C++_primer_plus学习笔记 第6章 分支语句和逻辑运算符

本章内容包括：

• if语句
• if else语句
• 逻辑运算符：&&、||和！
• cctype字符函数库
• 条件运算符：？：
• switch语句
• continue语句和break语句
• 读取数字的循环
• 基本文件的输入和输出

if 语句

• if有两种格式：if和if else

//if 语法和while相似：
if (test_condition)
    statement

• 如果测试条件为true，程序执行statement语句，后者可以是一条语句也可以是语句块{}。
• 如果测试条件为false，程序跳过statement语句
• if测试条件被强制转换为bool值，0为false，非0为true

//程序清单6.1
//有多少字符和空格
#include <iostream>

using namespace std;

int main(void)
{
    char ch;
    int total = 0;
    int spaces = 0;
    cin.get(ch);

    while (ch != '.')
    {
        if (ch == ' ')
            ++spaces;
        ++toatl:
        cin.get(ch);
    }
    cout << spaces << " spaces, " << total << " characters total in sentense\n";   

    return 0;
}

if else 语句

• if语句让程序决定是否执行特定的语句或语句块
• if else 让程序决定执行两条语句或语句块中的哪一条

if (test_condition)
    statement1
else
    statement2

//程序清单6.2
//字符++

#include <iostream>

using namespace std;

int main(void)
{
    char ch;
    cout << "Type,and I shall repeat.\n";
    cin.get(ch)

    while(ch != '.')
    {
        if (ch == '\n')
            cout << ch;
        else
            cout << ++ch;
        cin.get(ch);
    }

    return 0;
}

格式化if else语句

• if else 如果需要多条语句，就需要大括号{}括起来，组成语句块

if else if else结构

//程序清单
//猜数字

#include <iostream>

using namespace std;
const int Fave = 27;

int main(void)
{
    int n;
    cout << "Enter a number (1-100) to find my favorite number: ";
    do
    {
        cin >> n;
        if (n < Fave)
            cout << "Too low -- guess again: ";
        else if (n > Fave)
            cout << "Too high -- guess again: ";
        else
            cout << Fave << " is my favorite!\n";
    }while (n != Fave);

    return 0;
}

 逻辑表达式

• OR（||）或、AND（&&）与、NOT（！）非

逻辑OR运算符：|| （有真为真，全假为假）

• ||的优先级比关系运算符低
• 但建议加括号，以防忘记，阅读直观
• 5 >3 || 5 > 10 ——> (5 >3) || (5 > 10)

//程序清单6.4
//不分大小写

#include <iostream>

using namespace std;

int main(void)
{
    cout << "Reformat hard disk, continue? <y/n>: ";
    char ch;
    cin >> ch;
    if (ch == y || ch == Y)
        cout << "Warned!\a\a\n";
    else if (ch == n || ch == N)
        cout << "Wise choice...bye\n";
    else
        {
        cout << "That wasn't a y or n! Input again: ";
        cin >> ch;
        }

    return 0;
}

逻辑AND运算符：&& （有假为假，全真为真）

• &&的优先级比关系运算符低
• 但建议加括号，以防忘记，阅读直观
• 5 >3 && 5 > 10 ——> (5 >3) && (5 > 10)

//程序清单6.5
//由于两种不同原因结束while循环

#include <iostream>

const int ArSize = 6;
using namespace std;

int main(void)
{
    float naaq[ArSize];
    cout << "Enter the NAAQs, program terminates when make " << ArSize;
    cout << " entries or enter a negative value.\n";

    int i = 0;
    float temp;
    cout << "First value: ";
    cin >> temp;

    while (i < ArSize && temp >= 0)
    {
        naaq[i] = temp;
        ++i;
        if (i < Arsize)
        {
            cout << "Next value: ";
            cin >> temp;
        }
    }
    if (i == 0)
        cout << "No data.";
    else
    {
        cout << "Enter your NAAQ: ";
        float you;
        cin >> you;
        int count = 0;
        for (int j = 0; j < i; j++)
            if (naaq[i] > you)
                ++count;
        cout << count << " new age than you do.\n";

