While coding addition of two polynomials in c++ , I could compute the addition when no and order of terms are same . It shows error when its diff,why?

Question

Here I have tried to code addition of two single variable polynomials in c++ . The code runs fine when both the inputs are in sync with the other . But when it ain't it shows some crazy stuff happening and prints complete gibberish . I am pasting my code here with the output . Any suggestions would be helpful .

My Code

#include <iostream>

using namespace std;

class Term
{
public :
    int coeff;
    int exp;
};

class Poly
{
private:
    int n;
    Term *terms;
public:
    Poly(int n)
    {
        this->n  = n;
        terms = new Term[n];
    }

    void create();
    
    void Display();
    Poly* add(Poly &p2);
    ~Poly()
    {
        delete []terms;
    }
};

void Poly::create()
{
    cout << "Enter Terms " << endl;
    for(int i = 0; i<n;i++)
    {
     cin >> terms[i].coeff >> terms[i].exp ;
   
    }
}

void Poly::Display()
{
    for(int i = 0;i<n;i++)
    {
        cout <<terms[i].coeff<<"x"<<terms[i].exp <<" + ";
    }
}

Poly* Poly :: add(Poly &p2)
{
    int i,j,k;
    Poly* sum  = new Poly(n+p2.n);
    i = j = k = 0 ;

    while(i<n && j<p2.n)
    {
        if(terms[i].exp>p2.terms[j].exp)
        {
            sum->terms[k++] = terms[i++];
        }
        else if(terms[i].exp<p2.terms[j].exp)
        
           sum->terms[k++] = terms[j++];
        else
           sum->terms[k].exp = terms[i].exp;
        sum->terms[k++].coeff = terms[i++].coeff +p2.terms[j++].coeff ;
    }

    for(;i<n;i++)
    {
        sum->terms[k++] = terms[i];
    }

    for(;j<p2.n;j++)
    {
        sum->terms[k++] = p2.terms[j];
    }
    
    sum->n  = k;
    return sum ;
}

int main()
{
    int n,m;
    cout << "Enter no.of terms for 1st Polynomial" << endl ;
    cin >> n;
    Poly p1(n);
    p1.create();
    cout << "Enter no.of terms for 2nd Polynomial" << endl ;
    cin >> m ;
    Poly p2(m) ;
    p2.create();
    Poly *p3 ;
    
    p3 = p1.add(p2);
    cout << endl;
    p1.Display();
    cout << endl;
    cout << "-------" << endl ;
    p2.Display();
    cout << endl;
    cout << " ------------" << endl ;
    p3->Display();
    return 0;
}

Output 1:(passing 1st test Case)

Enter no.of terms for 1st Polynomial
5
Enter Terms
2 5
3 4
6 3
7 2
8 1
Enter no.of terms for 2nd Polynomial
5
Enter Terms
3 5
8 4
1 3
5 2
6 1

2x5 + 3x4 + 6x3 + 7x2 + 8x1 +
-------
3x5 + 8x4 + 1x3 + 5x2 + 6x1 +
------------
5x5 + 11x4 + 7x3 + 12x2 + 14x1 +
Process returned 0 (0x0)   execution time : 48.853 s
Press any key to continue.

Output II(Failing the Test Case 2)

Enter no.of terms for 1st Polynomial
5
Enter Terms
3 5
2 4
8 3
4 2
6 1
Enter no.of terms for 2nd Polynomial
4
Enter Terms
7 5
4 3
8 1
2 0

3x5 + 2x4 + 8x3 + 4x2 + 6x1 +
-------
7x5 + 4x3 + 8x1 + 2x0 +
------------
10x5 + 2x4 + 12x-1 + 4x2 + 14x0 + 2x0 +
Process returned 0 (0x0)   execution time : 56.271 s
Press any key to continue.

Any help suggestions to solve the problem would be appreciated . Thank You .

Answer 1

The main mistake in your code is this line:

        sum->terms[k++].coeff = terms[i++].coeff +p2.terms[j++].coeff ;

and, more precisely, its location in the whole routine.
Namely, it is located past the if - else if - else construct. As a result, it is executed despite exponents satisfy the 'less-than', the 'greater-than' or the 'equal' condition; and it should be done in the last case only.

The relevant part of code should be enclosed with another pair of curly braces:

    while(i<n && j<p2.n)
    {
        if(terms[i].exp>p2.terms[j].exp)
            sum->terms[k++] = terms[i++];
        else if(terms[i].exp<p2.terms[j].exp)
           sum->terms[k++] = terms[j++];
        else
        {
           sum->terms[k].exp = terms[i].exp;
           sum->terms[k++].coeff = terms[i++].coeff +p2.terms[j++].coeff ;
        }
    }

Another error is in the else if branch: you use the j index to the this->terms array:

sum->terms[k++] = terms[j++];

while you should use the p2.terms:

sum->terms[k++] = p2.terms[j++];