I'm working on a project where I need to declare a std::array as a private variable in a class, but the size of this array should be returned from a memberfunction called getNumberOfStudents().
Do you have any idea how could I solve this problem?
I will appreciate your help.
class Student{
private:
std::array<int,getNumberOfStudents()> arr;
public:
int getNumberOfStudents(){
std::cout<<"How many students do you wanna add? ";
std::cin>>numberOfStudent;
return numberOfStudent;
}
};
As an example, say I have a Book class and a BookInventory class. The book class has member variables "IDstring" for example:
class Book {
private:
string IDstring; // this is made up. A string of letters and nums
int numPages;
public:
Book(string IDstring, int numPages);
void setIDstring(string IDstring);
string getIDstring();
void setNumPages(int numPages);
int getNumPages();
};
etc... and a the BookInventory class which stores the books:
class BookInventory {
private:
int numBooks;
BookArray **book;
public:
BookInventory(int stock);
void sortBooks();
void insertBook(Book *book);
};
Something like that.
How Would I sort the books by their string attribute IDstring (which contains both letters and numbers i.e. 15C6HQP9), in alphabetical (or numerical when appropriate) order).
In, the example above, I create a array of pointers to pointers with BookArray **book;, because a larger project than the one above requires this type of array.
This if for my homework.
I have a class called Student that takes 3 parameters (id, name, class) and I want to store each student in an array called Roster (which can only have 7 students).
The user will provides input to add or remove students. Thus, I have to manage the array by creating or deleting students. So if the user specify the student ID, I have to remove him for the array.
I tried to use a fixed array, but I'm struggling to make it works. Is there a better way to implement this?
I must not use a vector or any STL container.
student.h
#ifndef STUDENT_H
#define STUDENT_H
#include <iostream>
#include <string>
static const int SIZE = 7;
class Student {
private:
int student_id;
std::string name;
std::string classification;
public:
Student(int, std::string, std::string); // constructor; initialize the list to be empty
~Student();
void print();
};
#endif
student.cpp
#include <iostream>
#include <string>
#include "student.h"
#define PROMPT "class> "
using namespace std;
Student::Student(int a, string b, string c){
student_id = a;
name = b;
classification = c;
}
Student::~Student(){
//delete Student
}
void Student::print(){
cout<<"Enrolled:"<<endl;
cout<<student_id<<"-"<<name<<"-"<<classification<<endl;
}
main.cpp
#include <iostream>
#include <string>
//#include <sstream>
#include "student.h"
#define PROMPT "class> "
using namespace std;
//**** Implement Error Handling ****\\
enum errorType {
UNKNOWN_ERROR,
INPUT_ERROR,
HANDLER,
NUM_ERRORS
};
// error messages
string errorMessage[NUM_ERRORS] = {
"Unknown Error\n",
"Input Error\n",
};
// error handler
void handleError(errorType err) {
if(err > 0 && err < NUM_ERRORS)
cout<< "Error: "<< errorMessage[err];
else cout<< "Error: "<< errorMessage[UNKNOWN_ERROR];
}
//**** END Error Handling ****\\
void enroll(Student newStudent){
cout<<"test";
Student roster[SIZE];
for(int i=0;i<SIZE;i++){
newStudent->roster[i];
}
}
void handleInput() {
int id; string n, c;
cin>>id>>n>>c;
Student newStudent(id,n,c);
newStudent.print();
enroll(newStudent);
//cout<<"hello3"<<endl;
return;
}
int main() {
//Student newStudent; /* <-- why doesn't this work?!*/
string input = "";
bool finished = false;
cout<<PROMPT; // prompt the user
while(!finished) {
if(input!="") cout<<PROMPT;
cin>>input;
if(input=="enroll") {
cout<<PROMPT<<"Enroll student:"<<endl;
handleInput();
}
else if(input=="drop") {
cout<<PROMPT<<"Enter ID:"<<endl;
}
else if(input=="roster") {
cout<<"This will print formatted list of students"<<endl;
}
else if(input=="quit") {
finished=true;
}
else handleError(errorType(1));
}
}
Since it is a homework, I'd like to point out some mistakes you did because it is important to understand what you are doing in the first place.
You must not program by coincidence, but by trying to understand exactly what's going on. By doing that you will become better and better and the answers should fall in place.
What you've done
So, from what you are describing, the array is fixed. Thus it is a good idea to use a constant as you did (SIZE).
