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和break语句
- 读取数字和循环
- 基本文件输入/输出
设计智能程序的一个关键是使程序具有决策能力.
6.1 if语句
- 程序清单6.1 if.cpp
6.1.1 if else语句
- 程序清单6.2 ifelse.cpp
6.1.2 格式化if else语句
- 由于C++是自由格式语言,因此只要使用大括号将语句括起,对大括号的位置没有任何限制.
6.1.3 if else if else结构
- 程序清单6.3 ifelseif.cpp
6.2 逻辑表达式
6.2.1 逻辑OR运算符:||
- 程序清单6.4 or.cpp
6.2.2 逻辑AND运算符:&&
- 程序清单6.5 and.cpp
6.2.3 用&&来设置取值范围
- 程序清单6.6 more_and.cpp
6.2.4 逻辑NOT运算符:!
- 程序清单6.7 not.cpp
6.2.5 逻辑运算符细节
6.2.6 其他表示方式
6.3 字符函数库cctype
程序清单6.8 cctypes.cpp
// cctypes.cpp -- using the ctype.h library
#include <iostream>
#include <cctype> // prototypes for character functions
int main()
{
using namespace std;
cout << "Enter text for analysis, and type @"
" to terminate input.\n";
char ch;
int whitespace = 0;
int digits = 0;
int chars = 0;
int punct = 0;
int others = 0;
cin.get(ch); // get first character
while (ch != '@') // test for sentinel
{
if(isalpha(ch)) // is it an alphabetic character?
chars++;
else if(isspace(ch)) // is it a whitespace character?
whitespace++;
else if(isdigit(ch)) // is it a digit?
digits++;
else if(ispunct(ch)) // is it punctuation?
punct++;
else
others++;
cin.get(ch); // get next character
}
cout << chars << " letters, "
<< whitespace << " whitespace, "
<< digits << " digits, "
<< punct << " punctuations, "
<< others << " others.\n";
// cin.get();
// cin.get();
return 0;
}6.4 ?:运算符
- 条件运算符(?:),它是C++中唯一一个需要3个操作数的运算符.
expression1?expression2:expression3;- 1
- 程序清单6.9 condit.cpp
6.5 switch语句
- 程序清单6.10 switch.cpp
6.5.1 将枚举量用作标签
- 程序清单6.11 enum.cpp
6.5.2 switch和if else
- switch并不是为处理取值范围而设计的.
6.6 break和continue语句
- continue语句用于循环中,让程序跳过循环体中余下的代码,并开始新一轮循环.
- 程序清单6.12 jump.cpp
- 和C语言一样,C++也有goto语句.
6.7 读取数字的循环
- 程序清单6.13 cinfish.cpp
- 程序清单6.14 cingolf.cpp
6.8 简单文件输入/输出
6.8.1 文本I/O和文本文件
6.8.2 写入到文本文件中
- 使用文件输出的主要步骤如下:
- 包含头文件fstream.
- 创建一个ofstream对象.
- 将该ofstream对象同一个文件关联起来.
- 就像使用cout那样使用该ofstream对象.
程序清单6.15 outfile.cpp
// outfile.cpp -- writing to a file
#include <iostream>
#include <fstream> // for file I/O
int main()
{
using namespace std;
char automobile[50];
int year;
double a_price;
double d_price;
ofstream outFile; // create object for output
outFile.open("carinfo.txt"); // associate with a file
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;
// display information on screen with cout
cout << fixed;
cout.precision(2);
cout.setf(ios_base::showpoint);
cout << "Make and model: " << automobile << endl;
cout << "Year: " << year << endl;
cout << "Was asking $" << a_price << endl;
cout << "Now asking $" << d_price << endl;
// now do exact same things using outFile instead of cout
outFile << fixed;
outFile.precision(2);
outFile.setf(ios_base::showpoint);
outFile << "Make and model: " << automobile << endl;
outFile << "Year: " << year << endl;
outFile << "Was asking $" << a_price << endl;
outFile << "Now asking $" << d_price << endl;
outFile.close(); // done with file
// cin.get();
// cin.get();
return 0;
}- 注意:方法close()不需要使用文件名作为参数,这是因为outFile已经同特定的文件关联起来.如果您忘记关闭文件,程序正常终止时将自动关闭它.
- 警告:打开已有的文件,以接受输出时,默认将它其长度截短为零,因此原来的内容将丢失.
6.8.3 读取文本文件
- 重要的是,声明一个ifstream对象并将其同文件关联起来后,便可以想使用cin那样使用它.
- 程序清单6.16 sumafile.cpp
// sumafile.cpp -- functions with an array argument
#include <iostream>
#include <fstream> // file I/O support
#include <cstdlib> // support for exit()
const int SIZE = 60;
int main()
{
using namespace std;
char filename[SIZE];
ifstream inFile; // object for handling file input
cout << "Enter name of data file: ";
cin.getline(filename, SIZE);
inFile.open(filename); // associate inFile with a file
if (!inFile.is_open()) // failed to open file
{
cout << "Could not open the file " << filename << endl;
cout << "Program terminating.\n";
// cin.get(); // keep window open
exit(EXIT_FAILURE);
}
double value;
double sum = 0.0;
int count = 0; // number of items read
inFile >> value; // get first value
while (inFile.good()) // while input good and not at EOF
{
++count; // one more item read
sum += value; // calculate running total
inFile >> value; // get next value
}
if (inFile.eof())
cout << "End of file reached.\n";
else if (inFile.fail())
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(); // finished with the file
// cin.get();
return 0;
}- 警告:Windows文本文件的每行都以回车字符和换行符结尾;通常情况下,C++在读取文件时将这两个字符转换为换行符,并在写入文件时执行相反的转换.有些文本编辑器(如Metrowerks CodeWarrior IDE编译器),不会自动在最后一行末尾加上换行符.因此,如果读者使用的是这种编辑器,请在输入最后的文本按下回车键,然后再保存文件.
6.9 总结
6.10 复习题
6.11 编程练习
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