    return 0;
}

用&&来设置取值范围（优秀、良好、及格、不及格）

//程序清单6.6
//评级

#include <iostream>
using namespace std;
const char * qualify[4] = {"pefect.\n", "good.\n", "not bad.\n", "bad.\n"};

int main (void)
{
    int index;
    float score;
    cout << "Enter your score: ";
    cin >> score;

    if ((score > 90) && (score <= 100))
        index = 0;
    else if ((score > 70) && (score <= 90))
        index = 1;
    else if ((score > 60) && (score <= 70))
        index = 2;
    else
        index = 3;

    cout << score << "' is " << qualify[index] << endl;

    return 0;
}

 逻辑NOT运算符：！（真假取反）

• if （！（x > 5））等同于if（x <= 5）

//程序清单6.7
//提醒输入超出范围

#include <iostream>
#include <climits>

using namespace std;

bool is_int(double);
int main(void)
{
    double num;
    cout << "Enter an integer value: ";
    cin >> num;

    while(!is_int(num))
    {
        cout << "Out of range --try again: ";
        cin >> num;
    }

    int val = int (num);
    cout << "You enter the integer: " << val << endl;

    return 0;
}

bool is_int(double x)
{
    if ((x <= INT_MAX) && (x >= INT_MIN))
        return true;
    else
        return false;
}

• 将可能的int值作为double来取，double取值范围更大，取值合适后在强制类型转换
• ！运算符优先级高于所有关系运算符和算数运算符

!(x >5)
!x >5

•  AND优先级高于OR

age > 30 && age < 45 || weight > 300
//等同于
((age > 30) && (age < 45)) || (weight > 300)

字符函数库cctype

• 简化诸如确定字符是否为大写、数字、标点符号等工作

//程序清单6.8

#include <iostream>
#include <cctype>

using namespace std;

int main(void)
{
    cout << "Enter text to analysis, and @ to terminate.\n";
    char ch;
    int whitespaces = 0;
    int digits = 0;
    int chars = 0;
    int punct = 0;
    int others = 0;

    cin.get(ch);
    while(ch != '@')
    {
        if (isalpha(ch))
            chars++;
        else if (isspace(ch))
            whitespace++;
        else if (isdigit(ch))
            digits++;
        else if (ispunct(ch))
            punct++;
        else 
            others++;
        cin.get(ch);

    cout << "letters: " << chars << endl;
    cout << "whitespace: " << whitespace << endl;
    cout << "punctuations: " << punct << endl;
    cout << "others: " << others << endl;

    return 0;
}

？：运算符（两者取其一）

• expression1 ？expression2 ：expression3
• expression1为真，条件表达式值为expression2，否则，条件表达式值为expression3

(5 > 3) ? 10 : 12        //(5 > 3) 为真，表达式值为10
(3 == 9) ? 25 : 18       //(3 == 9)为假，表达式值为18

//程序清单6.9
//两个数比大小

#include <iostream>

using namespace std;

int main(void)
{
    int a, b;
    cout << "Enter two integers: ";
    cin >> a >> b;
    int c = (a > b) ? a : b;
    cout << "The larger of " << a << " and " << b << " is " << c << endl;

    return 0;
}

• 它与下面的语句等效

int c;
if (a > b)
    c = a;
else
    c = b;

• 条件运算符最适合与简单关系和简单表达式的值。
• 当代码变得复杂时，使用if else语句来表达可能更为清晰。

switch语句（多选一）

switch (integer_expression)
{
    case label1 : statement(s)
    case label2 : statement(s)
    ...
    default : statement(s)
}

• integer_expression的值为4，则程序执行标签case 4
• integer_expression必须是结果为整数的表达式
• 标签必须是整数常量表达式（int、char或枚举量）
• integer_expression不与任何标签匹配，程序跳到default一行
• 程序执行完某个case不会停止，会继续往下执行，要停止必须使用break语句

