Increasing class pointer array - c++

Question

In my code, I want to add one student info into my class pointer array and the array size will increase each time a new student is added. Here is my code:
My header file:
class Student{
public:
int studentID;
char studentName[20];
int currentEnrollment;
Student();
void AddStudent(Student *tempStudent[], int countStudent, int sizeOfArray);}
My Class definition file:
void Student::AddStudent(Student *tempStudent[], int countStudent, int sizeOfArray)
{
for (int i = countStudent; i < sizeOfArray; i++)
{
cout << "Please enter student id (4 digits only): ";
cin >> tempStudent[i]->studentID;
cout << "Please enter student name: ";
cin >> tempStudent[i]->studentName;
}
}
My Main.cpp file
int *totalStudent = new int;
*totalStudent = 1;
int i, j, countStudent = 0;
int sizeOfArray = *totalStudent;
Student *newStudent[*totalStudent];
//Each time a new student is added, I will allocate a new memory for the array element, then add student Info using function.
for (i = countStudent; i < *totalStudent; i++)
{
newStudent[i] = new Student;
newStudent[i]->AddStudent(newStudent, countStudent, sizeOfArray);
countStudent++;
*totalStudent++;
}
When I run my code, I get an undefined reference error, so I do not know If I am able to increase my array or not. I intend to use C++ syntax so I use new and delete. Thank you for your help.
P.S: Here is my new code and it runs great, the only missing is the studentID for the first element in array.
In my main class:
int numStudent = 0;
int i, j, countStudent = 1;
Student *newStudent = new Student[countStudent];
AddStudent(newStudent, countStudent, numStudent);
My Student.h
class Student{
public:
int studentID;
char studentName[20];
int currentEnrollment;
};
Student AddStudent(Student *newStudent, int &countStudent, int &numStudent);
and My Student.cpp
Student AddStudent(Student *newStudent, int &countStudent, int &numStudent)
{
Student tempStudent;
cout << "Please enter student id (4 digits only): ";
cin >> tempStudent.studentID;
cout << "Please enter student name: ";
cin >> tempStudent.studentName;
newStudent[numStudent] = tempStudent;
numStudent++;
if (numStudent == countStudent)
{
Student *newStudentSize = new Student[countStudent + 1];
for (int i = 0; i < numStudent; i++)
{
newStudentSize[i] = newStudent[i];
}
delete []newStudent;
return *newStudentSize;
countStudent += 1;
}
}
Running this code will give me the following result:
StudentID: 11
StudentName: Dat
StudentID: 23
StudentName: Michael
Printing:
StudentID: 0
StudentName: Dat
StudentID: 23
StudentName: Michael

Answer 1

While it doesn't make sense to increase the array for each new student (it's inefficient) here's one way you can do it (I didn't even try to make your code compile because it has a number of issues and is unnecessarily complicated). Note that tempStudent (in the code snippet below) doesn't even have to be created using new. This solution stores Student objects in the students array (although it's easy to modify it to store Student object pointers instead). That said, usually, you'll just want to create an array of large enough size to accomodate all students (just set studentCount to some appropriate number and not 1 like in the example below).
class Student{
public:
int studentID;
char studentName[20];
int currentEnrollment;
Student(){};
};
int main(){
int studentCount=1;
Student * students = new Student[studentCount];
int numStudents=0;
bool done=false;
char finished='N';
while (!done){
//Student *tempStudent = new Student();
//create a Student on the stack
Student tempStudent;
cout << "Please enter student id (4 digits only): ";
//No input checking is done here
cin >> tempStudent.studentID;
No input checking is done here
cout << "Please enter student name: ";
cin >> tempStudent.studentName;
students[numStudents] = tempStudent;
numStudents++;
cout << "Stop entering students: Y or N";
cin >> finished;
done = finished=='Y' or finished=='y' ? true : false;
if(numStudents==studentCount){
students = ReallocateStudents(students, studentCount, studentCount*2);
studentCount *= 2;
}
}//end while
//print the students info
for(int i=0;i<numStudents;i++){
Student st = students[i];
cout << st.studentID << " " << st.studentName << std::endl;
}
//deallocate the students array or if you place this in the main like you did, the program will exit immediately so there's no need to deallocate
return 0;
}
Student * ReallocateStudents(Student* st, int oldSize, int newSize){
Student * newStudents = new Student[newSize];
//copy the existing values from the old array to the new one
for(int i=0;i<oldSize;i++){
newStudents[i] = st[i];
}
delete [] st; //delete the old array
return newStudents;
}
UPDATE:
Since you don't want to do everthing in the main(), just create a free AddStudents function and do everything there. Alternatively, you can create a
static function inside the Student class. It makes no sense to create AddStudent as a member of Student because that would require you to use an instance of Student to add a new instance, which makes for poor design (not to mention technical issues etc).
int main(){
// some code here
Students * allStudents = AddStudents();
//print students
}//end main
Students * AddStudents(){
int studentCount=1;
Student * students = new Student[studentCount];
int numStudents=0;
bool done=false;
char finished='N';
while (!done){
//create a Student on the stack
Student tempStudent;
cout << "Please enter student id (4 digits only): ";
//No input checking is done here
cin >> tempStudent.studentID;
No input checking is done here
cout << "Please enter student name: ";
cin >> tempStudent.studentName;
students[numStudents] = tempStudent;
numStudents++;
cout << "Stop entering students: Y or N";
cin >> finished;
done = finished=='Y' or finished=='y' ? true : false;
if(numStudents==studentCount){
students = ReallocateStudents(students, studentCount,
studentCount*2);
studentCount *= 2;
}
}//end while
return students;
}
This is simple and easy to both understand and maintain so I suggest using this approach.

Answer 2

addStudent does not do anything with the Student object it belongs to. So there is no need to put it in the 'Student' class. (Or you could rather rewrite it so it does something with the student object it belongs to). And it currently does not "add" anything, so the name is confusing.
What is technically wrong with it depends on what you want it to do. Currently it initializes student objects expected to already exist and pointed to by an array, from a specific array index, to the end of the array. That could well be a useful function, if that is what you want it to do. But you then must call it correctly with an array of pointers that point to valid Student objects, which you currently do not.
Currently in main you have a loop that initializes pointers in an array. And each time you initialize a pointer, you call AddStudent(..). The problem is that 'AddStudent()' tries to initialize ALL the student pointed to by your array.
This has two major problems (In addition to all the other problems with your loop).
Each time you create a new student, all your existing students will be
initialized again with new input from std::cin. (So for n students, you will
try to do n*n initializations)
While the loop in main is running, not all pointers in your array points
to existing Student objects. This may result in important data being
overwritten, a program crash or something totally different and unexpected.
You should sit back and reevaluate how you want to do things. Trying to fix single bugs in your existing code, one after another, will just create more bugs.
Just a hint to get you started:
class Student
{
public:
int studentID;
char studentName[20];
int currentEnrollment;
Student();
void init_from_cin();
};
And in your class definition file:
void Student::init_from_cin()
{
cout << "Please enter student id (4 digits only): ";
cin >> studentID;
cout << "Please enter student name: ";
cin >> studentName;
}
If you create a new Student like this:
Student *new_student = new Student;
new_student->init_from_cin();
Then after calling init_from_cin(), the Student object pointed to by new_student should be initialized.
How to create and initialize multiple Student objects in a loop, is left as exercise for the reader. But when you do it, you should understand what your lower and upper bounds of your loop are supposed to be. And you should also understand why moving the upper bound further away while your loop is running is a bad idea.
And never forget that sane programming languages start array indexing with 0.