I/O流——对象和类入门(例子代码)

//5.1简单的文件输入/输出//从文件infile.dat读取3个数字，求和将结果写入文件outfile.dat//infile.dat和outfile.dat事先已经在根目录下//(该程序的一个更好的版本在5.2给出)#include<fstream>int main(){using namespace std;ifstream in_stream;ofstream out_stream;in_stream.open("infile.dat");out_stream.open("outfile.dat");int first,second,third;in_stream>>first>>second>>third;out_stream<<"The sum of the first 3/n"<<"number in file.dat/n"<<"is "<<(first+second+third)<<endl;in_stream.close();out_stream.close();return 0;}

 

//5.2同时检查open调用的文件输出和输出//从文件infile.dat中读取3个数字，求和并将其写到文件outfile.dat中#include<fstream>#include<iostream>#include<cstdlib>int main(){using namespace std;ifstream in_stream;ofstream out_stream;in_stream.open("infile.dat");if(in_stream.fail()){cout<<"Input file opening failed./n";exit(1);}out_stream.open("outfile.dat");if(out_stream.fail()){cout<<"Output file opening failed./n";exit(1);}int first,second,third;in_stream>>first>>second>>third;out_stream<<"The sum of the first 3/n"<<"number in infile.dat/n"<<"is "<<(first+second+third)<<endl;in_stream.close();out_stream.close();system("pause");return 0;}

2010.12.22

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今天更换了C++编译器，用code::blocks,感觉挺好的，而且是免费的！

//5.3在文件末尾追加数据//将数据追加到data.txt文件的末尾#include <iostream>#include<fstream>#include<cstdlib>int main(){ using namespace std; ofstream fin; fin.open("data.txt"); if(fin.fail()) { cout<<"Input file opening failed./n"; exit(1); } fin<<"1 2 bucket my shoe./n" <<"3 4 shut the door./n"; fin.close(); cout<<"Opening data.txt for appending./n"; ofstream fout; fout.open("data.txt",ios::app); if(fout.fail()) { cout<<"Input file opening failed./n"; exit(1); } fout<<"5 6 pick up sticks./n" <<"7 8 ain't C++ great!/n"; fout.close(); cout<<"End of appending to file./n"; system("pause"); return 0;}

——2010.12.24

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//5.4输入一个文件名//从用户指定的文件读取3个数字，累加这些数字，再将结果写到用户指定的另一个文件#include <iostream>#include<fstream>#include<cstdlib>void initialization();//初始化要读取的文件int main(){ using namespace std; char in_file_name[16],out_file_name[16]; ifstream in_stream; ofstream out_stream; ofstream initialization_stream; cout<<"I will sum three numbers taken from an input/n" <<"file and write the sum to an output file./n"; cout<<"Enter the input file name (maximum of 15 characters):/n"; cin>> in_file_name; cout<<"Enter the output file name (maximum of 15 characters):/n"; cin>> out_file_name; cout<<"I will read numbers from the file " <<in_file_name<<" and/n" <<"place the sum in the file " <<out_file_name<<endl; //创建一个 以in_file_name命名的文件，并初始化数据 initialization_stream.open(in_file_name); if(initialization_stream.fail()) { cout<<"initialization file failed./n"; exit(1); } initialization_stream<<"1/n"<<"2/n"<<"3/n"<<"4/n"; initialization_stream.close(); // 打开文件并执行操作 in_stream.open(in_file_name); if(in_stream.fail()) { cout<<"Input file opening failed./n"; exit(1); } out_stream.open(out_file_name); if(out_stream.fail()) { cout<<"Output file opening failed./n"; exit(1); } int first,second,third; in_stream>>first>>second>>third; out_stream<<"The sum of the first 3/n" <<"numbers in "<<in_file_name<<endl <<"is "<<(first+second+third) <<endl; in_stream.close(); out_stream.close(); cout<<"Enter of Program./n"; return 0;}

 