//程序清单6.10

#include <iostream>

using namespace std;

void showmenu(void);
void report(void);
void comport(void);

int main(void)
{
    showmenu();
    int choice;
    cin >> chioce;

    while (choice != 5)
    {
        switch(choice)
        {
            case 1 : cout << "\a\n";
                     break;
            case 2 : report();
                     break;
            case 3 : cout << "Boss was in all day.\n";
                     break;
            case 4 : comfort();
                     break;
            default: cout << "That's not a choice.\n";
        }
        shormenu();
        cin >> choice;
    }

    return 0;
}

void showmenu(void)
{
    cout << "Enter 1, 2, 3, 4 or 5:\n";
            "1) alarm        2) report\n";
            "3) alibi        4) comfort\n";
            "5) quit\n";
}

void report(void)
{
    cout << "Sales up! Expenses down!\n";
}

void comfort(void)
{
    cout << "Finest CEO!";
}

    //正是由于程序执行完某个case不会停止，会继续往下执行的特性
    //可以为大小写提供相同语句
    char choice;
    cin >> chioce;

    while (choice != 'Q' && choice != 'q')
    {
        switch(choice)
        {
            case 'a' :
            case 'A' : cout << "\a\n";
                       break;
            case 'r' : 
            case 'R' : report();
                       break;
            case 'l' : 
            case 'L' : cout << "Boss was in all day.\n";
                       break;
            case 'c' : 
            case 'C' : comfort();
                       break;
            default: cout << "That's not a choice.\n";
        }
        shormenu();
        cin >> choice;
    }

将枚举量用作标签

• 当switch语句将int值和枚举量标签进行比较时，将枚举量提升为int
• 在while循环测试条件中，也会将枚举量提升为int类型

//程序清单6.11
//switch语句和枚举

#include <iostream>

using namespace std;

enum {red, orange, yellow, green, blue, violet, indigo};

int main(void)
{
    cout << "Enter color code (0-6): ";
    int code;
    cin >> code;
    while ((code >= red) && (code <= indigo))
    {
        switch (code)
        {
            case red    : cout << "Her lips were red.\n"; break;
            case orange : cout << "Her hair were orange.\n"; break;
            case yellow : cout << "Her shoes were yellow.\n"; break;
            case green  : cout << "Her nails were green.\n"; break;
            case blue   : cout << "Her sweatsuit were blue.\n"; break;
            case violet : cout << "Her eyes were violet.\n"; break;
            case indigo : cout << "Her mood were indigo.\n"; break;
        }
        cout << "Enter color code (0-6): ";
        cin >> code;
    }
    cout << "Bye\n";

    return 0;
}

 switch和if else

• switch：选项过多
• if else：设计取值范围、浮点测试、两个变量比较

break和continue语句

 

//程序清单6.12

#include <iostream>

using namespace std;

const int ArSize = 80;

int main(void)
{
    char line[ArSize];
    int spaces = 0;
    cout << "Enter a line of text:\n";
    cin.getline(line, ArSize);
    cout << "Complete line:\n" << line << endl;
    cout << "line through first period:\n";

    for (int i = 0; line[i] != '\0'; i++)
    {
        cout << line[i];
        if (line[i] == '.')
            break;
        if (line[i] != ' ')
            continue;
        spaces++;        //缩进是因为continue后的表达式会跳过
    }
    cout << endl << spaces << " spaces\n";

    return 0;
}

• for循环中，continue语句使程序 直接跳到更新表达式处，然后跳到测试表达式处
• while循环中，continue使程序直接跳到测试表达式处，因此while循环中位于continue之后的更新表达式都将跳过
• 该程序可以不使用continue语句：

if (line[i] == ' ')
    spaces++;

读取数字的循环（要输入数字但是误输单词，怎么办？）

会发生4种情况：

• n的值保持不变
• 不匹配的对象将被留在输入队列中
• cin对象中的一个错误标记被设置
• 对cin方法的调用将返回false（如果被转换为bool类型）

//程序清单6.13
//当用户输入的不是数字时，程序不再读取输入

#include <iostream>

using namespace std;

const int Max = 5;

int main(void)
{
    double fish[Max];
    cout << "Enter fish's weight.\n";
    cout << "Enter up to " << Max << " fish, <q> to terminate.\n";
    cout << "fish #1: ";

    int i;
    while (i < Max && cin >> fish[i])
    {
        if (++i < Max)
            cout << "fish #" << i+1 << ": ";
    }

