第六章

 1 #include<iostream>
 2 using namespace std;
 3 int main()
 4 {
 5     char ch;
 6     int spaces=0,total=0;
 7     cin.get(ch);
 8     while(ch !='.')//注意使用单引号。
 9     {
10         if(ch ==' ')spaces++;
11         total++;
12         cin.get(ch);
13     }
14     cout<<spaces<<" sapces, "<<total<<" characters total in sentence"<<endl;
15     return 0;
16 }

||和&&

 1 #include<iostream>
 2 using namespace std;
 3 const int size=6;
 4 int main()
 5 {
 6     float a[size];
 7     cout<<"enter value,when you make "<<size<<" enteries or enter a negetive"<<endl;
 8     int i=0;
 9     float temp;
10     cout<<"frist value";
11     cin>>temp;
12     while(i<size&&temp>=0)
13     {
14         a[i]=temp;
15         i++;
16         if(i<size)
17         {
18             cout<<"next value:";
19             cin>>temp;
20         }
21     }
22     if(i==0)cout<<"no data"<<endl;
23     else
24     {
25         cout<<"enter your naaq";
26         float you;
27         cin>>you;
28         int count=0;
29         for(int j=0;j<i;j++)if(a[j]>you)count++;
30         cout<<count;
31         cout<<" of your neighbors have greater awarense of the new age then you do";
32     }
33     return 0;
34 }

||是顺序点，也就是说，先修改左侧的值，在对右侧的值进行判定。

如 i++<||i==j;

&&也是顺序点。

？运算符：

x=(x>y)?x:y;

switch语句

1 switch（interger-expression）
2 {
3     case label1:statement(a);
4     case label2:statement(b);
5     …
6     case labeln:statement(n);
7 default:statement(s);
8 }

interger-expression为一个结果为整数值的表达式。

每个标签都必须是整数。

依次执行之后的所有语句，可以使用break停止。

 1 #include<iostream>
 2 using namespace std;
 3 void showmenu();
 4 void report();
 5 void comfort();
 6 int main()
 7 {
 8     char choice;
 9     cin>>choice;
10     while(choice!='q'&&choice!='Q')
11     {
12         switch(choice)
13         {
14         case 'a':
15         case 'A':cout<<"a and A";
16                 break;
17         case'b':
18         case'B':cout<<"b and B";
19                 break;
20         case'c':
21         case'C':cout<<"c and C";
22                 break;
23         case'd':
24         case'D':cout<<"d and D";
25                 break;
26         default:cout<<"that is not a choice"<<endl;
27         }
28         cin>>choice;
29     }
30     return 0;
31 }

将枚举量用作标签

当switch语句将int值与枚举标签进行比较时，将枚举量提升为int。

 1 #include<iostream>
 2 using namespace std;
 3 enum{red,orange,yellow,green,blue,violet,indigo};
 4 int main()
 5 {
 6     cout<<"enter color code(1-6)";
 7     int code;
 8     cin>>code;
 9     while(code>=red&&code<=indigo)
10     {
11         switch(code)
12         {
13         case red:cout<<"red"<<endl;break;
14         case orange:cout<<"orange"<<endl;break;
15         case yellow:cout<<"yellow"<<endl;break;
16         case green:cout<<"green"<<endl;break;
17         case blue:cout<<"blue"<<endl;break;
18         case violet:cout<<"violet"<<endl;break;
19         case indigo:cout<<"indigo"<<endl;break;
20         }
21         cout<<"enter the code(0-6)";
22         cin>>code;
23     }
24     return 0;
25 }

break和continue语句

continue跳过循环体剩余的部分，开始新一轮的循环，

break跳出循环。

 1 #include<iostream>
 2 const int size=80;
 3 int main()
 4 {
 5     using namespace std;
 6     char line[size];
 7     int spaces=0;
 8     cout<<"enter a line of text";
 9     cin.get(line,size);
10     cout<<"complete line"<<endl<<line<<endl;
11     cout<<"line throught first today"<<endl;
12     for(int i=0;line[i]!='
';i++)
13     {
14         cout<<line[i];
15         if(line[i]=='.')break;
16         else if(line[i]!=' ')continue;
17         else spaces++;
18     }
19     cout<<endl<<spaces<<" spaces"<<endl;
20     return 0;
21 }

