Implementing arrays in C++

I suppose you want to put 2 single-Dimensional arrays with N elements each into a 2D array with N rows and 2 Columns, then the following code might be useful.

int arr[100][2];

cout<<"\n Enter the Number of students: ";
cin>>n;

for(int i=0; i<n; i++)
{
  arr[i][0]=students[i];
  arr[i][1]=Grade1[i];
}

// Now use bubble sort to sort the second column of array arr

for(int i=0; i<n; i++)
{
   for(int j=i; j<n; j++)
   {
      if(arr[i][1] < arr[j][1])
      {
         int t1;
         t1=arr[i][1];
         arr[i][1]=arr[j][1];
         arr[j][1]=t1;
         t1=arr[i][0];
         arr[i][0]=arr[j][0];
         arr[j][0]=t1;

      }      
   }
} 

Start with a simple implementation of bubble sort (eg: take a look at pseudocode available on Wikipedia), and then use student grade to compare ordering

Your can store student and grade in array like this. n-th element of grade array represent grade for n-th student

int studentIds[10];
int grade[10];

And on your bubble sort, use condition similar like following to compare student based on his/her grade

// iterate students using two indices i & j
...
if(grade[i] < grade[j]) {
  // .. bubble element i to the right
}
...

If you can use modern c++ tools like std::vector and other cool business like std::sort (which sorts an std::vector), it'd be much cleaner than doing it yourself with raw arrays. Learning to use container classes and the built-in algorithms of the language will serve you well.

#include <vector>
#include <algorithm>
#include <iostream>
struct Student //structure to hold student ID and grade info
{
    Student(int id, int grade):id(id),grade(grade){}
    int id;
    int grade;
};
//comparison operator to use with std::sort
bool operator <(const Student& lhs, const Student& rhs)
{
    return lhs.grade < rhs.grade;
}
int main()
{
    //You are starting with raw arrays...
    int studentID[5]{1,2,3,4,5};
    int grade[5]{90,91,73,62,87};

    //convert to std::vector of Student objects
    //not safe indexing into raw array, just an example
    std::vector<Student> students;
    for(unsigned int i=0;i<5;++i){
        students.push_back(Student(studentID[i],grade[i]));}

    //sort the vector (using the less-than operator provided above)
    std::sort(students.begin(),students.end());
    //reverse the vector, making it high-to-low order
    std::reverse(students.begin(),students.end());

    //print the result (uses std::for_each with a lambda function)
    std::for_each(students.begin(),students.end(),
    [](Student s){std::cout<<"ID: "<<s.id<<" grade: "<<s.grade<<std::endl;});
}

Output:

ID: 2 grade: 91
ID: 1 grade: 90
ID: 5 grade: 87
ID: 3 grade: 73
ID: 4 grade: 62