accessing an array through pointer

Question

I wanted to create a database like class through OOP. In this class, there are 3 arrays which act as columns in a table.I insert data through insert() function and prints data through printEntries() function. But that function can't access the arrays to retrieve data.

#include <iostream>
#include <sstream>
#include <fstream>
#include <string> 
using namespace std;

class subject{

   public:
      int numOfStudents;
      string subjectId;
      int * marksArray;
      int * indexArray;
      char * gradeArray;
      int index ; // index for inserting data

      subject(int students , string subjectName){

      numOfStudents = students;
      subjectId = subjectName;
      this->index =0 ;
      //creating and pointing to arrays
      int  A[numOfStudents];      marksArray = A;
      int  B[numOfStudents];      indexArray = B;
      char C[numOfStudents];      gradeArray = C;

  }

 void insert(int studentId , int mark , char grade){
 indexArray[index] = studentId;
 marksArray[index] = mark;
 gradeArray[index] = grade; 

 this->index = this->index +1;    
}

int getNumberOfEntries(){
    return index ;
}

 void printEntries(){
        cout<< indexArray[0] << "  O" << marksArray[0] << "  " << gradeArray[0] << endl; 
        cout<< indexArray[1] << "  OOO" << marksArray[1] << "  " << gradeArray[1] << endl; 
        cout<< indexArray[2] << "  OO" << marksArray[2] << "  " << gradeArray[2] << endl;  
      }
  };

 int main(int argc, char const *argv[]){ 
  subject S(10,"Mathematics");
  cout<<S.subjectId<<endl;
  cout<<S.numOfStudents<<endl;
  S.insert(35,34,'A');
  S.insert(33,34,'B');
  S.insert(54,34,'C');
  S.insert(21,34,'D');
  S.insert(14,34,'F');
  S.printEntries();
  return 0;
 }

output is :

Mathematics 10 35 O34 A 0 OOO0

1) C++ does not support VLAs; 2) Using a pointer/reference to a locally-defined variable outside of the scope in which it was defined is undefined behavior; 3) Consider templating your array size if you know the size at runtime and it is not huge, otherwise use std::vector; 4) Member functions that do not modify the object (such as getNumberOfEntries and printEntries) should be marked const. — paddy
– paddy, Commented Nov 20, 2018 at 4:16

Chris Mc · Accepted Answer · 2018-11-20 04:39:41Z

2

As pointed out by @paddy in the comments to your question, your issue is within the constructor.

//creating and pointing to arrays
int  A[numOfStudents];      marksArray = A;
int  B[numOfStudents];      indexArray = B;
char C[numOfStudents];      gradeArray = C;

What you are doing is saving the address of the very first element and the rest disappears after you leave the constructor. you are lucky that it's even allowing you to save the first insert and to quote the comment what you are doing is "defined is undefined behavior"

Replacing the pointers with std::vectors and inserting you elements there will be must easier.

class subject
{
public: // It's considered bad practive to have public data members in a class
  std::vector<int> marksVector;
  std::vector<int> indexVector;
  std::vector<char> gradeVector;
  // ...

  void insert(int studentId , int mark , char grade){
     indexVector.push_back(studentId);
     marksVector.push_back(mark);
     gradeVector.push_back(grade);

     // No longer need to worry about index for next insert
  }
};

answered Nov 20, 2018 at 4:39

Chris Mc

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2 Comments

Nipuna Chandimal Over a year ago

After pushing data into database like object in this. I want to manipulate data .Which means i want to access rows in columns (indexes in arrays by index) is it possible by using vectors ??

user9254539 Over a year ago

@Nipunachandimal Vectors can pretty much do everything that arrays can, so yes.

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