However, as we can see below you a declaring an array of size SIZE in the function. By doing that, your array is like a temporary variable, because its scope is inside the function. Each time you call this function, the array will be declared again and then deleted at the exit. So it should be declared outside.
void enroll(Student newStudent)
{
cout<<"test";
Student roster[SIZE]; // Here 'roster' will be available only inside the function.
for(int i=0;i<SIZE;i++)
{
newStudent->roster[i]; // Here there is few mistakes see my explanation below*
}
}
If we look at this part:
newStudent->roster[i];
First of all, the arrow '->' is used with pointers. The dot '.' is used with objects. In both case, it does the same thing, access to public members of Student.
Since you passed
void enroll(Student newStudent)
you should use '.' instead.
newStudent.SomeOfYourMembers;
If the parameter was a pointer to a Student
void enroll(Student *newStudent)
Then, you'd have to use the arrow '->' like you did.
Back to the original statement:
newStudent->roster[i];
This means, you want to access to 'roster' array at position 'i' inside your Student object (newStudent). As you can see in your code, roster is not declared inside Student (and should not be since you want an array of Students), so that won't work.
Guidelines
As I mentionned, your array should be outside the function, so at a higher scope.
Then, if you need an array of student, basically, 'roster[i]' will give you access to the student 'i'. Thus, if you want to print the student, you would do something like that:
roster[i].print();
This would be valid because 'print()' is defined as public.
In order to store a student inside the array, you can do something like:
roster[i] = new Student(0 /* id*/, "name", "classification");
But don't forget, each time you use new, you have to balance it with a delete. And if you are creating the student like this in a loop, you will have to clean them the same way:
for(int i = 0; i < SIZE; ++i)
{
delete roster[i];
}
Good luck!
Don't hesitate if there is there anything that I could clarify. I hope this helps!
Edit: In reply to your first comment.
Concerning the roster array
No, it is not mandatory to create a class roster you could declare roster in the main.cpp.
The key concept is that by defining
Student roster[SIZE];
the array will contains objects of type Student.
What roster[i].print() means is that you are printing one of the Student of that array, in fact the one at position 'i'.
Concerning the print() function
What is powerfull with Object Oriented language, each object will have the same print() function. So, you do not need to convert the array to string.
However, if you want a string to be printed out (or returned) you can write the code inside the print() function that will do this job.
The advantage of this, is that if further on you need to change your array in some ways, your print() function will always work.
Concerning the Delete
When you are doing something like this on an array that contains objects:
delete roster[i];
It will delete the object at the position 'i'. Thus, the destructor of that Student 'i' will be called. If your object Student would contains other object, you would have to delete them in the destructor.
Further notices
Since ID is an input that you are storing into a string, you will have to convert the ID to the same type of the student_id, which is a int. Then you can always write a loop for each student and check their ID to delete the proper one.
Concerning the container, a fixed array might not be the best to achieve this job. You might want to look the LinkedList concept.
It doesn't make much sense for enroll to be a member function, so
I'd wrap the roster into a class to get automatic clean up of my
pointers.
#include <cstddef>
struct Student {};
class Roster
{
private:
static const size_t size = 7;
// non-copyable
Roster(const Roster&);
Roster& operator=(const Roster&);
public:
Roster() {
for(unsigned i = 0; i < size; ++i) {
roster_[i] = NULL;
}
}
~Roster() {
for(unsigned i = 0; i < size; ++i) {
delete roster_[i];
}
}
// enroll by copy
bool enroll(const Student& s) {
for(unsigned i = 0; i < size; ++i) {
if(roster_[i] == NULL) {
roster_[i] = new Student(s);
return true;
}
}
// out of space
return false;
}
// enroll by taking ownership
bool enroll(Student* s) {
for(unsigned i = 0; i < size; ++i) {
if(roster_[i] == NULL) {
roster_[i] = s;
return true;
}
}
// out of space
return false;
}
private:
// data
Student* roster_[size];
};
int main()
{
Roster r;
Student s;
r.enroll(s);
Student* sp = new Student();
r.enroll(sp);
return 0;
}
What about this?
Student * roster[2];
roster[0] = new Student(5,"first","2A");
roster[1] = new Student(2,"Second","5B");
Ps:
Enrol and Size shouldn't be members of the student class.
Print should ideally be externalized and a ToString function should be added instead.
