C/ C++ Basics (II) Flow of Control

1. C/C++ Basics (II)Flow of Control Berlin Chen 2002 Textbook: Walter Savitch, "Absolute C++," Addison Wesley, 2002

2. Outline • Boolean Expressions (布林運算式) • Building, Evaluating, Precedence Rules • Branching (分支) Mechanisms • if-else • switch • Nesting if-else • Loops (迴圈) • While, do-while, for • Nesting loops

3. Boolean Expressions • Comparison Operators (比較運算子) • Logical Operators • Logical AND (&&) • Logical OR (||) • Logical NOT (!)

4. Evaluating Boolean Expressions • Data type bool • Returns true or false • true, false are predefined library constants • Truth tables OR AND NOT

5. Precedence of Operators

6. Precedence Examples • Arithmetic before logical x + 1 > 2 || x + 1 < -3 means: (x + 1) > 2 || (x + 1) < -3 • Short-circuit evaluation (x >= 0) && (y > 1) • Be careful with increment operators! (x > 1) && (y++) • Integers as boolean values All non-zero values true (非零整數視為true) Zero value false (零視為false) 當若已知 X<0 (即左邊的布林運算式為false時)，則可以不用判斷(執行)右邊的布林運算式

7. Branching Mechanisms

8. if-else Statements • Choice of two alternate statements based on condition expression • Example:if (hrs > 40) grossPay = rate*40 + 1.5*rate*(hrs-40);else grossPay = rate*hrs;

9. if-else Statements • Formal syntax: if (boolean_expression) yes_statementelse no_statement • Note each alternative is only ONEstatement! • To have multiple statements execute ineither branch  use compound statement 用左右大括弧 “{“ “}”包成單一的區塊敘述，裡面可以有一個以上的敘述 複合敘述

10. if-else Statements 複合(區塊)敘述 • Compound (Block) Statement • if-else statements only ‘get’ one statement per branch • Must use compound statement { } for multiples • Also called a ‘block’ statement • Each block should have block statement • Even if just one statement • Enhances readability

11. if-else Statements • Compound Statement in Action • Note indenting in this example if (myScore > yourScore){ cout << " I win!\n "; wager = wager + 100;}else{ cout << " I wish these were golf scores.\n "; wager = 0;}

12. if-else Statements • Common Pitfalls • Operator ‘=‘ vs. operator ‘==‘ • One means ‘assignment’ (=) • One means ‘equality’ (==) • VERY different in C++! • Example: if (x = 12)  Note operator used! Do_Somethingelse Do_Something_Else

13. if-else Statements • The Optional else • else clause is optional • If, in the false branch (else), you want ‘nothing’ to happen, leave it out • Example:if (sales >= minimum) salary = salary + bonus;cout << " Salary = $ " << salary; • Note: nothing to do for false condition, sothere is no else clause! • Execution continues with cout statement

14. if-else Statements • Nested Statements • if-else statements contain smaller statements • Compound or simple statements (we’ve seen) • Can also contain any statement at all,including another if-else statement • Example:if (speed > 55)if (speed > 80) cout << " You are really speeding! ";else cout << " You are speeding. "; • Note proper indenting(縮排)!

15. if-else Statements • Multiway if-else • Not new, just different indenting • Avoids ‘excessive’ indenting • Syntax: if (Boolean_expression_1) Statement_1 elseif (Boolean_expression_2) Statement_2 . . elseif (Boolean_expression_n) Statement_n else Statement_For_All_Other_Possibilities

16. if-else Statements • Multiway if-else Example if ((temperature <-10) && (day==SUNDAY) ) cout << “stay at home”; //and day != SUNDAY elseif (temperature <-10) cout << “stay at home, but call work”; elseif (temperature <=0) cout << “Dress warm.”; //temperature >=-10 else //temperature >10 cout << “Work hard and play hard.”;

17. Conditional Operator(條件運算子) • Also called ‘ternary (三重的) operator’ • Allows embedded conditional in expression • Essentially ‘shorthand if-else’ operator • Example:if (n1 > n2) max = n1;else max = n2; • Can be written:max = (n1 > n2) ? N1 : n2; • ‘?’ and ‘:’ form this ‘ternary’ operator 條件運算子雖較簡潔，但乍看之下較 If-else 不清楚

18. The switch Statement • A new statement for controlling multiple branches • Uses controlling expression which returns either a bool value (true or false), , an int, a char,or enum constant switch無法執行浮點數測試！

