fstream ifstream我不明白如何加载数据文件到我的程序 [英] fstream ifstream I don't understand how to load a data file into my program

问题描述

我的教授很聪明，但希望像我这样的完整的noob只是知道如何编程 c ++ 。我不明白 fstream 函数是如何工作的。

我将有一个包含三列数据的数据文件。我将不得不用对数确定每一行数据是否代表一个圆，矩形或三角形 - 这部分很容易。我不明白的部分是如何 fstream 函数工作。

我认为我：

  #include< fstream> 
  

那么我应该声明我的文件对象吗？

  ifstream Holes; 
  

然后我打开它：

  ifstream.open孔; //？ 
  

我不知道什么是正确的语法，我找不到简单的教程。一切似乎比我的技能可以处理更先进。

此外，一旦我读入数据文件，什么是正确的语法将数据放入数组？

我只是声明一个数组例如 T [N] 和 cin fstream object <$ c>基本 ifstream code>用法：

  #include< fstream& // for std :: ifstream 
 #include< iostream> // for std :: cout 
 #include< string> // for std :: string和std :: getline 
 
 int main（）
 {
 std :: ifstream infile（thefile.txt）; //构造对象并打开文件
 std :: string line; 
 
 if（！infile）{std :: cerr<< 打开文件时出错！\\\
; return 1; } 
 
 while（std :: getline（infile，line））
 {
 std :: cout< 文件说，<线<< '.\\\
; 
} 
} 
  

让我们进一步假设我们要处理每行根据一些模式。我们使用字符串流：

  #include< sstream> // for std :: istringstream 
 
 // ...如前面的
 
 while（std :: getline（infile，line））
 {
 std :: istringstream iss（line）; 
 double a，b，c; 
 
 if（！（iss>> a>> b> c））
 {
 std :: cerr< 无效的行，skipping.\\\
; 
 continue; 
} 
 
 std :: cout<< 我们获得三个值[< a<< ，< b<< ，< c<< ] .\\\
; 
} 
  




My professor is very smart but expects complete noobs like me to just know how to program c++. I don't understand how the fstream function works.

I will have a data file with three columns of data. I will have to determine with a logarithm whether each line of data represents a circle, rectangle or triangle - that part is easy. The part I don't understand is how the fstream function works.

I think I:

#include < fstream > 

then I should declare my file object?

ifstream Holes;

then I open it:

ifstream.open Holes; // ?

I don't know what the proper syntax is and I can't find straightforward tutorial. Everything seems way more advanced than my skills can handle.

Also, once I've read-in the data file what is the proper syntax to put the data into arrays?

Would I just declare an array e.g. T[N] and cin the fstream object Holes into it?

解决方案

Basic ifstream usage:

#include <fstream>   // for std::ifstream
#include <iostream>  // for std::cout
#include <string>    // for std::string and std::getline

int main()
{
    std::ifstream infile("thefile.txt");  // construct object and open file
    std::string line;

    if (!infile) { std::cerr << "Error opening file!\n"; return 1; }

    while (std::getline(infile, line))
    {
        std::cout << "The file said, '" << line << "'.\n";
    }
}

Let's go further and assume we want to process each line according to some pattern. We use string streams for that:

#include <sstream>   // for std::istringstream

// ... as before

    while (std::getline(infile, line))
    {
        std::istringstream iss(line);
        double a, b, c;

        if (!(iss >> a >> b >> c))
        {
            std::cerr << "Invalid line, skipping.\n";
            continue;
        }

        std::cout << "We obtained three values [" << a << ", " << b << ", " << c << "].\n";
    }

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