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1 Chapter 4 Control Statements: Part 1 杨明 yangming2002@seu.edu.cn

2 OBJECTIVES Basic problem-solving techniques. To develop algorithms through the process of top-down, stepwise refinement ( 自顶向下 - 逐步求精 ). To use the if and if...else selection statements( 选择语句 ) to choose among alternative actions. To use the while repetition statement ( 循环语句 ) to execute statements in a program repeatedly. Counter-controlled( 计数器控制 ) repetition and sentinel-controlled( 标记值控制 ) repetition. To use the increment( 自增 ), decrement( 自减 ) and assignment( 赋值 ) operators 2

3 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 3

4 编程解决问题 4.1 Introduction Step 1: Before writing a program: Have a thorough understanding of the problem Carefully plan an approach for solving it Step 2: While writing a program: Know what building blocks( 组件 ) are available Use good programming principles (1) 算法设计 (2) 编程实现 4

5 4.1 Introduction 1. 算法设计算法 (Algorithm) 伪代码 (Pseudocode) 自顶向下 - 逐步求精 (Top-down, Stepwise refinement) Ch 编程实现组件 : 控制语句 (1 顺序 -3 选择 -3 循环 ) 编程原则 : 结构化编程 (Structured Programming) 5

6 4.1 Introduction 算法 (Algorithm): 求解特定问题的一组有限的操作序列 Actions( 操作 ) to be executed The order( 顺序 ) in which these actions are to be executed 程序 (Program): 用计算机语言所要求的规范描述出一系列动作, 它表达了程序员要求计算机执行的操作 6

7 4.1 Introduction 起床脱睡衣洗澡穿衣搭车上班吃早饭在计算机程序中, 指定语句执行顺序称为程序控制 (Program Control) 在 C++ 语言中落实程序控制的语句称为控制语句 (Control Statements) 7

8 4.1 Introduction Pseudocode( 伪代码 ) 介于算法和程序语言之间, 辅助程序员进行算法设计的非正式语言目的 : 辅助算法设计特点 : 不是真正的编程语言, 无法编译执行独立于编程语言, 与具体语言无关, 无需考虑特定语言的语法使用日常英语描述, 仅包含字符 8

9 伪代码仅描述可执行语句变量的声明不会被包含在伪代码中 6, 如 int i; 4. int main() 伪代码仅描述对算法重要的语句, 而不是 5. { 6. int number1; // first integer to add 全部程序例如 return 0; Figure 4.1. Pseudocode for the addition program 4.1 Introduction 1 Prompt the user to enter the first integer 2 Input the first integer 3 4 Prompt the user to enter the second integer 5 Input the second integer 7 Add first integer and second integer, store result 8 Display result 3. #include <iostream> // allows program to perform input and output 7. int number2; // second integer to add 8. int sum; // sum of number1 and number std::cout << "Enter first integer: "; // prompt user for data 11. std::cin >> number1; // read first integer from user into number1 12. std::cout << "Enter second integer: "; // prompt user for data 13. std::cin >> number2; // read second integer from user into number sum = number1 + number2; // add the numbers; store result in sum 16. std::cout << "Sum is " << sum << std::end1; // display sum; end line return 0; // indicate that program ended successfully 19. } // end function main 9

10 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 10

11 4.2 Control Structures 结构化程序设计的概念是 E.W.Dijkstra 在 60 年代末提出的, 主要目的是控制编程中的复杂性该方法的要点是 : 1. 没有 goto 语句 2. 一个入口, 一个出口 3. 自顶向下逐步求精的分解 4. 主程序员组 1, 2 解决程序结构规范化问题 3 解决将大化小, 将难化简的求解方法问题 4 解决软件开发的人员组织结构问题 11

12 4.2 Control Structures 程序段 1 True 条件 False 条件 False 程序段 2 程序段 1 程序段 2 True 程序段顺序 Sequence 选择 Selection 循环 Repetition 12

13 4.2 Control Structures --- 顺序结构顺序结构 : 计算机顺序执行 C++ 语句 UML 活动图 (Activity Diagram): 用于描述系统工作流, 由一些专用符号组成活动状态符号 (Action State Symbol) 判断符号 ( 菱形,Diamond) 小圆圈 (Small Circle) 变迁箭头 (Transition Arrow) 等等 13

14 4.2 Control Structures --- 顺序结构顺序结构的 UML 活动图 (Activity Diagram) 实心圆圈开始符号初始状态注释 (notes) 变迁箭头活动顺序活动状态符号表示活动结束符号最终状态虚线相连 14