//5.6格式化输出，流作为函数参数//展示对输出进行格式化的命令//读取rawdata.dat文件中的所有数字//用美观的格式将数字写到屏幕，同时写到neat.dat文件#include <iostream>#include <fstream>#include <cstdlib>#include <iomanip> //这是setw必需的using namespace std;void make_neat(ifstream& messy_file,ofstream& neat_file, int number_after_decimalpoint,int field_width);//流参数必须是传引用调用//前条件：messy_file和neat_file这两个流已用open函数连接到文件//后条件：与messy_file流连接的那个文件中的数字写出屏幕// 同时写到与neat_file流连接的那个文件//每个数字独占一行，并采用定点记数法(换言之，不采用e记数法)，//而且在小数点之后保留number_after_decimalpoint位小数；//每个数字的前面要么添加一个正号，要么添加一个负号，//而且每个数字都占用宽度为field_width的一个域(该函数不关闭文件)int main(){ ifstream fin; ofstream fout; fin.open("rawdata.dat"); if(fin.fail()) { cout<<"Input file opening failed./n"; exit(1); } fout.open("neat.dat"); if(fout.fail()) { cout<<"Output file opening failed./n"; exit(1); } make_neat(fin,fout,5,12); fin.close(); fout.close(); cout<<"End of program./n"; return 0;}//使用iostream、fstream和iomanip：void make_neat(ifstream& messy_file,ofstream& neat_file, int number_after_decimalpoint,int field_width){ neat_file.setf(ios::fixed); //不采用e记数法 neat_file.setf(ios::showpoint); //显示小数点 neat_file.setf(ios::showpos); //显示正号 neat_file.precision(number_after_decimalpoint); cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.setf(ios::showpos); cout.precision(number_after_decimalpoint); double next; while(messy_file>>next) //如果还有需要读取的数字，就满足条件 { cout<<setw(field_width)<<next<<endl; neat_file<<setw(field_width)<<next<<endl; }}

——2010.12.25

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//5.7检查输入//演示new_line和get_input函数#include <iostream>using namespace std;void new_line();//丢弃当前输入行上剩余的所有输入//同时丢弃行末的'/n'。这个版本只支持键盘输入void get_input(int& number);//后条件：为number变量被赋予一个有用户批准的值int main(){ int n; get_input(n); cout<<"Final value read in = "<<n<<endl <<"End of demonstration./n"; return 0;}//使用iostream:void new_line(){ char symbol; do { cin.get(symbol); }while(symbol != '/n');}//使用iostream:void get_input(int& number){ char ans; do { cout<<"Enter input number: "; cin>>number; cout<<"You entered "<<number <<" Is that correct?(yes/no): "; cin>>ans; new_line(); }while((ans != 'Y')&&(ans != 'y'));}

——2010.12.27

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//5.8编辑文本文件//该程序用于创建一个名为cplusad.dat的文件，该文件等同于cad.dat文件，//只是后者中的所有'C'都被替换为'C++'。假定cad.dat中除了代表C编程语言//的C之外，没有其他大写字母'C'#include <iostream>#include <fstream>#include <cstdlib>using namespace std;void add_plus_plus(ifstream& in_stream,ofstream& out_stream);//前条件：in_stream已经通过open调用连接到一个输入文件//out_stream也通过open连接到一个输出文件//后条件：与in_stream连接的文件的内容拷贝到与out_stream连接的文件中//但每个'C'都被替换为"C++"void initialization();//初始化，生成一个cad.datint main(){ initialization(); ifstream fin; ofstream fout; cout << "Begin editing files./n"; fin.open("cad.dat"); if(fin.fail()) { cout << "Input file opening failed./n"; exit(1); } fout.open("cplusad.dat"); if(fout.fail()) { cout << "Output file opening failed./n"; exit(1); } add_plus_plus(fin,fout); fin.close(); fout.close(); cout << "End of editing files./n"; return 0;}void add_plus_plus(ifstream& in_stream,ofstream& out_stream){ char next; in_stream.get(next); while(!in_stream.eof()) { if(next == 'C') { out_stream << "C++"; } else { out_stream << next; } in_stream.get(next); }}void initialization(){ ofstream ini_doc; ini_doc.open("cad.dat"); if(ini_doc.fail()) { cout << "Input file opening failed./n"; exit(1); } ini_doc << "C is one of the world 's most modern/n" << "programming languages. There is no /n" << "languages as versatile as C, and C is /n" << "fun to use."; ini_doc.close();}

——2010.12.29

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