从同一类中的另一个方法调用方法时出错

我似乎无法弄清楚我正在尝试构建的这个 GPA 计算器程序是怎么回事。问题出在Student类的printTranscript方法上。该方法调用同一类getGPA方法，该方法返回双精度值，输出是某个大数字而不是标准 GPA。

主.cpp：

#include <iostream>
#include "Student.cpp"
using namespace std;
int main(){
Student stud("Lebron", 23232);
stud.addCourse("Passing", 3, 'A');
stud.addCourse("Finals Record", 4, 'D');
stud.printTranscript();
return 0;
}

学生.h：

#include "Course.h"
#include <vector>
class Student{
private:
string name;
int studentID;
vector<Course> courses;
public:
Student(){
name = "No Name";
studentID = 0;
}
Student(string n, int ID){
name = n;
studentID = ID;
}
string getName(){
return name;
}
void setName(string n){
name = n;
}
int getID(){
return studentID;
}
void setID(int ID){
studentID = ID;
}
void addCourse(string, int, char);
void addCourse(Course);
double getGPA();
void printTranscript();
};

学生.cpp：

#include "Student.h"
void Student::addCourse(string name, int credits, char grade){
courses.push_back(Course(name,credits,grade));
}
void Student::addCourse(Course c){
courses.push_back(c);
}
double Student::getGPA(){
double gradePoints, totalCredits;
for(int i = 0; i < courses.size(); i++){
if(courses[i].getGrade() == 'A'){
gradePoints += (4.0 * courses[i].getCredits());
totalCredits += courses[i].getCredits();
}
else if(courses[i].getGrade() == 'B'){
gradePoints += (3.0 * courses[i].getCredits());
totalCredits += courses[i].getCredits();
}
else if(courses[i].getGrade() == 'C'){
gradePoints += (2.0 * courses[i].getCredits());
totalCredits += courses[i].getCredits();
}
else if (courses[i].getGrade() == 'D'){
gradePoints += (1.0 * courses[i].getCredits());
totalCredits += courses[i].getCredits();
}
}
return (gradePoints / totalCredits);
}
void Student::printTranscript(){
cout << "Transcript for: " << name << endl;
cout << "============================" << endl;
for(int i = 0; i < courses.size(); i++){
cout << "Course: " << courses[i].getID() << endl;
cout << "Grade: " << courses[i].getGrade() << endl;
cout << endl;
}
cout << "Overall GPA: " << getGPA() << endl;
}

课程：

#include <iostream>
#include <string>
using namespace std;
class Course{
private:
string courseID; // Course name
int numCredits; // Instructor
char letterGrade; // Textbook
public:
Course(string ID, int credits, char grade){ // Assign the course name.
courseID = ID;
numCredits = credits;
letterGrade = grade;
}
string getID(){
return courseID;
}
void setID(string ID){
courseID = ID;
}
int getCredits(){
return numCredits;
}
void setCredits(int credits){
numCredits = credits;
}
char getGrade(){
return letterGrade;
}
void setGrade(char grade){
letterGrade =  grade;
}
};

很抱歉代码很长，但我似乎无法弄清楚为什么 main 中的输出会给我：

Transcript for: Lebron James
============================
Course: Passing
Grade: A
Course: Finals Record
Grade: D
Overall GPA: 2.2321e+230

如果我在主类中将stud.getGPA作为一个单独的语句，它工作正常。当从同一类的另一个方法调用时，这里输出的大量数字是什么，以及如何解决这个问题？

很抱歉代码很长，但我不想错过任何东西，因为我仍处于C++的开始阶段。