15 4.2 Control Structures --- 选择结构单选 (single-selection statement) if. 选择或忽略某个 ( 组 ) 行为双选 (double-selection statement) if else 在两个 ( 组 ) 行为中选择其一多选 (multiple-selection statement) switch 在多个 ( 组 ) 行为中选择 15

16 4.2 Control Structures --- 循环结构 while 执行某个 ( 组 ) 行为 0 或多次 (0+) for 执行某个 ( 组 ) 行为 0 或多次 (0+) do...while 执行某个 ( 组 ) 行为 1 或多次 (1+) 16

17 4.2 Control Structures --- 关键字返回一个对象或者类型所占的内存字节数所有的 C++ 关键词都只包含小写字母! 17

18 4.2 Control Structures --- 程序的组成 C++ 程序有七种控制结构顺序结构 3 种选择结构 3 种循环结构每个控制结构可以用活动图表示, 只有一个入口和一个出口, 即仅有一对 UML 开始符号 + 结束符号 18

19 4.2 Control Structures --- 程序的组成所有 C++ 程序都是根据程序算法对这七种控制结构的组合控制结构的两种组合方式 : 堆叠 (control-statement stacking): 一个控制结构的出口与另一个的入口连接嵌套 (control-statement nesting): 一个控制结构中包含另一个控制结构 19

20 4.2 Control Structures --- 堆叠控制结构 A 控制结构 A 控制结构 B 控制结构 B 20

21 4.2 Control Structures --- 嵌套控制结构 A 控制结构 B 21

22 4.2 Control Structures --- 程序的组成总结任何 C++ 程序都是根据程序算法对这七种控制结构 ( 顺序 + 3 选择 + 3 循环 ) 进行两种方式 ( 堆叠 + 嵌套 ) 组合而成 22

23 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 23

24 1 4.3 if Selection Statement 作用 : 根据判断条件 condition, 执行或者忽略某个 ( 组 ) 动作编程任务 : 当学生成绩大于等于 60 分, 输出合格 2 Pseudocode( 伪代码 ): If student's grade is greater than or equal to 60 Print "Passed" 3 C++ 代码 : Indent 缩进 if ( grade >= 60 ) cout << "Passed"; if ( grade >= 60 ) cout << "Passed"; 24

25 4.3 if Selection Statement condition, 可以是 bool 类型的关系 (>,<,>=,<=) 或者等价 (==,!=) 表达式也可以是任何数值表达式表达式的值为 0, 则表示 false 表达式的值非 0(nonzero), 则表示 true int a = 3, b = 3;// b=-3; if (a+b) cout<<"true"<<endl; 25

26 4.3 if Selection Statement 如果条件满足时, 需执行一组动作呢? if ( grade >= 60 ) { cout << "Passed"; counter++; } 程序中任何可以放置单条语句的地方, 都可以放置语句块! { } 内的语句称为复合语句 (Compound Statements) 或者语句块 (Block) { } 内可以是 >=0 条语句, 一般是多条语句 0, 不执行任何语句, 称为 null 语句 ( 空语句 ) 1, 通常 { } 可省略, 特例摇摆 else 26

27 4.3 if Selection Statement 开始符号判断符号 Guard Condition 守护条件 Action 结束符号 27

28 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 28

29 4.4 if...else Double-Selection Statement 29

30 1 4.4 if...else Double-Selection Statement 作用 : 根据条件值为 true 或者 false, 分别执行不同的行为编程任务 : 当学生成绩大于等于 60 分, 输出合格, 否则输出不合格 2 Pseudocode( 伪代码 ): If student's grade is greater than or equal to 60 Print Passed if ( grade >= 60 ) Else cout << "Passed"; Print Failed else 3 C++ 代码 cout << "Failed"; 30

31 4.4 if...else Double-Selection Statement if (a >= b){ largest = a; smallest = b; } if (a < b){ largest = b; smallest = a; } if (a >= b){ largest = a; smallest = b; } else{ largest = b; smallest = a; } 31

32 4.4 if...else Double-Selection Statement --- 条件操作符?: Conditional Operator( 条件操作符 ) C++ 中唯一的三元运算符 (ternary operator) 操作数 1? 操作数 2 : 操作数 3 操作数 1: 条件操作数 2: 条件为 true 时, 整个条件表达式的值操作数 3: 条件为 false 时, 整个条件表达式的值 string s = grade >= 60? "Passed" : "Failed"; cout << s; cout << ( grade >= 60? "Passed" : "Failed" ); P103 Fig 4.20 运算符优先级表格 32

