Jack F Jack F - 1 month ago 10
C++ Question

derivative of the polynomial in a linked list

For some reason when i am trying to do my derivative it just does a derivative of the one item not the whole polynomial.

struct term{
double coef;
unsigned deg;
struct term * next;
};


I have a struct and then also a class Polynomial with deep copy constructor and = constructor.
In a private class i have a
term* ptr


here is my code for the derivative

void Polynomial::derivative (Polynomial *p){
term *x;
if ( ptr == NULL)
return ;
term *temp;
temp = ptr;
while (temp != NULL){
if ( ptr == NULL){
ptr = new term ;
x = ptr ;
}
else{
x -> next = new term ;
x = x -> next ;
}
x-> coef = temp -> coef * temp -> deg;
x-> deg = temp -> deg - 1;
temp = temp -> next;

}
ptr=x;
}


so when i try to derivative of
3x^4 + 3x^4 + 6x^7 + 3x^4 + 3x^4 + 6x^7 + 2x^9
i get
18x^8


I was looking over the code and have no idea why does it do that for just the last term, since it is a while loop and should go from beginning till NULL and do the derivative.

Answer

You're getting the last term because of these two lines:

in your else condition:

x = x -> next

and your final assignment:

ptr = x;

Consequently, this also leaks memory, since all those pretty terms you allocated prior are now in the ether. You were leaking the old ones anyway, so this really needs a rethink regardless.

I strongly recommend that since your Polynomial class supports full copy construction and assignment operation, you create a new derivative polynomial from this one, and return that. if the caller wants this one transformed they can poly = poly.derivative(); themselves.

Example of derivative generator (as opposed to transformer). And as a bonus, eliminates all constant terms when generating the derivative.

Polynomial Polynomial::derivative() const
{
    Polynomial poly;

    const term *p = ptr;
    while (p)
    {
        if (p->deg != 0) // skip constant terms 
        {
            // add term to poly here (whatever your method is called)
            poly.addTerm(p->coef * p->deg, p->deg-1);
        }
        p = p->next;
    }
    return poly;
}

This allows this kind of generation: (note p1 is unchanged by derivative()):

Polynomial p1;
... populate p1...
Polynomial p2prime = p1.derivative();

And for something really enjoyable:

Polynomial p1;
... populate p1...
Polynomial p2prime2 = p1.derivative().derivative();

Anyway, I hope that makes sense.