如何从文本文件输出数据?(学生成绩单计划）

有许多领域使事情变得更加困难，但最大的问题涉及您的输入尝试。当您使用以下方法进行输入时，您的输入肯定会失败：

while(!inputFile.eof())
{
Student st;
inputFile >> st.ID;
inputFile >> st.Name;
inputFile >> st.Course;
inputFile >> st.Credit;
inputFile >> st.Score;
studArr[n] = st;
n++;
}

请参阅：为什么！循环条件中的 eof(( 总是错误的。本质上，在读取最后一个 VALIDinputFile >> st.Score;后，eofbit没有设置，因此您在inputFile >> st.ID;失败的地方再次循环，使st中的所有值都不确定，您不检查每个输入的结果，因此您分配studArr[n] = st;损坏您的studArr。

确保您不会落入该陷阱的一种方法是继续写入>>重载来处理结构的输入。在这种情况下，很容易做到：

struct Student {
int ID = -1;
std::string Name = "";
std::string Course = "";
int Credit = -1;
int Score = -1;
/* overloading the >> and << to read and write your struct helps */
friend std::istream& operator >> (std::istream& is, Student& s) {
is >> s.ID >> s.Name >> s.Course >> s.Credit >> s.Score;
return is;
}
...
};

现在，读取和验证所有输入变得非常简单：

#define MAXS 99     /* if you need a constant, #define one (or more) */
...
int main (int argc, char **argv) {
Student studArr[MAXS], tmp;
size_t n = 0;
std::ifstream f(argc > 1 ? argv[1] : "dat/StudentRecords.txt");
while (n < MAXS && f >> tmp)
studArr[n++] = tmp;
...

如果按照建议，您将 STL 容器(如std::vector(用于学生存储，而不是基本数组类型，则验证将进一步简化为while (f >> tmp)(您还将使用不同的方法(如.push_back(tmp)而不是直接分配(

整理输入后，虽然您可以对数组进行排序，但实际上没有必要。一个简单的方法是遍历您的学生数据并在另一个数组(或向量等(中收集唯一的ID值。然后，您只需将唯一 ID 作为外循环进行循环，并将学生数组的元素作为内循环进行循环，并处理 ID 与当前外循环值匹配的每个学生。重写您的 GPA 映射逻辑和一个函数来循环您的学生数组，执行所描述的操作可以按如下方式完成：

/* simple map GPA function that returns grade points given score */
int mapgpa (int score)
{
if (score < 60)
return 0;
else if (score < 70)
return 1;
else if (score < 80)
return 2;
else if (score < 90)
return 3;
else if (score >= 90)
return 4;
return 0;
}

以及为每个学生找到 GPA 的逻辑：

/* a function to handle outputting the student grades
* in your desired format.
*/
void dogrades (Student *s, size_t n)
{
int uniqueID[MAXS] = {0};               /* array to hold unique IDs */
size_t seen = 0;                        /* number of unique IDs seen */
for (size_t i = 0; i < n; i++) {        /* loop collecting unique IDs */
for (size_t j = 0; j < seen; j++)
if (uniqueID[j] == s[i].ID)
goto next;
uniqueID[seen++] = s[i].ID;
next:;
}
for (size_t j = 0; j < seen; j++) {     /* loop ever unique IDs */
int heading = 0,        /* simple flag indicating name printed */
sumcredits = 0,     /* variable to hold sum of credits */
sumpoints = 0;      /* variable to hold sum of gradepoints */
for (size_t i = 0; i < n; i++) {    /* loop over each stuct element */
int gradepts = 0;               /* var to map score->gradepoits */
if (s[i].ID == uniqueID[j]) {   /* if struct element == ID */
if (!heading) {             /* if no heading output, do it */
std::cout << s[i].ID << " " << s[i].Name << "nn";
heading = 1;            /* set flag indicating done */
}
gradepts = mapgpa(s[i].Score);  /* get gradepts from score */
/* output current course and gradepoints */
std::cout << s[i].Course << " "
<< s[i].Credit << " "
<< s[i].Score << " "
<< gradepts << ".0nn";
sumcredits += s[i].Credit;              /* sum credits */
sumpoints += s[i].Credit * gradepts;    /* sum gradepoints */
}
}
std::cout.precision(3);     /* set precsion and output GPA */
std::cout << "======================nn"
<< "GPA " << (double)sumpoints/sumcredits << "nn"
<< "======================nn";
}
}

