c++ - 如何从 C++ 文件中读取特定数量的字符？

Question

如果我们有一个存储日期的文件，后面跟着 24 个数字，我们需要做的是计算这些数字的平均值，并将输出放在另一个文件中。此外，文件中的这些数字是 char 形式而不是 int。我无法弄清楚如何读取每个数字并将其相加。 假设该文件由 10 行这样的行组成。 那么输出文件必须有 10 行日期，后跟平均值。

#include<fstream>
#include<iomanip>
#include<string>
using namespace std;

int main()
{
    fstream objtread,objtwrite;
    objtread.open("temperature.txt",ios::in);
    objtwrite.open("average.txt",ios::out);
    char temp[10];
    float temperature;


    while(objtread)
    {   float sum=0.0;

        objtwrite<<getline(temp,10);
        while()
        sum = sum + get(temperature);
        objtwrite<<setprecision(3)<<sum;

    }


}

Answer 1

您的代码无法编译，例如 while() 无效且 get() 未定义

calculate the average of these numbers and put the output in another file

目前你只计算总和，你错过了除法以获得平均值

the output file must have 10 lines of dates followed by the average value.

你没写日期

---

一种简单的方法是逐行读取输入文件行，然后使用 stringstream 逐个提取日期和值

来自您的代码的建议:

#include <iostream>
#include<fstream>
#include<iomanip>
#include<string>
#include <sstream> 

using namespace std;

int main()
{
  fstream objtread,objtwrite;

  objtread.open("temperature.txt",ios::in);
  objtwrite.open("average.txt",ios::out);

  if (!objtread.is_open())
    cerr << "cannot open temperature.txt" << endl;
  else if (!objtwrite.is_open())
    cerr << "cannot open average.txt" << endl;
  else {
    string line;

    while (line.clear(), getline(objtread, line), line.length() > 10) {
      string date = line.substr(0, 10);
      stringstream ss(line.substr(10));
      double sum = 0, value;
      int n = 0;

      while (ss >> value) {
        sum += value;
        n += 1;
      }

      objtwrite << date << ' ' << setprecision(3) << ((n == 0) ? 0.0 : sum/n) << endl;
    }
  }
}

编译和执行:

/tmp % g++ -pedantic -Wextra -Wall a.cc
/tmp % cat temperature.txt 
2019/02/28 20.44 20.25 20.14 20.02 19.87 19.66 19.41 19.13 19.51 20.44 21.82 22.92 23.49 23.60 22.72 22.54 22.28 21.62 20.34 19.05 20.39 19.72 19.11 19.12
2019/03/01 19.12 19.12 19.12 19.00 19.04 19.14 19.55 19.77 20.06 20.16 20.77 21.03 21.09 21.53 21.44 21.15 20.79 20.26 19.76 19.43 20.55 20.39 20.26 20.21
2019/03/02 20.13 19.91 19.63 19.68 19.81 20.07 20.41 20.16 20.99 21.91 22.47 23.36 23.26 23.27 23.20 23.20 22.80 21.95 21.34 21.18 21.07 21.16 21.15 21.14
2019/03/03 21.07 20.97 20.92 20.95 20.97 20.98 20.98 21.15 21.45 22.00 22.37 23.28 23.31 23.05 23.18 23.06 22.94 22.66 21.64 20.67 21.00 20.47 20.07 19.75
2019/03/04 19.41 19.02 18.76 19.16 18.99 18.90 18.80 18.86 19.50 20.57 20.87 21.00 21.94 22.81 23.11 23.10 22.01 22.32 21.84 21.63 21.52 21.57 21.60 21.61
2019/03/05 21.57 21.49 21.44 21.37 21.34 21.44 21.53 21.85 22.54 23.08 23.49 23.54 23.67 24.30 24.39 25.75 24.11 23.82 23.27 22.83 21.78 21.79 21.79 21.76
2019/03/06 21.72 21.72 21.75 21.93 22.09 22.11 21.68 21.09 21.62 21.81 22.35 22.25 22.72 23.34 23.21 23.49 23.19 22.73 22.43 22.01 18.50 17.11 16.95 16.70
2019/03/07 16.62 16.48 16.44 16.43 16.53 16.87 17.23 17.71 18.15 18.47 19.02 19.39 19.62 19.86 19.96 19.66 19.25 18.82 18.36 17.97 16.84 17.00 17.17 17.22
2019/03/08 17.17 16.78 16.29 15.73 15.38 15.38 15.53 15.78 16.15 16.66 17.29 17.95 18.38 18.63 18.83 18.96 19.05 19.08 19.10 19.14 18.36 18.63 18.89 19.14
2019/03/09 19.34 19.56 19.76 19.91 20.04 20.19 20.30 20.42 20.69 21.06 21.72 22.17 22.61 23.37 23.66 23.44 23.31 23.07 22.76 22.41 16.59 16.72 16.81 16.87
2019/03/10 16.86 16.88 16.85 16.70 16.58 16.45 16.31 16.10 16.20 16.44 16.75 17.07 17.21 17.19 17.27 17.05 16.84 16.75 16.69 16.59 16.53 16.43 16.14 15.98
2019/03/11 16.07 16.17 16.31 16.42 16.46 16.48 16.47 16.54 17.30 18.37 19.64 20.53 21.11 21.42 21.50 21.35 20.68 20.37 19.87 19.50 17.63 17.39 17.25 17.79
2019/03/12 18.37 18.61 18.39 17.70 17.40 17.99 17.14 17.20 19.31 20.27 20.94 21.80 22.30 22.54 22.58 22.45 22.15 21.42 20.37 19.32 19.61 19.29 19.01 18.79
2019/03/13 18.56 18.36 18.23 18.14 18.11 18.04 17.89 18.32 20.07 22.39 23.13 23.71 23.98 23.96 23.89 23.67 23.38 22.94 22.08 21.66 21.53 21.46 21.37 21.24
/tmp % ./a.out
/tmp % cat average.txt
2019/02/28 20.7
2019/03/01 20.1
2019/03/02 21.4
2019/03/03 21.6
2019/03/04 20.8
2019/03/05 22.7
2019/03/06 21.4
2019/03/07 18
2019/03/08 17.6
2019/03/09 20.7
2019/03/10 16.7
2019/03/11 18.4
2019/03/12 19.8
2019/03/13 21.1