Using dynamic allocation to create an array and insert elements into it

Question

I want to make a library management system as an assignment

class student
{
          char ID_number[30];
          char Student_name[30];
      public:
          void create_student()
          {
                    cout<<"\nEnter The ID Number ";
                    cin>>ID_number;
                    cout<<"\n\nEnter The Name of The Student: ";
                    cin>>Student_name;

                    cout<<"\n\nStudent Created Successfully"<<endl;
          }
          void show_student()
          {
                    cout<<"\nID Number: "<<ID_number;
                    cout<<"\nStudent Name: ";
                    cin.getline(Student_name,30);


          }

How would i go about using dynamic allocation to make every new entry go into an array and use pointers to show a certain student?

I am really bad at this particular part, thanks in advance!

Well sadly for this assignment i can only use dynamic allocation I think i can do it but not without listing a specific array size — 00qq00
– 00qq00, Commented May 12, 2017 at 8:49
vector provide dynamic allocation. You don't need to specify the size of it — Ed Nio
– Ed Nio, Commented May 12, 2017 at 8:50
Sorry i haven't been very clear, i need to use new[] and delete[] not vectors — 00qq00
– 00qq00, Commented May 12, 2017 at 8:54

Mr.C64 · Accepted Answer · 2017-05-12 09:04:33Z

1

Since you are coding in C++, and not in C, I would suggest using a string class, like std::string, instead of raw char arrays, e.g.:

std::string student_name;

If you want to store a list of students in memory, I'd suggest using the std::vector container, e.g.:

std::vector<Student> students;

You can use the vector::push_back method to add new students to the container.

EDIT If you want to use new[] and delete[] as a learning exercise, you may allocate a big array of students, and then use an integer index inside the array, pointing to the first free slot for inserting new students. E.g.:

int capacity = 100;
Student* students = new Student[capacity];

int student_count = 0;

To add a new student, you copy it in the array index given by student_count, and then you increment this variable by one, pointing to the next free slot in the pre-allocated array.

You must pay attention to not overflow the capacity of the array. Once the pre-allocated array is full, if you want to add a new student you need to allocate a new array with bigger capacity. You can use a 2x or 1.5x scale factor to calculate the new capacity.

Dont't forget to release the memory previously allocated with delete[].

If you do that, you are basically kind of implementing std::vector from scratch (even if this standard container has additional advanced features, like move semantics, etc.).

edited May 12, 2017 at 9:04

answered May 12, 2017 at 8:49

Mr.C64

43.3k15 gold badges97 silver badges170 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

00qq00 Over a year ago

But string doesn't let it read more than on word after the space no? I want it to read a full line of characters i.e "john doe", if i use string and cin>>student_name; it'll just get "john" and ignore the rest And i can only use dynamic allocation in this assignment, actually this is the whole point of the assignment sadly. Edit: to be clear i need to use new[] and delete[] no vectors

Mr.C64 Over a year ago

@00qq00: Good question. Use getline(cin, student_name);.

00qq00 Over a year ago

Thanks this works!, any idea how to use new[] delete[] instead of vectors?

00qq00 Over a year ago

Thanks, i'll go ahead and try

doctorlove · Accepted Answer · 2017-05-12 09:27:54Z

0

If by dynamic you mean you want to create "a few" stduent records, but aren't sure how many you don't need to worry about new or delete. Just use a vector.

std::vector<student> students;
while(!bored)
{
    student latest;
    latest.creat_student(); // this looks a bit odd, perhaps it should be a free function...
    vector.push_back(latest);
    //TODO - see if I am bored yet
}

Now we have students. You said you wanted to use pointers to display them. Or rather a certain student. You haven't described how you will choose a "certain student", so let's show them all first

for (auto item : students)
{
    item.show_student();
}

Disappointingly, your show_student method isn't declared const, which you should consider.

To index to a particular student, you could use students[17]. Or which ever index you want. You might be able to use std algorithms to find students with particular properies, if you provide a way to get the properties (and of course consider using std::string instead of char *)

If you really want to use a raw array of student, then something like this works, but you will get into reallocs etc if 5 or whatever you choose isn't enough. Which is a compelling reason for choosing a std::vector.

student * them = new student[5];
for (size_t i = 0; i < 5; ++i)
{
    student latest;
    latest.create_student();
    them[i] = latest;
}

for (size_t i = 0; i < 5; ++i)
{
    them[i].show_student();
}
delete[] them;

edited May 12, 2017 at 9:27

answered May 12, 2017 at 9:18

doctorlove

19.4k3 gold badges49 silver badges65 bronze badges

2 Comments

00qq00 Over a year ago

Okay so with new and delete i have to declare the size of my array every time?

doctorlove Over a year ago

Exactly - just added an edit. Vector deals with this for you, and reallocates when it needs to. If you use a raw array you need to do this yourself.

Ed Nio · Accepted Answer · 2017-05-12 09:04:10Z

0

std::vector<student> or std::vector<student*> if you want a pointer.

Adding a new student :

Student* newStudent = new Student(name);

vector.append(newStudent);

Reading a student called "Victor" and then deleting it:

Student* victor;
for(int i = 0; i<vector.size();i++) {
    if(vector.at(i)->getName() == "Victor") {
        victor = vector.at(i);
        break;
    }
 }
//Then you can delete it
delete(victor);

std::vector

edited May 12, 2017 at 9:04

answered May 12, 2017 at 8:53

Ed Nio

5933 gold badges11 silver badges28 bronze badges

5 Comments

00qq00 Over a year ago

I can only use new[] and delete[] so any ideas how and where to insert them?

Ed Nio Over a year ago

I am not sure of what do you mean with new and delete.

00qq00 Over a year ago

public: void * operator new[] (size_t) { return 0; } void operator delete[] (void*) { } }; int main() { MyClass *pMyClass = new MyClass[5]; delete [] pMyClass; } I need to use something like this

doctorlove Over a year ago

That would work if you insist on using a c-style array, sorta by why define your own?

Ed Nio Over a year ago

Is it a student exercice ? If it is @Mr.C64 is better than mine.

Collectives™ on Stack Overflow

Using dynamic allocation to create an array and insert elements into it

3 Answers 3

4 Comments

2 Comments

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

2 Comments

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related