    double total = 0.0;
    for (int j = 0; j < i; j++)
        total += fish[i];

    if (i == 0)
        cout << "No fish.\n";
    else
        cout << "Average weight: " << total / i << endl;

    cout << "Done.\n";

    return 0;
}

//程序清单6.14
//当用户输入的不是数字时，程序提示错误，让用户继续输入

#include <iostream>

using namespace std;
const int Max = 5;

int main(void)
{
    int golf[Max];
    cout << "Enter golf scores.\n";
    cout << "Enter " << Max << " rounds.\n";

    int i;
    for (i = 0; i < Max, i++)
    {
        cout << "round #" << i+1 < ": ";
        while (!(cin >> golf[i]))        //判断输入是否正确，如果错误，执行循环
        {
            cin.clear();
            while (cin.get() != '\n')
                continue;
            cout << "Enter a number: ";
        }
    }

    double total = 0.0;
    for (i = 0; i < Max; i++)
        total += golf[i];

    cout << "Average score = " << total / Max << " in " << Max << " rounds.\n";

    return 0；
}

 简单文件输入/输出

写入到文本文件中

 

//程序清单6.15
//写入文件

#include <iostream>
#include <fstream>                        //1、包含文本流

using namespace std;

int main(void)
{
    char automobile[50];
    int year;
    double a_price;
    double d_price;

    ofstream outFile;                     //2、为类ofstream创建outFile对象
    outFile.open("carinfo.txt");          //3、将outFile对象和carinfo.txt文件关联

    cout << "Enter the make and model of automobile: ";
    cin.getline(automobile, 50);
    cout << "Enter the model year: ";
    cin  >> year;
    cout << "Enter the original asking price: ";
    cin >> a_price;
    d_price = 0.913 * a_price;

    cout << fixed;                        //用小数，不用科学计数
    cout.precision(2);                    //保留小数点后两位
    cout.setf(ios_base::showpoint);       //小数点后为0也显示

    cout << "Make and model: " << automobile << endl;
    cout << "Year: " << year << endl;
    cout << "Was asking $" << a_price << endl;
    cout << "Now asking $" << d_price << endl;

                                             //4、outFile和cin用法一模一样
    outFile << fixed;                        //用小数，不用科学计数
    outFile.precision(2);                    //保留小数点后两位
    outFile.setf(ios_base::showpoint);       //小数点后为0也显示

    outFile << "Make and model: " << automobile << endl;
    outFile << "Year: " << year << endl;
    outFile << "Was asking $" << a_price << endl;
    outFile << "Now asking $" << d_price << endl;

    outFile.close();        //与outFile.open()成对

    return 0;
}

 读取文本文件

//程序清单6.16
//读取文件

#include <iostream>
#include <fstream>                //1、file I/O support
#include <cstdlib>                //support for exit()

using namespace std;
const int SIZE = 60;

int main(void)
{
    char filename[SIZE];
    ifstream inFile;                            //2、为类ifstream创建对象inFile
    cout << "Enter name of data file: ";
    cin.getline(filename, SIZE);
    inFile.open(filename);                      //3、将对象inFile与文件filename关联

    if (!inFile.is_open())                      //判断文件打开是否成功，若否执行循环
    {
        cout << "Could not open the file: " << filename << endl;
        cout << "Terminating.\n";
        exit(EXIT_FAILURE);                     //出错退出，正常用return
    }   
    cout << "Sucess open the txt file.\n";

    double value;
    double sum = 0.0;
    int count = 0;

    inFile >> value;
    while (inFile.good())                        //读取成功且没到文件尾
    {
        ++count;
        sum += value;
        inFile >> value;
    }

    if (inFile.eof())                            //如果到文件尾
        cout << "End of file reached.\n";
    else if (inFile.fail())                      //如果读取失败
        cout << "Input terminated by data mismatch.\n";
    else
        cout << "Input terminated for unknown reason.\n";

    if (count == 0)
        cout << "No data processed.\n";
    else
    {
        cout << "Items read: " << count << endl;
        cout << "Sum: " << sum << endl;
        cout << "Average: " << sum / count << endl;
    }
    inFile.close();                              //与inFile.open()成对

    return 0;
}