33 4.4 if...else Double-Selection Statement --- 条件操作符操作数? Action1 : Action2 以操作数为条件, 当条件为 true 时执行 Action1, 为 false 时执行 Action2 grade>=60? cout<<"passed" : cout<<"failed"; 34

34 4.4 if...else Double-Selection Statement --- 条件操作符 string s = grade >= 60? "Passed" : "Failed"; cout << s; cout << ( grade >= 60? "Passed" : "Failed" ); grade>=60? cout<<"passed":cout<<"failed"; 35

35 4.4 if...else Double-Selection Statement 36

36 4.4 if...else Double-Selection Statement --- 嵌套 If if student's ( studentgrade grade >= 90 is ) greater // 90 and than above or gets equal "A" to 90 cout Print << "A"; "A" else Else // <90 if ( studentgrade If student's >= grade 80 ) // is greater gets "B" than or equal to 80 等 cout << Print "B"; "B" 价 if ( studentgrade >= 90 ) // 90 and above gets "A" else Else // <80 cout << "A"; if ( studentgrade If student's else >= grade if 70 ( studentgrade ) // is greater gets >= "C" than 80 ) // or equal gets to "B" 70 cout << Print "C";"C" cout << "B"; else Else // <70 else if ( studentgrade >= 70 ) // gets "C" if ( studentgrade If student's cout >= grade << 60 "C"; ) // is greater gets "D" than or equal to 60 cout << "D"; Print else "D" if ( studentgrade >= 60 ) // gets "D" else // Else less than cout 60 gets << "D"; "F" 方法一 Print "F" cout << "F"; else // less than 60 gets "F" Pseudocode cout << "F"; 方法二 37

37 4.4 if...else Double-Selection Statement --- 摇摆 else 问题需求 :x>5 且 y>5 时, 输出 "x and y are > 5"; x 不大于 5 时, 输出 "x is <= 5" x=3 时, 应输出 "x is <= 5" if ( x > 5 ) if ( y > 5 ) cout << "x and y are > 5"; else cout << "x is <= 5"; Dangling else: 为了避免多义性,C++ 编译器将 else 与之前最近的 if 关联 38

38 4.4 if...else Double-Selection Statement --- 摇摆 else 问题 if ( x > 5 ) if ( y > 5 ) cout << "x and y are > 5"; else cout << "x is <= 5"; 编译器的理解 :else 子句与前面最近的没有 else 子句的 if 配对 if ( x > 5 ) if ( y > 5 ) cout << "x and y are > 5"; else cout << "x is <= 5"; x>5 而且 y>5 x>5 而且 y<=5 39

39 4.4 if...else Double-Selection Statement --- 摇摆 else 问题 if ( x > 5 ) if ( y > 5 ) cout << "x and y are > 5"; else 编译器的理解 :else 子句与前面最近的没有 else 子句的 if 配对正确的写法 cout << "x is <= 5"; if ( x > 5 ) if ( y > 5 ) cout << "x and y are > 5"; else cout << "x is <= 5"; ( 破坏 else 与第二个 if 的关联 ) if ( x > 5 ) { if ( y > 5 ) cout << "x and y are > 5"; } else cout << "x is <= 5"; 40

40 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 41

41 4.5 while Repetition Statement 需求 : 程序设计中经常会需要对相同的操作重复执行多次循环 (loop): 根据某个条件的满足与否决定是否重复执行一个 ( 组 ) 行为循环一般由四个部分组成 : 循环初始化循环条件循环体下一次循环准备 42

42 4.5 while Repetition Statement While there are more items on my shopping list, Purchase next item and cross it off my list 循环初始化 :Prepare the shopping list 循环条件 : there are more items on my shopping list 循环体 : Purchase next item,cross it off my list 下一次循环准备 : cross it off my list 43

43 4.5 while Repetition Statement --- while 循环作用 : 在满足一定条件时重复执行某个 ( 组 ) 操作 while (< 条件 >) < 语句 > 表示条件的关系或逻辑表达式简单语句或复合语句 { } 编程任务 : 寻找 3 的第一个大于 100 的指数值循环初始化 int product = 3; while ( product <= 100) product = 3 * product; 循环条件循环体和下一次循环准备 44

44 4.5 while Repetition Statement --- while 循环合并符号 (Merge Symbol): 将多个行为流合并为一个流, 没有对应的 C++ 语句 1. 入流 - 出流 (n-1,1-n) 2. 守护条件 (Guard Condition) 45