19. The switch Statement • Syntax: switch (Controlling_Expression){ case Constant_1: Statement_Sequence_1 break; case Constant_2: Statement_Sequence_2 break; . . case Constant_n: Statement_Sequence_n break; default : Default_Statement_Sequence } You need not place a break statement in each case. If you omit a break, that case continuous until a break (or the end of the end of the switch statement) is reached. case的標籤必須為常數！

20. The switch Statement switch (vehicleClass){ case 1: cout<<“Passenger car.”; toll=0.50; break; case 2: cout<<“Bus.”; toll=1.50; break; case 3: cout<<“Truck.”; toll=2.00; break; default : cout<<“Unknown vehicle class!”; } • Example int vehicleClass; double toll;cout << “Enter vehicle class:”; cin >> vehicleClass; If you forgot this break, then passenger car will pay $1.50.

21. The switch Statement • Multiple case labels • Execution ‘falls through’ until break • switch provides a ‘point of entry’ • Example:case ‘A’:case ‘a’: cout << " Excellent: you got an ‘A’!\n ";break;case ‘B’:case ‘b’: cout << " Good: you got a ‘B’!\n ";break; • Note multiple labels provide same ‘entry’

22. The switch Statement • switch Pitfalls/Tip • Forgetting the break; • No compiler error • Execution simply ‘falls thru’ other cases untilbreak; • Biggest use: Menus • Provides clearer ‘big-picture’ view • Shows menu structure effectively • Each branch is one menu choice

23. The switch Statement • switch Menu Example • Switch statement ‘perfect’ for menus: switch (response){case ‘1’: // Execute menu option 1break;case ‘2’: // Execute menu option 2break;case 3’: // Execute menu option 3break;default: cout << “Please enter valid response.”;}

24. Enumeration Types • A Type whose values defined by a list of constants of type int • Handy for defining a list of identifiers to use as the case labels in a switch statement 可以兩個以上的 named constants 有相同值 enum MonthLength { JAN_LENGTH=31, FEB_LENGTH=28, MAR_LENGTH=31, APR_LENGTH=30, MAY_LENGTH=31, JUN_LENGTH=30, JULY_LENGTH=31, AUG_LENGTH=31, SEP_LENGTH=30, OCT_LENGTH=31, NOV_LENGTH=30, NOV_LENGTH=31 }; named constants 從0開始，依序加1 enum Direction { NORTH=0, SOUTH=1, EAST=2, WEST=3 }; enum Direction { NORTH, SOUTH, EAST, WEST }; TWO=18; THREE=19; FIVE=-2; enum MyEnum { ONE=17, TWO, THREE, FOUR=-3, FIVE };

25. Loops • 3 Types of loops in C++: • while • Most flexible • No ‘restrictions’ • do-while • Least flexible • Always executes loop body at least once • for • Natural ‘counting’ loop

26. while Loop • Syntax • A while statement with a single statement body • A while statement with a multistatement body while(Boolean_Expression) statement //測試部分 1 //主體部分 2 while(Boolean_Expression) { statement_1 statement_2 . }

27. while Loop • while Loop Example • Consider:count = 0; // Initializationwhile (count < 3) // Loop Condition{ cout << " Hi "; // Loop Body count++; // Update expression} • Loop body executes how many times?

28. do-while Loop • Syntax • A do-while statement with a single statement body • A do-while statement with a multistatement body do statement while(Boolean_Expression); 1 //主體部分 2 //測試部分 do { statement_1 statement_2 . } while(Boolean_Expression); 記得最後要有分號 ” ; ”

29. do-while Loop • do-while Loop Example count = 0; // Initializationdo{ cout << " Hi "; // Loop Body count++; // Update expression} while (count < 3); // Loop Condition • Loop body executes how many times? • do-while loops always execute body at leastonce!

