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Accelerated C++ 的练习 4-6 要求我编写一个程序来读取并计算学生的总成绩。我有以下代码：

// source file for Student_info-related functions
#include "Student_info.h"
#include "grade.h"

using std::istream;  using std::vector;
using namespace std;

bool compare(const Student_info& x, const Student_info& y)
{
    return x.name < y.name;
}

istream& read(istream& is, Student_info& s)
//void read(istream& is, Student_info& s)
{

    double midterm, final;

    // read and store the student's name and midterm and final exam grades
    is >> s.name >> midterm >> final;

    std::vector<double> homework;
    read_hw(is, homework);  // read and store all the student's homework grades

    s.grade = grade(midterm, final, homework);

    cout << "name is: " << s.name << endl;
///    cout << "midterm is: " << midterm << endl;
///    cout << "final is: " << final << endl;
    cout << "grade is: " << s.grade << endl;
    cout << "" << endl;

    return is;
}

// read homework grades from an input stream into a 'vector<double>'
istream& read_hw(istream& in, vector<double>& hw)
{
    cout << "I get into read_hw!" << endl;

    if (in) {
        // get rid of previous contents
        hw.clear();

        // read homework grades
        double x;
        while (in >> x)
            hw.push_back(x);

        // clear the stream so that input will work for the next student
        in.clear();
    }
    cout << "I get (just before) out of read_hw!" << endl;
    return in;
}

从 main() 调用上述内容：

int main()
{
    vector<Student_info> students;
    Student_info record;
    string::size_type maxlen = 0;       // the length of the longest name

    // read and store all the students data.
    // Invariant: students contains all the student records read so far
    //          maxlen contains the length of the longest name in students
    while (read(cin, record)) {
        cout << "Get into read() while loop" << endl;
        // find length of longest name
        maxlen = max(maxlen, record.name.size());
        students.push_back(record);
        cout << "Get to end of read() while loop" << endl;
    }
    cout << "I get out of read ok!!" << endl;  // WE NEVER GET THIS FAR

这是输出：

(Canopy 32bit) joes-imac:4-6 david$ make test 
./main <data/gradesTest
I get into read_hw!
I get (just before) out of read_hw!
name is: Moo
grade is: 100

Get into read() while loop
Get to end of read() while loop
I get into read_hw!
I get (just before) out of read_hw!
name is: Moore
grade is: 79.4

Get into read() while loop
Get to end of read() while loop
I get into read_hw!
I get (just before) out of read_hw!
name is: Norman
grade is: 72.8

Get into read() while loop
Get to end of read() while loop
I get into read_hw!
I get (just before) out of read_hw!
libc++abi.dylib: terminate called throwing an exception
/bin/sh: line 1: 20610 Abort trap: 6           ./main < data/gradesTest
make: *** [test] Error 134
(Canopy 32bit) joes-imac:4-6 david$

为什么会失败？为什么当程序失败（数据输入末尾没有空行或类似的东西）时，我们似乎进行了一次空白运行（即没有读入任何数据）？

4

1 回答 1

2

您对最后一个循环的输入到底是什么？midterm您是否试图通过发送和值的非数字值来结束循环final？当您使用这些数字计算成绩时，您的错误似乎正在发生。我的猜测是尝试这样做：

if (is)
{
    s.grade = grade(midterm, final, homework);
}
于 2013-07-31T17:13:38.397 回答