Polynomial multiplication recursively - c++

Question

Here is my code, why is it showing segmentation fault? Can someone correct my mult4 function... I know that problem is mult4 and i am not sure is function mult4 is correct solution for polynomial recursively multiplication...
#include <iostream>
#include <cstdlib>
using namespace std;
class Polynomial
{
public:
Polynomial()
{
deg=0;
coeff = new float[deg+1];
coeff[0]=0;
}
Polynomial(int n)
{
deg = n;
coeff = new float[deg+1];
for(int i = 0; i<=deg;i++)
coeff[i] = rand()/(float)RAND_MAX;
}
Polynomial(float *array, int n)
{
deg=n;
coeff=array;
}
Polynomial(const Polynomial& P)
{
deg = P.deg;
coeff = new float[deg+1];
for(int i=0;i<=deg;i++) coeff[i] = P.coeff[i];
}
~Polynomial()
{
delete []coeff;
}
Polynomial operator+(const Polynomial& P) const;
Polynomial operator-(const Polynomial& P) const
{
int i;
Polynomial c;
if(deg>=P.deg)
{
c.deg=deg;
for(i=deg;i>=0;i--)
c.coeff[i]=coeff[i];
for(i=P.deg;i>=0;i--)
c.coeff[i]=c.coeff[i]-P.coeff[i];
}
else
{
c.deg=P.deg;
for(i=P.deg;i>=0;i--)
c.coeff[i]=-P.coeff[i];
for(i=deg;i>=0;i--)
c.coeff[i]=c.coeff[i]+coeff[i];
}
return c;
}
Polynomial& operator=(const Polynomial& P)
{
if (this!=&P)
{
delete [] coeff;
deg = P.deg;
coeff = new float[deg+1];
for(int i=deg;i>=0;i--) coeff[i] = P.coeff[i];
}
return (*this);
}
float operator[](float x) // evaluation of polynomial
{
float rez=0;
for(int i=deg;i>=0;i--)
rez=rez*x+coeff[i];
return rez;
}
int degree() const
{
return deg;
}
public:
float *coeff; // array- representation of polynomial
int deg; //degree of polynomial
friend ostream& operator<<(ostream &out, const Polynomial& P) //overload operatpr<<
{
for ( int i =P.deg; i >= 0; i-- )
{
out << P.coeff[i] << "x^" << i << " ";
if(i>=1)
{
if(P.coeff[i-1]>0) out<<"+";
}
}
out << endl;
return out;
}
};
Polynomial Polynomial::operator+(const Polynomial& P) const
{
int i;
Polynomial c;
if(deg>=P.deg)
{
c.deg=deg;
for(i=deg;i>=0;i--)
c.coeff[i]=coeff[i];
for(i=P.deg;i>=0;i--)
c.coeff[i]=c.coeff[i]+P.coeff[i];
}
else
{
c.deg=P.deg;
for(i=P.deg;i>=0;i--)
c.coeff[i]=P.coeff[i];
for(i=deg;i>=0;i--)
c.coeff[i]=c.coeff[i]+coeff[i];
}
return c;
}
Polynomial shift(Polynomial A, int x)
{
Polynomial c(A.deg+x);
int k=0;
for(int i=c.deg;i>=0;i--)
c.coeff[i]=0;
for(int i=A.deg;i>=0;i--)
{
c.coeff[c.deg-k]=A.coeff[i];
k++;
}
return c;
}
Polynomial mult4( Polynomial P, Polynomial Q)
{
if(P.deg==1)
{
Polynomial qw(2);
qw.coeff[0]=P.coeff[0]*Q.coeff[0];
qw.coeff[2]=P.coeff[1]*Q.coeff[1];
qw.coeff[1]=P.coeff[1]*Q.coeff[0]+Q.coeff[1]*P.coeff[0];
return qw;
}
else
{
Polynomial p1(((P.deg)/2)-1);
Polynomial p2(P.deg-(P.deg)/2);
Polynomial q1(((Q.deg)/2)-1);
Polynomial q2(Q.deg-(Q.deg)/2);
int k=0;
for(int i=p1.deg;i>=0;i--)
{
p1.coeff[i]=P.coeff[i];
}
for(int i=p2.deg;i>=0;i--)
{
p2.coeff[i]=P.coeff[P.deg-k];
k++;
}
for(int i=q1.deg;i>=0;i--)
{
q1.coeff[i]=Q.coeff[i];
}
k=0;
for(int i=q2.deg;i>=0;i--)
{
q2.coeff[i]=Q.coeff[P.deg-k];
k++;
}
Polynomial t1,t2,t3;
t1=mult4(p1+p2,q1+q2);
t2=mult4(p1,q1);
t3=mult4(p2,q2);
return(t2+shift(t1-t2-t3,P.deg/2)+shift(t3,2*(P.deg/2)));
}
}
int main()
{
float *p=new float[4];
for(int i=3;i>=0;i--)
p[i]=-1.1+i;
float *k=new float[5];
for(int i=4;i>=0;i--)
k[i]=-1.2+i;
Polynomial P(p,3);
Polynomial K(k,4);
cout<<mult4(P,K);
return 0;
}

Answer 1

You have several errors in your code.
You need a copy constructor. Familiarize yourself with The Rule Of Three.
There are many lines where you set the deg of a Polynomial but don't allocate memory for coeff. For example, in shift, you have:
Polynomial shift(Polynomial A, float x)
{
Polynomial c;
int k=0;
c.deg=A.deg+x;
for(int i=c.deg;i>=0;i--)
c.coeff[i]=0;
What you need is:
Polynomial shift(Polynomial A, float x)
{
// Polynomial c(A.deg+x);
// Not sure why you whether c.deg should be A.deg+x or just A.deg.
Polynomial c(A.deg);
int k=0;
for(int i=c.deg;i>=0;i--)
c.coeff[i]=0;
In the operator= function, you are changing the index incorrectly. In stead of using:
for(int i=deg;i>=0;i++) coeff[i] = P.coeff[i];
you need to use:
for(int i=deg;i>=0;i--) coeff[i] = P.coeff[i];
// ^^ Decrement, not increment
In mult4, your use of
p1.deg=((P.deg)/2)-1;
seems flawed. What happens when P.deg/2 is equal to 0? Perhaps you need to use:
p1.deg=(P.deg)/2;
Setting of q1.deg must be similarly updated.
I might have missed other errors.