有人可以帮我完成这个计划任务吗？ (C++)

Question

So, the purpose of this program is to input data from a.txt file that has a long list of student IDs and three grades, and output this data into another.txt file, but with the IDs and the averaged grades. I am a bit stumped about how to make my program read the next line after it reads one line. Right now how the code is it just repeatedly prints the first line.

int main()

{

int id;
int score1;
int score2;
int score3;
ofstream output;
ifstream input;

int grade = 0.3 * score1 + 0.3 * score2 + 0.4 * score3;

  char letgrade;

if (grade >= 90.0)
char letgrade = 65;
else if (grade >= 80.0)
char letgrade = 66;
else if (grade >= 70.0)
char letgrade = 67;
else if (grade >= 60.0)
char letgrade = 68;
else char letgrade = 69;

output.open("studentGrade.txt");

do 
{

input.open("StudentInfo.txt");

input >> id >> score1 >> score2 >> score3;

output << id << " " << letgrade << endl;

input.close();

}

while (!input.eof());

output.close();

return 0;

 }

From what I understand, the getline function comes in handy here. It's just I need help regarding how to use it.

Answer 1

You are repeatedly reopening the file anew in input.open("StudentInfo.txt");

Your do {} while loop should be something more like the following ( although you need to be careful with while (.stream.eof) {} ):

input.open("StudentInfo.txt");
while (!input.eof()) {
  input >> ...;
  output << ... << endl;
}

---

The code posted has other issues a teacher would likely characterize as deficiencies: unorganized procedures, bloated predicate ( if expression), failure to adhere to a recognizable variable and coding style - but nothing that a quick review of material presented in lecture or section notes on files, loops and a quick read of a well-regarded C++ style guide won't fix.

C++ is a tough language and countless veterans have "blown their feet off" in spite of an excellent command of the language. As a novice, you should expect that answers to questions (such as this one) to require a 30-45 minutes of additional research to define terms, place it in a wider context or develop intuition for the machinery.

Answer 2

minimal working version, lots of comments.

#include <fstream>
#include <iostream>

// this here's a function. it's a bit of code that accepts 3 parameters,
// does stuff with them, prints a message, then returns a value.
int computeGradeAverage(int a, int b, int c)
    {
    int avg =  ( a + b + c ) / 3;
    std::cout << "computed average of " << a << ", " << b << ", " << c << ", is " << avg << std::endl;
    return avg;
    }

// the main function. typically programs have many functions, and the C/C++
// languages (and java etc etc) require a single 'main' function to start the program at
// (it can be up top, or at bottom of file, or in the middle; doesn't matter).
int main()
    {

    // a) allocate named variable of given types to hold values.
    int id;
    int score1;
    int score2;
    int score3;
    int gradeaverage;
    std::ofstream output;
    std::ifstream input;

    // b) open input and output files
    input.open("StudentInfo.txt");
    output.open("studentGrade.txt");

    // c) loop over each line of input file
    do
        {
        // c.i) read data (one line) in
        input >> id >> score1 >> score2 >> score3;

        // c.ii) do calculation on input data
        gradeaverage = computeGradeAverage(score1, score2, score3);

        // c.iii) write data out
        output << id << " " << gradeaverage << std::endl;
        }

    // d) stop looping if we have reached the end of the input file (eof = end of file)
    // ... 'input' is the file object; input.eof() is call of one of its member function s
    // ... the 'eof()' function will return true (if at end) or false (if there's more data).
    // ... "while(!thing)" means "while NOT thing" .. so the 'while ( ! input.eof()  )' means:
    //  ..  do { "the things" } while " input file not finished "
    while (!input.eof());
    // There are several ways of doing this, like while(!eof) { .. } and others.

    // e) close the files. while it'll work without doing this, it's a very important
    // habit to put away things after use; deallocate memory, close files, commit transactions,
    // free (as in liberate, not beer) any acquired resources, etc etc.
    input.close();
    output.close();

    // f) return a value to the caller. In this case, the caller is the operating system,
    // and at least with unixy programs, '0' means 'okay', and other numbers would mean
    // some error occurred. For example, the 'g++' compiler returns 1, on any error.
    return 0;
    }