clear（）方法重置错误输入标记，同时也重置文件尾。

假设要将一系列数字读取到数组中，并允许在数组填满之前结束输入。可以使用cin；

但是万一用户输入的是单词呢？

下面的程序如果数组被填满或者输入非数字输入，程序结束。

 1 #include<iostream>
 2 const int Max=5;
 3 int main()
 4 {
 5     using namespace std;
 6     double fish[Max];
 7     cout<<"enter the weight of your fishes"<<endl;
 8     cout<<"you may enter uo to "<<Max<<" fishes<q to terminate>"<<endl;
 9     cout<<"fish #1";
10     int i=0;
11     while(i<Max&&cin>>fish[i])//cin位于测试条件中被转换成bool型，如果输入成功返回true，否则返回false
12     {
13         if(++i<Max)cout<<"fish #"<<i+1<<":";
14     }
15     double total=0;
16     for(int j=0;j<i;j++)total+=fish[j];
17     if(i==0)cout<<"no fish"<<endl;
18     else cout<<total/i<<"= average weight of "<<i<<" fish"<<endl;
19     return 0;
20 }

下面程序是输入错误继续输入的。

 1 #include<iostream>
 2 const int Max=5;
 3 using namespace std;
 4 int main()
 5 {
 6     int golf[Max];
 7     cout<<"enter thegolf scores"<<endl;
 8     cout<<"you must enter "<<Max<<" round"<<endl;
 9     int i;
10     for(i=0;i<Max;i++)
11     {
12         cout<<"round #"<<i+1<<":";
13         while(!(cin>>golf[i]))
14         {
15             cin.clear();
16             while(cin.get()!='
')continue;//读取行尾之前的所有输入，从而删除这一行的错误输入。
17             cout<<"enter a number";
18         }
19     }
20     double total=0;
21     for(i=0;i<Max;i++)
22     total+=golf[i];
23     cout<<total/Max<<"=average score "<<Max<<" round"<<endl;
24     return 0;
25 }

写入到文本文件中

头文件fstream

自己声明ofstream变量（对象）

使用命名空间std；使用完文件后要关闭close（）

 1 #include<iostream>
 2 #include<fstream>
 3 using namespace std;
 4 int main()
 5 {
 6     char automobile[50];
 7     int year;
 8     double a_price;
 9     double b_price;
10 
11     ofstream outfile;
12     outfile.open("carinfo.txt");
13 
14     cout<<"enter the make and model of automobile:";
15     cin.getline(automobile,50);
16     cout<<"enter the model year";
17     cin>>year;
18     cout<<"enter the original asking price:";
19     cin>>a_price;
20     b_price=0.913*a_price;
21 
22     cout<<fixed;
23     cout.precision(2);//精度
24     cout.setf(ios_base::showpoint);
25     cout<<"make and model:"<<automobile<<endl;
26     cout<<"year:"<<year<<endl;
27     cout<<"was asking $"<<a_price<<endl;
28     cout<<"was asking $"<<b_price<<endl;
29 
30     outfile<<fixed;
31     outfile.precision(2);
32     outfile.setf(ios_base::showpoint);
33     outfile<<"make and model:"<<automobile<<endl;
34     outfile<<"year:"<<year<<endl;
35     outfile<<"was asking $"<<a_price<<endl;
36     outfile<<"was asking $"<<b_price<<endl;
37 
38     outfile.close();
39     return 0;
40 }

声明一个ofstream对象，然后使用open（）将其与特定对象关联起来。

读取文本文件

可以使用is_open（）检查文件是否被成功打开。

1 inFile.open(“bowling.txt”);
2 if(!inFile.is_open())
3 {
4     exit(EXIT_FAILURE);
5 }

程序例

 1 #include<iostream>
 2 #include<fstream>
 3 #include<cstdlib>
 4 const int SIZE=60;
 5 using namespace std;
 6 int main()
 7 {
 8     char filename[SIZE];
 9     ifstream infile;
10     cout<<"enter name of data file ";
11     cin.getline(filename,SIZE);
12     infile.open(filename);
13     if(!infile.is_open())
14     {
15         cout<<"coule not open the file"<<filename<<endl;
16         cout<<"program terminating"<<endl;
17         exit(EXIT_FAILURE);
18     }
19     double value;
20     double sum=0;
21     int count =0;
22     infile>>value;
23     while(infile.good())
24     {
25         count++;
26         sum+=value;
27         infile>>value;
28     }
29     if(infile.eof())cout<<"end of file reached"<<endl;
30     else if(infile.fail())cout<<"input terminated by data mismatch"<<endl;
31     else cout<<"input terminated for unknow reason"<<endl;
32     if(count==0)cout<<"no data"<<endl;
33     else 
34     {
35         cout<<"items read "<<count<<endl;
36         cout<<"sum"<<sum<<endl;
37         cout<<"average "<<sum/count<<endl;
38     }
39     infile.close();
40     return 0;
41 }

方法good（）

在没有任何错误时返回true。