45 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 46

46 4.6 Formulating Algorithms: Counter- Controlled Repetition( 计数器控制的循环 ) 计数器控制的循环用一个称作的计数器 (Counter) 的变量控制一组语句重复执行的次数, 这种技术叫计数器控制的循环计数器控制循环又被称作确定循环 (definite repetition), 因为在循环体执行之前已经知道它将被执行的次数 47

47 4.6 Formulating Algorithms: Counter- Controlled Repetition( 计数器控制的循环 ) 编程任务班里有 10 个学生参加考试, 根据提供的考 Set total to zero 试成绩 Set (0 grade 到 100 counter 的整数值 to one ), 初始化变量计算和显示计数器全部学生成绩之和, 并计算全班平均成绩 While grade counter is less than or equal to ten Pseudocode( 伪代码 ) Prompt the user to enter the next grade Input the next grade Add the grade into the total Add one to the grade counter 循环体 Set the class average to the total divided by ten Print the total of the grades for all students in the class Print the class average 53

48 4.6 Formulating Algorithms: Counter- Controlled Repetition( 计数器控制的循环 ) 类定义 (Page 87) Fig. 4.8: 接口文件 GradeBook.h Fig. 4.9: 成员函数定义文件 GradeBook.cpp 类使用 (Page 90) Fig. 4.10: 在 main 函数中创建并使用类对象 54

49 4.6 Formulating Algorithms: Counter- Controlled Repetition( 计数器控制的循环 ) 50 total = 0; // initialize total 51 gradecounter = 1; // initialize loop counter // processing phase 54 while ( gradecounter <= 10 ) // loop 10 times 55 { 56 cout << "Enter grade: "; // prompt for input 57 cin >> grade; // input next grade 58 total = total + grade; // add grade to total 59 gradecounter = gradecounter + 1; // increment by 1 60 } // end while 63 average = total / 10; 55

50 4.6 Formulating Algorithms: Counter- Controlled Repetition( 计数器控制的循环 ) 变量需要初始化总和变量通常被初始化为 0 未初始化变量会包含垃圾值 (garbage value), 也称为未定义值 (undefined value), 为该变量对应内存地址中最后存放的值根据使用情况, 计数器变量通常应先初始化为 0 或 1 56

51 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 57

52 问题描述 : 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 开发通用的计算平均成绩的程序, 在每次程序运行时可以处理任意数量的成绩. 解决办法 : 用一个特殊值作为标记值 (sentinel value, 也称信号值 signal value 哑值 dummy value 或标志值 flag value), 表示数据输入结束 (end of data entry) 标记值必须能与可接受的有效输入值有所区分! 58

53 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 定义标记控制的循环 (sentinel controlled repetition) 也称为不确定循环 (indefinite repetition), 因为执行循环之前无法事先知道重复次数 59

54 4.7 Formulating Algorithms: Sentinel-Controlled Repetition -- 自顶向下, 逐步求精 TOP: 程序总体功能的描述逐步求精 : 进行功能细化, 细化的操作根据顺序结构依次执行设计出的算法通常包含三部分 : 初始化阶段 (initialization phase) 初始化程序变量处理阶段 (processing phase) 输入调整数据结束阶段 (termination phase) 计算并输出结果 60

55 1 2 3 Determine the class average for the quiz 细化 1 Initialize variables 2 Input, sum and count the quiz grades 3 Calculate and print the total of all student grades and the class average 细化 Initialize total to zero; Initialize counter to zero Prompt the user to enter the first grade 输入第几个完成几个输入 Input the first grade (possibly the sentinel) While the user has not yet entered the sentinel Add this grade into the running total Add one to the grade counter Prompt the user to enter the next grade Input the next grade (possibly the sentinel) If the counter is not equal to zero Set the average to the total divided by the counter Print the total of the grades for all students in the class Print the class average else Print "No grades were entered" 61

56 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 类定义 (Page 93) Fig. 4.12( 接口 GradeBook.h) Fig. 4.13( 成员函数定义 GradeBook.cpp) 类使用 (Page 95) Fig ( 在 main 函数中创建类对象 ) 62

57 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 08 using std::fixed; // ensures that decimal point is displayed #include <iomanip> // parameterized stream manipulators 11 using std::setprecision; // sets numeric output precision std::fixed 设置浮点数输出格式, 定点输出 <iomanip> 带参数的流操作算子 std::setprecision 设置输出精度 63

