第六章

#include<iostream>
using namespace std;
int main()
{
    char ch;
    int spaces=0,total=0;
    cin.get(ch);
    while(ch !='.')//注意使用单引号。
    {
        if(ch ==' ')spaces++;
        total++;
        cin.get(ch);
    }
    cout<<spaces<<" sapces, "<<total<<" characters total in sentence"<<endl;
    return 0;
}

||和&&

#include<iostream>
using namespace std;
const int size=6;
int main()
{
    float a[size];
    cout<<"enter value,when you make "<<size<<" enteries or enter a negetive"<<endl;
    int i=0;
    float temp;
    cout<<"frist value";
    cin>>temp;
    while(i<size&&temp>=0)
    {
        a[i]=temp;
        i++;
        if(i<size)
        {
            cout<<"next value:";
            cin>>temp;
        }
    }
    if(i==0)cout<<"no data"<<endl;
    else
    {
        cout<<"enter your naaq";
        float you;
        cin>>you;
        int count=0;
        for(int j=0;j<i;j++)if(a[j]>you)count++;
        cout<<count;
        cout<<" of your neighbors have greater awarense of the new age then you do";
    }
    return 0;
}

||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

switch（interger-expression）
{
    case label1:statement(a);
    case label2:statement(b);
    …
    case labeln:statement(n);
default:statement(s);
}

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。

#include<iostream>
using namespace std;
void showmenu();
void report();
void comfort();
int main()
{
    char choice;
    cin>>choice;
    while(choice!='q'&&choice!='Q')
    {
        switch(choice)
        {
        case 'a':
        case 'A':cout<<"a and A";
                break;
        case'b':
        case'B':cout<<"b and B";
                break;
        case'c':
        case'C':cout<<"c and C";
                break;
        case'd':
        case'D':cout<<"d and D";
                break;
        default:cout<<"that is not a choice"<<endl;
        }
        cin>>choice;
    }
    return 0;
}

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。

#include<iostream>
using namespace std;
enum{red,orange,yellow,green,blue,violet,indigo};
int main()
{
    cout<<"enter color code(1-6)";
    int code;
    cin>>code;
    while(code>=red&&code<=indigo)
    {
        switch(code)
        {
        case red:cout<<"red"<<endl;break;
        case orange:cout<<"orange"<<endl;break;
        case yellow:cout<<"yellow"<<endl;break;
        case green:cout<<"green"<<endl;break;
        case blue:cout<<"blue"<<endl;break;
        case violet:cout<<"violet"<<endl;break;
        case indigo:cout<<"indigo"<<endl;break;
        }
        cout<<"enter the code(0-6)";
        cin>>code;
    }
    return 0;
}

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。

#include<iostream>
const int size=80;
int main()
{
    using namespace std;
    char line[size];
    int spaces=0;
    cout<<"enter a line of text";
    cin.get(line,size);
    cout<<"complete line"<<endl<<line<<endl;
    cout<<"line throught first today"<<endl;
    for(int i=0;line[i]!='
';i++)
    {
        cout<<line[i];
        if(line[i]=='.')break;
        else if(line[i]!=' ')continue;
        else spaces++;
    }
    cout<<endl<<spaces<<" spaces"<<endl;
    return 0;
}

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。

#include<iostream>
const int Max=5;
int main()
{
    using namespace std;
    double fish[Max];
    cout<<"enter the weight of your fishes"<<endl;
    cout<<"you may enter uo to "<<Max<<" fishes<q to terminate>"<<endl;
    cout<<"fish #1";
    int i=0;
    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
    {
        if(++i<Max)cout<<"fish #"<<i+1<<":";
    }
    double total=0;
    for(int j=0;j<i;j++)total+=fish[j];
    if(i==0)cout<<"no fish"<<endl;
    else cout<<total/i<<"= average weight of "<<i<<" fish"<<endl;
    return 0;
}

下面程序是输入错误继续输入的。

#include<iostream>
const int Max=5;
using namespace std;
int main()
{
    int golf[Max];
    cout<<"enter thegolf scores"<<endl;
    cout<<"you must enter "<<Max<<" round"<<endl;
    int i;
    for(i=0;i<Max;i++)
    {
        cout<<"round #"<<i+1<<":";
        while(!(cin>>golf[i]))
        {
            cin.clear();
            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。
            cout<<"enter a number";
        }
    }
    double total=0;
    for(i=0;i<Max;i++)
    total+=golf[i];
    cout<<total/Max<<"=average score "<<Max<<" round"<<endl;
    return 0;
}

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（）

#include<iostream>
#include<fstream>
using namespace std;
int main()
{
    char automobile[50];
    int year;
    double a_price;
    double b_price;

    ofstream outfile;
    outfile.open("carinfo.txt");

    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;
    b_price=0.913*a_price;

    cout<<fixed;
    cout.precision(2);//精度
    cout.setf(ios_base::showpoint);
    cout<<"make and model:"<<automobile<<endl;
    cout<<"year:"<<year<<endl;
    cout<<"was asking $"<<a_price<<endl;
    cout<<"was asking $"<<b_price<<endl;

    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<<"was asking $"<<b_price<<endl;

    outfile.close();
    return 0;
}

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。

inFile.open(“bowling.txt”);
if(!inFile.is_open())
{
    exit(EXIT_FAILURE);
}

程序例

#include<iostream>
#include<fstream>
#include<cstdlib>
const int SIZE=60;
using namespace std;
int main()
{
    char filename[SIZE];
    ifstream infile;
    cout<<"enter name of data file ";
    cin.getline(filename,SIZE);
    infile.open(filename);
    if(!infile.is_open())
    {
        cout<<"coule not open the file"<<filename<<endl;
        cout<<"program terminating"<<endl;
        exit(EXIT_FAILURE);
    }
    double value;
    double sum=0;
    int count =0;
    infile>>value;
    while(infile.good())
    {
        count++;
        sum+=value;
        infile>>value;
    }
    if(infile.eof())cout<<"end of file reached"<<endl;
    else if(infile.fail())cout<<"input terminated by data mismatch"<<endl;
    else cout<<"input terminated for unknow reason"<<endl;
    if(count==0)cout<<"no data"<<endl;
    else 
    {
        cout<<"items read "<<count<<endl;
        cout<<"sum"<<sum<<endl;
        cout<<"average "<<sum/count<<endl;
    }
    infile.close();
    return 0;
}

方法good（）

在没有任何错误时返回true。