如何一次从一个文件读取输入，类似于在C ++中使用cin/scanf从控制台读取输入? [英] How to read an input from a file one at a time similar to reading input from console using cin/scanf in c++?

问题描述

我正在尝试从文件中逐个读取输入，而不是从标准输入中给出输入.

I am trying to read input from a file one by one instead of giving it from standard input.

我所拥有的，目前正在使用！

What I have, which is currently working!

   for(int i = 0; i <CLASS_SIZE; i++)
    {
      for(int j = 0; j <10 ; j++)
        {
            scanf("%d", &grade);
            studentsInClass[i].setGrade(j,grade);
        }  
    }

来自控制台的当前输入是:

current input from console is this:

91 92 85 58 87 75 89 97 79 65
88 72 81 94 90 61 72 75 68 77
75 49 87 79 65 64 62 51 44 70

我希望直接从文件中读取此输入！因此，我尝试逐行将其读取为文件流，然后尝试将该行保存到字符串向量中，以便可以在内部循环中提取每行的单个数字.

I want this input to be read directly from a file! So, I tried reading it as a file stream, line by line and then tried to save that line into a vector of strings, so that I can extract the single number per line in an inner loop.

std::ifstream inputfile("input.txt");
std::string line;
std::vector<std::string> v;

for(int i = 0; i <CLASS_SIZE; i++)
{
  for(int j = 0; j <10 ; j++)
    {
        if(inputfile.is_open()){
          while( std::getline(inputfile, line) ){
                std::cout << line << '\n';
                std::istringstream iss(line);
                iss>>v;
                for(std::vector<std::string>::iterator it = v.begin(); it != v.end(); ++it) {
                  studentsInClass[i].setGrade(j, std::stoi(it, nullptr, 2));
             }
          }
        inputfile.close();
        }
    }
}

但是出现以下错误！因为我不知道正确的方法！

but getting following error! Because I don't know the right way to do it!

error: cannot bind ‘std::basic_istream<char>’ lvalue to ‘std::basic_istream<char>&&’
                     iss>>v;

及以下错误，因为我正在尝试将字符串转换为整数，因为我的程序需要将字符串消耗为整数，所以我再次在语法上做错了事.

and below error, because I am trying to convert the string to an integer as my program needs to consume as integer and again I am doing something wrong with the syntax.

error: no matching function for call to ‘stoi(std::vector<std::__cxx11::basic_string<char> >::iterator&, std::nullptr_t
, int)’
                       studentsInClass[i].setGrade(j, std::stoi(it, nullptr, 2));

任何人都可以帮助解决此问题吗?

can anyone help to fix this issue?

推荐答案

首先，我将对使您满意的代码进行必要的最小更改:

As a first approximation, I'd make the most minimal changes necessary to the code that makes you happy:

std::ifstream infile("filename");

for(int i = 0; i <CLASS_SIZE; i++)
{
  for(int j = 0; j <10 ; j++)
    {          
        // scanf("%d", &grade);
        infile >> grade;
        studentsInClass[i].setGrade(j,grade);
    }  
}

鉴于您知道每个学生的确切成绩，使用 getline 几乎不会(如果有).如果输入是面向行的，那将很有用.例如，考虑这样的输入文件:

Given that you know the exact number of grades for each student, you gain little (if anything) from using getline. That would be useful primarily if the input was line-oriented. For example, consider an input file like this:

91 92 85 58 87 75 89 97 79 65 88 
72 81 94 90 61 72 75 68 77
75 49 87 79 65 64 62 51 44 70

按现状，您显然仍希望这是3位学生的成绩，每位10年级，所以第一行的最后一个成绩( 88 )属于第二个学生，而不是第一位.

As it stands, you still apparently want this read as 3 students with 10 grades apiece, so the last grade on the first line (the 88) belongs to the second student, not the first.

使用 getline ，然后分别解析每行，如果您希望将此输入数据解释为第一位学生具有11年级，第二位学生具有9年级(第三位是10年级)，则是有意义的.

Using getline and then parsing each line individually would make sense if you wanted this input data interpreted as the first student having 11 grades, and the second student 9 grades (and the third 10 grades).

这涵盖了您提出的问题，但是我对代码仍然不太满意.就个人而言，我更喜欢编写一个学生类，该类知道如何从流中读取自己的数据:

That covers the question you asked, but I'm still not particularly happy about the code. Personally, I'd prefer to write a student class that knew how to read its own data from a stream:

class student { 
    std::vector<int> grades;
public:

   // other stuff, of course.

   friend std::istream &operator>>(std::istream &is, student &s) {
       int grade;
       for (int i=0; i<10; i++) {
           is >> grade;
           s.grades.push_back(grade);
       }
       return is;
    }
};

然后，当我们想读取多个学生的数据时，我们将执行以下操作:

Then when we want to read data for multiple students, we'd do something like this:

std::vector<students> the_class;
std::ifstream infile("grades.txt");

for (int i=0; i<CLASS_SIZE; i++) {
    student s;

    // use `operator>>` above to read all 10 grades for one student.
    infile >> s;
    the_class.push_back(s);
}

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