58 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 59 gradecounter = 0; // initialize loop counter // processing phase 62 // prompt for input and read grade from user 63 cout << "Enter grade or -1 to quit: "; 64 cin >> grade; // input grade or sentinel value // loop until sentinel value read from user 67 while ( grade!= -1 ) // while grade is not { 69 total = total + grade; // add grade to total 70 gradecounter = gradecounter + 1; // increment counter // prompt for input and read next grade from user 73 cout << "Enter grade or -1 to quit: "; 74 cin >> grade; // input grade or sentinel value 75 } // end while 64

59 4.7 Formulating Algorithms: Sentinel- Controlled Repetition( 标记控制的循环 ) 55 double average; // number with decimal point for average 78 if ( gradecounter!= 0 ) // if user entered at least one grade { 80 // calculate average of all grades entered 81 average = static_cast< double >( total ) / gradecounter; // display total and average (with two digits of precision) 84 cout << "\ntotal of all " << gradecounter << " grades entered is " 85 << total << endl; 86 cout << "Class average is " << setprecision( 2 ) << fixed << average 87 << endl; 88 } // end if 89 else // no grades were entered, so output appropriate message 90 cout << "No grades were entered" << endl; 65

60 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 浮点类型 55 double average; // number with decimal point for average 80 // calculate average of all grades entered 81 average = static_cast< double >( total ) / gradecounter; C++ 常用浮点数类型 (Floating-Point Number) float 类型 ( 单精度 ), 7 位有效数字, 4 个字节 double 类型 ( 双精度 ), 15 位有效数字, 8 个字节计算机不能 100% 精确表示小数, 如 Pi 值单精度浮点数的精度比双精度低, 但是所需内存少, 运算速度快 Floating-Point Constant 程序代码中的小数 ( 如 ), 缺省作为 double 处理 66

61 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 类型转化 55 double average; // number with decimal point for average 80 // calculate average of all grades entered 81 average = static_cast< double >( total ) / gradecounter; 显式类型转换 (explicit conversion) 类型转换操作符 total 中存放的值仍是整数, 而计算时建一个临时的 double 类型的浮点数值 (total 的 double 类型的值 ) 除以 gradecounter 一元运算符 (unary operator) 操作数 67

62 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 类型转化 55 double average; // number with decimal point for average 80 // calculate average of all grades entered 81 average = static_cast< double >( total ) / gradecounter; C++ 进行表达式计算时, 要求操作数类型一致隐性类型转换 (implicit conversion) gradecounter 必须提升 (Promote) 为 double 之后进行计算, 将浮点数除法得到的结果赋给 average 68

63 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 类型转化 69

64 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出 Welcome to the grade book for CS101 C++ Programming! Enter grade or -1 to quit: 97 Enter grade or -1 to quit: 88 Enter grade or -1 to quit: 72 Enter grade or -1 to quit: -1 Total of all 3 grades entered is 257 Class average is Press any key to continue 84 cout << "\ntotal of all " << gradecounter << " grades entered is " 85 << total << endl; 86 cout << "Class average is " << setprecision( 2 ) << fixed << average 87 << endl; 70

65 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出 setprecision( int n ) 带参数的流操作算子, parameterized stream manipulator 必须 : #include <iomanip> 1 n 缺省值为 6 2 设置 fixed 或 scientific 输出时, 参数 n 指定小数点后的位数, 四舍五入 : 若未设置 fixed 或 scientific 输出, 则 n 指定输出的有效数字位, 四舍五入仅设置输出格式, 原值不变 71

4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出 1. cout << 31.4159265 << endl; 2. cout << setprecision(3) << 3.14159265 << endl; 3.

66 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出 1. cout << << endl; 2. cout << setprecision(3) << << endl; 3. cout << fixed << setprecision(3) << << endl; 4. cout << scientific << setprecision(3) << << endl; 5. cout << << endl; 72

67 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出两个流操作算子 fixed: 使浮点数值以定点格式 ( 而不是科学计数法 scientific, 浮点 ) 输出 showpoint: 即使数值为整数, 仍强制打印小数点和尾部的 0, 如和 endl 相同, 无参数, 使用 <iostream> 不需要头文件 <iomanip> 73

68 4.7 Formulating Algorithms: Sentinel- Controlled Repetition 格式化输出 Welcome to the grade book for CS101 C++ Programming! Enter grade or -1 to quit: 97 Enter grade or -1 to quit: 88 Enter grade or -1 to quit: 72 Enter grade or -1 to quit: -1 Total of all 3 grades entered is 257 Class average is Press any key to continue 74