You should use the inline constructor initialization instead:
Student(int a,string b,string c):id(a),name(b),class(c){}
You've used the keyword class as a variable name of type string. You shouldn't do that. Does it even compile like that?
enroll should have two arguments: void enroll( Student enrollee, Student Roster[]). You should probably change the name of Roster to roster because it's not a class and typically class names are capitalized.
If your array will only ever have 7 students then you could use some sentinel value to mark that the current student as an invalid student. Perhaps the id will be -1 to mark this. It means basically that you need some way to keep track of which spots in the array you can still use. If you don't do this then declaring an array of Students will get you an array of students with garbage member variables. You wouldn't be able to tell which students are real ones and which are just place holders for when someone new enrolls in the class. I would create a default constructor of Student and initialize its member variables like this:
id=-1;
name="";
name_of_class="";
I changed the name of your string class to avoid confusion.
After all that, enroll would look something like this:
void Student::enroll( Student enrolee, Student roster[]){
//search through roster to check for the first student with an
//id of -1
//if there are no students with id of -1, produce an error message
//that the class is full
//overwrite the student with id of -1 with the id, name, and
//name_of_class of enrollee
}
Although I'm not sure what exactly string class is there for. Does it store what class the Student is in? Is it their year in school like Freshman, Sophomore?
If you're suppose to use dynamic allocation of roster, though, it's a different story, but you said it will only ever have seven students.
In a data-structure, how do you insert a function?
struct Student_info {
std::string name;
double midterm, final;
unsigned int& counter;
std::vector<double> homework;
double overall = grade(students[counter]);
};
always get this type of error:-
a. "variable" was not declared in this code.
b. "Student_info::counter" cannot appear in a constant-expression.
c. an array reference cannot appear in a constant-expression.
d. a function call cannot appear in a constant-expression
edit:-
oopps, i mean student_info contain in a vector, wait, why that's info needed anyway... Dx
oh, and btw, this is from Accelerated C++, a book obviously, and I'm trying to answer one of its exercise, then I need to know this part, not found any on the book Dx
the question is 4-6. Rewrite the Student_info structure to calculate the grades immediately and store only the final grade.
You can NOT dynamically insert a function into a structure.
You can declare a structure that has a method()
struct Student_info
{
void doDomethingToStudent()
{
// Manipulate the object here.
}
// STUFF
};
Also you can not initialize member like above.
double overall = grade(students[counter]);
Here you need to create constructor that will initialize members.
struct Student_info
{
Student_info(std::string& studentName, unsigned int& externalCounter)
: name(studentName)
, midterm(0)
, final(0)
, counter(externalCounter)
, homework()
// It is not clear if overall is a normal memeber
// Or a static member of the class
, overall(grade(students[counter]))
{}
// STUFF
};
int main()
{
unsigned int counter = 0;
Student_info bob("Bob", counter);
}
My question is how to access and modify a 2D array defined in one class that is friends with another class. Below are some details on my question:
In class A I declare and allocate the appropriate space for my 2D array (pointer-to-pointer) u.
Class A
{
public:
friend class B;
long double **u;
int fun;
void make();
};
void A::make()
{
long double **u = new long double *[nx];
for (int i=0;i<nx;i++)
u[i] = new long double [ny];
int fun = 9;
}
Class A is friends with Class B; I need to use the array I declared in Class A in a function defined in class B. Below is my Class B:
class B
{
public:
void get(A*);
};
void B::get(A *pt)
{
using namespace std;
cout << pt->fun;
cout << pt->u[0][0];
}
I get a Bus error on my second cout pt->u[0][0]. Is there a simple way to use this setup I have to access my u[][] array? I think that I get the error because the pointer points to the 1st entry of my array, thus my whole 2D array is saved in memory as a single row (thinking aloud here). I'm a Fortran guy so this stuff is a little new to me.
Any help or "pointers" to other helpful threads would be appreciated.
Thank you !
Alberto
I think you get error because A::u is not initialized ( in method A::make you initialize a local variable u, not member. You need to change
void A::make()
{
long double **u = new long double *[nx]; // should be just u, or this->u.
There are some problems with your code: nx and ny don't seem to be defined anywhere, and in make you don't initialize A::fun at all, you instead set a local variable named fun which goes out of scope immediately.
As for your error, it sounds like the error stems from the fact that make() has not been called on pt. Ensure that make() is called on the instance you pass to get, otherwise the array u will not be allocated.