫 newGame() 以開啟下一回合 • (4) 另外寫一 PlayGuess 類別來測試 Guess2 類別，其 main 方法需 • 生成一個 Guess2 物件，並呼叫該物件的 newGame() 方法。

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