在dogrades函数中处理逻辑后，main()减少到：

int main (int argc, char **argv) {
Student studArr[MAXS], tmp;
size_t n = 0;
std::ifstream f(argc > 1 ? argv[1] : "dat/StudentRecords.txt");
while (n < MAXS && f >> tmp)
studArr[n++] = tmp;
dogrades (studArr, n);
}

把它放在一个可以编译的示例中，该示例将要读取的数据文件的名称作为第一个参数(或默认情况下从"dat/StudentRecords.txt"读取(，您可以执行以下操作：

#include <iostream>
#include <iomanip>
#include <fstream>
#define MAXS 99     /* if you need a constant, #define one (or more) */
struct Student {
int ID = -1;
std::string Name = "";
std::string Course = "";
int Credit = -1;
int Score = -1;
/* overloading the >> and << to read and write your struct helps */
friend std::istream& operator >> (std::istream& is, Student& s) {
is >> s.ID >> s.Name >> s.Course >> s.Credit >> s.Score;
return is;
}
friend std::ostream& operator << (std::ostream& os, const Student& s) {
os << std::setw(5) << s.ID << "  "
<< std::setw(8) << std::left << s.Name << "  "
<< std::setw(5) << s.Course << "  "
<< s.Credit << "  "
<< std::setw(3) << s.Score << 'n';
return os;
}
};
/* simple map GPA function that returns grade points given score */
int mapgpa (int score)
{
if (score < 60)
return 0;
else if (score < 70)
return 1;
else if (score < 80)
return 2;
else if (score < 90)
return 3;
else if (score >= 90)
return 4;
return 0;
}
/* a function to handle outputting the student grades
* in your desired format.
*/
void dogrades (Student *s, size_t n)
{
int uniqueID[MAXS] = {0};               /* array to hold unique IDs */
size_t seen = 0;                        /* number of unique IDs seen */
for (size_t i = 0; i < n; i++) {        /* loop collecting unique IDs */
for (size_t j = 0; j < seen; j++)
if (uniqueID[j] == s[i].ID)
goto next;
uniqueID[seen++] = s[i].ID;
next:;
}
for (size_t j = 0; j < seen; j++) {     /* loop ever unique IDs */
int heading = 0,        /* simple flag indicating name printed */
sumcredits = 0,     /* variable to hold sum of credits */
sumpoints = 0;      /* variable to hold sum of gradepoints */
for (size_t i = 0; i < n; i++) {    /* loop over each stuct element */
int gradepts = 0;               /* var to map score->gradepoits */
if (s[i].ID == uniqueID[j]) {   /* if struct element == ID */
if (!heading) {             /* if no heading output, do it */
std::cout << s[i].ID << " " << s[i].Name << "nn";
heading = 1;            /* set flag indicating done */
}
gradepts = mapgpa(s[i].Score);  /* get gradepts from score */
/* output current course and gradepoints */
std::cout << s[i].Course << " "
<< s[i].Credit << " "
<< s[i].Score << " "
<< gradepts << ".0nn";
sumcredits += s[i].Credit;              /* sum credits */
sumpoints += s[i].Credit * gradepts;    /* sum gradepoints */
}
}
std::cout.precision(3);     /* set precsion and output GPA */
std::cout << "======================nn"
<< "GPA " << (double)sumpoints/sumcredits << "nn"
<< "======================nn";
}
}
int main (int argc, char **argv) {
Student studArr[MAXS], tmp;
size_t n = 0;
std::ifstream f(argc > 1 ? argv[1] : "dat/StudentRecords.txt");
while (n < MAXS && f >> tmp)
studArr[n++] = tmp;
dogrades (studArr, n);
}

(注意：还添加了<<运算符的重定向，以简化原始结构信息的输出，如果您发现需要(

示例使用/输出

$ ./bin/studentrecords
12546 Amy
CS1 4 81 3.0
CS2 4 90 4.0
CS3 3 90 4.0
======================
GPA 3.64
======================
13455 Bill
CS1 4 76 2.0
CS2 4 85 3.0
CS3 3 75 2.0
======================
GPA 2.36
======================
14328 Jim
CS1 4 64 1.0
CS2 4 71 2.0
CS3 3 69 1.0
======================
GPA 1.36
======================
14388 Henry
CS3 3 80 3.0
======================
GPA 3
======================
15667 Peter
CS3 3 45 0.0
======================
GPA 0
======================

仔细查看，如果您有疑问，请告诉我(并认真考虑使用可用的 STL 容器，如std::vector和std::map(