How to check if object is in the array?

Question

I know that if there is even no object in array there is still some address. But I want to find solution to check if there is an object under the specific index we will ask. I need to have such mechanism to adding new Points to polygon. But before that I need to know if counter of object should grow or stay in the same value if there was an object. Maybe I should try fill all array with NULL?

main.cpp

#include <iostream>
#include "punkt.h"
#include "wielokat.h"

using namespace std;

int main(int argc, char *argv[])
{
    Punkt  p1("p1", 10, 20); // 10x20
    Punkt  p2("p2", 1, 1);   //1x1

    Wielokat w1 ("square", 4);

    w1.set(p1,0);
    w1.set(p2,0);
    w1.showWielokat();


    system("PAUSE");
    return EXIT_SUCCESS;
}

Wielokat.cpp

#include "punkt.h"
#include "wielokat.h"
#include <iostream>

using namespace std;

void Wielokat::increase(int n)
{
    m_ilosc = m_ilosc + n;
    m_tab = new Punkt * [m_ilosc];  
    cout<<"Dodaj "<<m_ilosc<<endl;
}

void Wielokat::decrease(int n)
{
    m_ilosc = m_ilosc - n;
    if(m_ilosc<0){ m_ilosc=0;}
    m_tab = new Punkt * [m_ilosc]; 
    cout<<"Odejmij "<<m_ilosc<<endl;
}
void Wielokat::set(Punkt p, int pos)
{
    //How to check if there was already object ?
    m_tab[pos] = new Punkt(p);
    m_counter++;
}

void Wielokat::showWielokat()
{
    for(int i=0; i<m_counter; i++){
        m_tab[i]->show();
    }

}
void Wielokat::crash(int pos){
    //after delete all elements moved one by one to the left side
    delete m_tab[pos];
    for(int i=pos; i<m_ilosc; i++){
        m_tab[i]=m_tab[pos+1];
    }
}
double Wielokat::getParimeter(){
    //here is function whih will count circuit 
}

Wielokat.h

class Wielokat {

    public:

    Wielokat(char* nazwa, int ilosc):m_nazwa(nazwa), m_ilosc(ilosc) 
    {
        m_tab = new Punkt * [m_ilosc]; 
        m_counter = 0;
    }

    Wielokat(const Wielokat& p): m_ilosc(p.m_ilosc), m_nazwa(strdup(p.m_nazwa))
    {}

    ~Wielokat()
    {
        for(int i=0; i<m_counter; i++){
            delete m_tab[i];
        }
        delete m_tab;
    }

    //Function:
    void increase(int n);
    void decrease(int n);
    void set(Punkt p, int pos);
    void crash(int pos);  //delete
    void showWielokat();
    double getParimeter();



    private:
    Punkt **m_tab;  //our tab of elemenst
    char* m_nazwa;
    int m_ilosc;
    int m_counter;
};

Jon · Accepted Answer · 2012-05-15 14:36:37Z

6

You are coding in C++, which means you can have an std::vector<Punkt> (or an std::vector<Punkt*>, if polymorphism were required). Don't reinvent the wheel; use it.

With an std::vector all of the manual allocation code is simply not required and you can check how many elements there are with vec.size().

Update: OK, so you can't use vector because this is homework.

The alternative solution is to zero out the memory of your array whenever you initialize it and then check if m_tab[i] == 0 before trying to use object i. Using memset, that would look like

// WARNING! INCOMPLETE/BUGGY CODE!
m_tab = new Punkt* [m_ilosc];
memset(m_tab, 0, m_ilosc * sizeof(m_tab[0]));

And since you are doing this from two places in the class, you should move this logic inside a separate private method.

Regarding the incomplete/buggy part, those two lines above have a few problems:

The "old" array (if one exists) is not delete[]d; this is a memory leak.
The values from the "old" array (if one exists) should be copied to the new array; the way things are now they are simply lost.

You should fix these as the next step.

edited May 15, 2012 at 14:36

answered May 15, 2012 at 14:06

Jon

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4 Comments

mathewM Over a year ago

But in this case i can't use std:vector.

mathewM Over a year ago

Yes, it is. I don't know maybe I should make all elements NULL or checking by sizeof if this will be smaller than object is inside?

mathewM Over a year ago

One more question: There is any way to delete single element of array ?

Jon Over a year ago

@mathewM: To remove it from the array, move all elements after it one position earlier and then set the position where the previously last element was to 0.

Component 10 · Accepted Answer · 2012-05-15 14:09:26Z

0

If you needv to check for values in a collection, you'd be better using an associative container such as a map or a set from STL. I see no reason from your code that you can swap this in pretty easily.

answered May 15, 2012 at 14:09

Component 10

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Comments

Attila · Accepted Answer · 2012-05-15 14:11:13Z

0

Besides Jon's advice, C++ does not hold your hands, so you will need to explicitly maintain the valiud elements inthe array: either by a) using a max-valid-index or b) using NULL values for invalid entries (preferably both)

Note: using NULL would only work for storing pointers in the array. Otherwise you will need to have an invalid value (e.g a Punkt with too large/too small coordinates)

answered May 15, 2012 at 14:11

Attila

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