使用默认类构造函数C ++遇到麻烦

Question

I am having some trouble understanding default constructor methods in C++ and how to write one to fit the needs of my methods. I was tasked with creating a simple array that could represent polynomials, such as a(n)x^(n) + a(n-1)x^(n-1) + … + a(0). It should be array of coefficients, with coefficient a(i) being stored in location i of the array. The coefficients are floating point values (potentially negative), so the array should be of type double. It should also be of size MAXPOLY (a constant variable set to 50), limiting the array to holding polynomials with maximum degree of MAXPOLY – 1 (or 49).
The full assignment instructions/description can be found here: Beginner polynomial program in C++

My issue lies in creating the default constructor for the polynomial object, Poly. The default class constructor is meant to initialize a polynomial to the constant 0, creating a class object that is a degree-0 polynomial of 0. The assignment also notes, however, that a post condition of this constructor is that all array elements of coeff[] are set to 0.0 and that is where my confusion ensues. Should first initialize the entire array for 0-MAXPOLY and then define the poly at 0? Or should one or the other be sufficient? How should I go about writing the constructor?

I apologize if this question seems rather obvious, but I am brand new to C++ and the textbook resources I have read on this subject have been unclear. Also, I think I may be overthinking and confusing myself. Any help would be greatly appreciated! Thank You!

Answer 1

I do believe you are overthinking the problem. Just as a polynomial with coeff[0] = 10 is "x = 10", one with coeff[0] = 0 is "x = 0", assuming the rest of the array is initialized to zero.

As a beginner in C++, keep in mind that array indexes are from 0 to N-1 where N is the size of the array.

Answer 2

You should store the coefficients in an array. You have two general options:

• Use a fixed-size array and also a counter of how many elements of the array are "in use"
• Use a variable-sized array

The second option is simpler and makes more sense, so your class would have a member

std::vector<double> coefficients;

and in the constructor initializer list you would have:

coefficients(1, 0.0)

meaning to initialize it to having 1 member of value 0.0 . Then you can find the degree of the polynomial by doing coefficients.size() - 1 .

Answer 3

Looking at the linked question, it appears you do not have a choice about what representation to use. Assuming this:

class Poly
{...
    // array for holding the coefficients of the poly
    double coeff[MAXPOLY];               
...
}

Then, your constructor needs to fill the array with zero values, like this:

// Class constructor
Poly::Poly ()
{ 
   //ADD YOUR CODE HERE
   for( int i = 0; i < MAXPOLY; i++ ) 
       coeff[i]=0.0;
}

This is the most explicit demonstration of what's going on, though you can shorten the code to initialize an array to something like this:

memset(coeff, 0, sizeof(coeff)); // this fills every byte of the array's memory space with zero.

functionally they produce the same thing.