30. while vs. do-while • Very similar, but… • One important difference: • Issue is ‘WHEN’ boolean expression is checked • while: checks BEFORE body is executed • do-while: checked AFTER body is executed • After this difference, they’re essentiallyidentical! • while is more common, due to it’s ultimate‘flexibility’

31. 迴圈更新 (loop update) 迴圈初始化 (loop initialization) 迴圈測試 (loop test) 一般以變數作為迴圈計數器 常使用遞增(++)或遞減運算子(--) for (Init_Action; Bool_Exp; Update_Action) Body_Statement 迴圈主體 (loop body) for Loop • for Loop Syntax • Like if-else, Body_Statement can be a compound/block statement (複合/區塊 敘述) • Much more typical

32. for Loop • for Loop 一般有下述執行步驟 • 設定一個迴圈控制初始值 //初始化 • 執行測試，決定迴圈是否繼續 //測試 • 執行迴圈的工作 //迴圈主體 • 更新迴圈控制所用的值 //更新 1 2 3 4 Initial Expression Update Expression Test Expression Body Statement

33. for Loop c++的程式風格是 for 和之後的 刮號 保留一個空白，與一般函數 呼較不同，強化控制敘述 與函數呼叫的不同 • for Loop Example for (count=0;count<3;count++) {int num; cout << " Please input a integer: "; cin>> num; count << " The integer you type is: " << num<< " \n "; } • How many times does loop body execute? • Initialization, loop condition and update all‘built into’ the for-loop structure! • A natural ‘counting’ loop 在複合(區塊)敘述中定義的變數， 當程式執行至此區塊的敘述變數 才存在，離開則變數空間被釋放， 及只有在區塊中才會知道這變數 大括弧構成一個複合敘述 這樣你就可以在迴圈主體 中加入任何數目的敘述了

34. for Loop • 更新迴圈控制所用的值 • 遞增 (++)和遞減(--)運算子 e.g. i++ or i-- • 複合設定 e.g. i+=5 or i+=n amount*=cnt1+cnt2 means: amount=amount*(cnt1+cnt2)

35. for Loop • 在for迴圈區塊敘述宣告變數 • 在複合(區塊)敘述中定義的變數，當程式執行至此區塊的敘述變數才存在，離開則變數空間被釋放，及只有在區塊中才會知道這變數

36. Comma Operator(逗號運算子) • 將兩個以上的運算式至於同一處(合成一個運算式) • Most often used in a for-loop for (i=0, j=10; i<j ; I++, j-=2) • Evaluate list of expressions, returning value of the last expression • Example: first = (first = 2, second = first + 1); • first gets assigned the value 3 • second gets assigned the value 3 • No guarantee what order expressions will be evaluated

37. Loop Issues • Loop’s condition expression can beANY boolean expression • Examples: while (count<3 && done!=0){ // Do something} for (index=0;index<10 && entry!=-99){ // Do something}

38. Loop Pitfalls: Misplaced ; • Watch the misplaced ; (semicolon) • Example:while (response != 0) ;{ cout << "Enter val: "; cin >> response;} • Notice the ‘;’ after the while condition! • Result here: INFINITE LOOP!

39. Loop Pitfalls: Infinite Loops • Loop condition must evaluate to false atsome iteration through loop • If not  infinite loop. • Example:while (1){ cout << " Hello ";} • A perfectly legal C++ loop  always infinite! • Infinite loops can be desirable • e.g.: ‘Embedded Systems’

40. The break and continue Statements • Flow of Control • Recall how loops provide ‘graceful’ and clear flow of control in and out • In RARE instances, can alter natural flow • break; • Forces loop to exit immediately. (放棄整個 loop) • continue; • Skips rest of loop body (放棄目前 loop iteration，直接執行下一個) • These statements violate natural flow • Only used when absolutely necessary!

41. Nested Loops • Recall: ANY valid C++ statements can beinside body of loop • This includes additional loop statements! • Called ‘nested loops’ • Requires careful indenting:for (outer=0; outer<5; outer++)for (inner=7; inner>2; inner--) cout << outer << inner; • Notice no { } since each body is one statement • Good style dictates we use { } anyway

42. Summary 1 • Recall: ANY valid C++ statements can beinside body of loop • This includes additional loop statements! • Called ‘nested loops’ • Requires careful indenting:(縮排)for (outer=0; outer<5; outer++)for (inner=7; inner>2; inner--) cout << outer << inner; • Notice no { } since each body is one statement • Good style dictates we use { } anyway

43. Summary 2 • do-while loops • Always execute their loop body at least once • For loop • A natural ‘counting’ loop • Loops can be exited early • break statement • continue statement • Usage restricted for style purposes

44. Homework-5 • Computer Program Write a program to accept two positive numbers form the console and output the sum of square of all numbers between them. The program terminates when one of the input number is not positive. e.g. 1. if the inputs are (4, 6), the answer will be 42+52+62 =77 2. if the inputs are (5, 1), the answer will be 12+22+32+42+52 =55 3. if the inputs are (-4, 6), (1, -20), (81, -90), etc., then the program will be terminated!