69 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 75

70 4.8 Formulating Algorithms: Nested Control Statements( 嵌套控制结构 ) 需求学校开了一门课, 让学生参加房地产经纪人资格考试去年,10 位同学读完这门课并参加了证书考试学校想知道学生考试情况, 请编写一个程序来总结这个结果已经得到了 10 个学生的名单, 每个姓名后面写 1 时表示考试通过, 写 2 时表示没有通过如果超过 8 个学生及格, 则打印消息 "Bonus to instructor!" 76

71 4.8 Formulating Algorithms: Nested Control Statements( 嵌套控制结构 ) 设计解决方法 1. 程序要处理 10 个考试成绩计数器控制循环 2. 每个考试成绩为数字 1 或 2, 每次程序读取考试成绩时, 如果不是 1, 则假设其为 2 双选 3. 另外使用两个计数器, 分别计算及格人数和不及格人数 4. 程序处理所有结果之后, 要确定是否有超过 8 个学生及格单选 77

72 4.8 Formulating Algorithms: Nested Control Statements( 嵌套控制结构 ) 1 Initialize passes to zero 2 Initialize failures to zero 3 Initialize student counter to one 4 5 While student counter is less than or equal to 10 6 Prompt the user to enter the next exam result 7 Input the next exam result 8 9 If the student passed 10 Add one to passes 11 Else 12 Add one to failures Add one to student counter Print the number of passes 17 Print the number of failures 19 If more than eight students passed 20 Print "Bonus to instructor" 78

73 4.8 Formulating Algorithms: Nested Control Statements( 嵌套控制结构 ) 在声明变量时赋初值, 可以帮助程序员避免数据未初始化问题控制结构的嵌套循环结构体内包含了双选 79

74 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 80

75 4.9 Assignment Operators( 赋值运算符 ) 赋值表达式缩写 c = c + 3; c += 3; +=: 加法赋值运算符二元运算符 : +, -, *, /, or % variable = variable operator expression; variable operator= expression; 81

76 4.9 Assignment Operators( 赋值运算符 ) 82

77 Topics 4.1 Introduction 4.2 Control Structures 4.3 if Selection Statement 4.4 if...else Double-Selection Statement 4.5 while Repetition Statement 4.6 Formulating Algorithms: Counter-Controlled Repetition 4.7 Formulating Algorithms: Sentinel-Controlled Repetition 4.8 Formulating Algorithms: Nested Control Statements 4.9 Assignment Operators 4.10 Increment and Decrement Operators 84

78 4.10 Increment and Decrement Operators ( 自增和自减运算符 ) 一元自增运算符 (increment operator) ++ 一元自减运算符 (decrement operator) -- Operator 名称示例说明 ++ Preincrement ( 前置自增 ) ++a 先将 a 加 1, 然后在表达式中使用 a 的新值 ++ Postincrement ( 后置自增 ) a++ 在 a 出现的表达式中使用当前值, 然后将 a 加 1 -- Predecrement ( 前置自减 ) --b 先将 b 减 1, 然后在表达式中使用 b 的新值 -- Postdrecrement ( 后置自减 ) b-- 在 b 出现的表达式中使用当前值, 然后将 b 减 1 85

79 passes = passes + 1; passes += 1; 4.10 Increment and Decrement Operators ( 自增和自减运算符 ) passes++; ++passes; passes = 10; a. x = passes++; b. y = ++passes; a. x=10, passes=11 b. passes=11, y=11 86

80 4.10 Increment and Decrement Operators ( 自增和自减运算符 ) 12. c = 5; // assign 5 to c 13. cout << c << endl; // print cout << c++ << endl; // print 5 then postincrement 15. cout << c << endl; // print cout << endl; // skip a line // demonstrate preincrement 20. c = 5; // assign 5 to c 21. cout << c << endl; // print cout << ++c << endl; // preincrement then print cout << c << endl; // print 6 87

81 4.10 Increment and Decrement Operators ( 自增和自减运算符 ) 高低 88

82 理解算法伪代码程序的概念掌握 " 自顶向下, 逐步求精 " 的结构化编程方法使用 if if..else 选择结构进行选择操作使用 while 循环结构理解嵌套的概念使用计数器控制循环与标记控制循环使用自增自减赋值运算符 Summary 89

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2/80 2 2/80 2 3/80 3 DSP2400 is a high performance Digital Signal Processor (DSP) designed and developed by author s laboratory. It is designed for multimedia and wireless application. To develop application