Array Initialization problems: Unexpected behavior - c++

Question

The following program builds perfectly. However, during execution, no matter what value of degree I provide, the program takes only 2 array elements as input. I suppose there might be a problem with the redeclaration of the arrays f[] and fDash[]. In JAVA, arrays can be easily redeclared using the new keyword. Is that possible in c++ too? If not, what is the alternative?
P.S. I am using CodeBlocks 13.12 and compiler settings are standard.
#include <iostream>
#include <cmath>
using namespace std;
class Polynomial
{
public:
void input(void);
void expression(void);
void derivative(void);
double value(double var);
double der(double var);
private:
int f[];
int fDash[];
int degree;
};
void Polynomial::input()
{
cout<<"Enter degree of polynomial:\t";
cin>>degree;
f[degree+1];
fDash[degree];
for(int i=0;i<=degree;i++)
{
cout<<"Enter coefficient of x^"<<i<<":\t";
cin>>f[i];
}
for(int i=0;i<degree;i++)
{
fDash[i]=f[i+1]*(i+1);
}
}
void Polynomial::expression()
{
cout<<f[0];
for(int i=1;i<=degree;i++)
{
cout<<" + "<<f[i]<<"*x^"<<i;
}
}
void Polynomial::derivative()
{
cout<<fDash[0];
for(int i=1;i<degree;i++)
{
cout<<" + "<<fDash[i]<<"*x^"<<i;
}
}
double Polynomial::value(double var)
{
double val=0.0;
for(int i=0;i<=degree;i++)
{
val+=f[i]*pow(var,i);
}
return val;
}
double Polynomial::der(double var)
{
double val=0.0;
for(int i=0;i<degree;i++)
{
val+=fDash[i]*pow(var,i);
}
return val;
}
int main()
{
double lb,ub,step,var,accum=0.0,rms;
int counter=0;
Polynomial p;
p.input();
cout<<"\n\n\nPolynomial is:\nf(x) = ";
p.expression();
cout<<"\n\n\nDerivative is:\nf'(x) = ";
p.derivative();
cout<<"\n\n\nEnter x0,x1,Step:\t";
cin>>lb;
cin>>ub;
cin>>step;
cout<<"\n\n\n====================================";
cout<<"\n\nx\t|\tf\t|\tf'\n\n\n";
var=lb;
while(var<=ub)
{
cout<<var<<"\t|\t"<<p.value(var)<<"\t|\t"<<p.der(var)<<"\n";
accum+=pow(p.value(var),2.0);
var+=step;
counter++;
}
cout<<"\n====================================";
accum/=counter;
rms=sqrt(accum);
cout<<"\nRMS energy of f(x) = "<<rms;
return 0;
}

Answer 1

This does not compile on clang, it fails with "error: field has incomplete type 'int []' int f[];" and likewise for fDash.
Let's see how you declared these fields:
int f[];
int fDash[];
In C++, you can declare arrays with statically defined sizes like so:
int f[5];
int fDash[6];
If you want dynamic arrays, which you need in this case, you'd have to declare
int* f;
int* fDash;
and allocate memory for them with
f = new int[5];
You also must release that memory somewhere like so
delete [] f;
But beware - managing your own memory like this is error prone and should be avoided. You should just use std::vector instead, which is the equivalent of java.util.ArrayList:
std::vector<int> f;
std::vector<int> fDash;
And modify your input function like so:
void Polynomial::input()
{
cout<<"Enter degree of polynomial:\t";
cin>>degree;
int input;
for(int i=0;i<=degree;i++)
{
cout<<"Enter coefficient of x^"<<i<<":\t";
cin>>input;
f.push_back(input);
}
for(int i=0;i<degree;i++)
{
fDash.push_back(f[i+1]*(i+1));
}
}

Answer 2

You don't use arrays correctly. You need to allocate memory if you want to use array of variable length. How use arrays in c++ see this and this, or use std::vector