Pointer to Structures & Arrays

[devroad]

I have been reviewing over this issue for the last 24 hours. I'm fairly new to C++ and, unfamiliar with pointers. I have read over the basics of pointers and how they work (along with structures) but, I don't seem to understand the premise of this question that I am presented with. I declare a structure called Fraction with integers num and den.
In main, declare an array of 20 Fraction pointers called fracPtrs, all initialized to 0, and initialize the size of the array (how many created so far) to 0.

I have a hard time understanding what it really means. At first I tried this:

     Fraction *fracPtrs[20];
     // Then, I used a for loop to initialize all values(num and den) to 0.
     // I also did this, inside the for loop.
     fracPtrs[i] = NULL;  // Initializing all pointers to NULL, or so I hoped? 

There was a run time error. After a little more work, I have come down to this :

  #include <iostream>
#include <iomanip>
using namespace std;

struct Fraction							// A structure for the fraction. Includes the numerator and denominator.
{
	int num, den;
};

int main()								// Start of the main function.
{
	int selectOption;					// Store the user selection.
	const int SIZE = 20;				// Size of the array.
	Fraction *fracPtrs;					// Declare a fracPtrs pointer of type Fraction.
	fracPtrs = new Fraction[SIZE];

	for(int i=0; i < SIZE; i++)			
	{
		fracPtrs[i].num = 0;
		fracPtrs[i].den = 0;
	}

	return 0;
}

I understand the declaration of the pointer, initially just specifies the type. We then set the pointer to the first address on the heap of the dynamically allocated memory. I don't understand the declaration an array of 20 Fraction pointers all initialized to 0 and then again, initializing the size of the array (ones created ) to 0 ?
Any insight would much appreciated.
Thank you :)

[Bourgond Aries]

About your run-time error, did you initialize in the same way? Your second code runs flawlessly.

#include <iostream>
#include <iomanip>


using namespace std;

struct Fraction							// A structure for the fraction. Includes the numerator and denominator.
{
	int num, den;
};

int main()								// Start of the main function.
{
	int selectOption;					// Store the user selection.
	const int SIZE = 20;				// Size of the array.
	Fraction *fracPtrs[SIZE];					// Declare a fracPtrs pointer of type Fraction.
	for (int i = 0; i < SIZE; ++i)
        fracPtrs[i] = new Fraction;

	for(int i=0; i < SIZE; i++)
	{
		fracPtrs[i]->num = 0;
		fracPtrs[i]->den = 0;
	}

    for (int i = 0; i < SIZE; ++i)
        delete fracPtrs[i];

	return 0;
}

[devroad]

I changed the second one. I was trying to see if someone could guide me in the right direction. The terminology used in the question is confusing me as I don't know which arrays to initialize to 0.

[devroad]

Bump, any ideas. anyone ?

[Bourgond Aries]

I don't think you are initializing, but assigning instead. If you want to initialize, use a class instead of a struct to initialize num and den in a constructor. I think you wanted to initialize the pointers to 0, I think I've done that here:

#include <iostream>


struct Fraction
{
	int num, den;
};


int main()
{
	enum {SIZE = 20};

	// Uninitialized array:
    {
        std::cout << "No initialization:\n";
        Fraction *fracPtrs[SIZE];
        for (int i = 0; i < SIZE; ++i)
            std::cout << i << ": " << std::hex << fracPtrs[i] << "\t";
    }

    // Initialized array
    {
        std::cout << "\n\nProper initialization:\n";
        Fraction *fracPtrs[SIZE] = {nullptr};
        for (int i = 0; i < SIZE; ++i)
            std::cout << i << ": " << std::hex << fracPtrs[i] << "\t";
    }

	return 0;
}

[devroad]

Thank you for your response. I figured it later, but I still checked my code and compared it to yours. The pointers do initialize to 0, but I had used a for loop. Your approach seems to be less complicated.