How to make arrays behave like vectors? - c++

Question

I want to have an array with initial size 4. The user can add as many elements(int type) to it. The function append() will be used for it. Also every time the size of the array is insufficient the same function will double its size and add the element to it.
#include <iostream>
using namespace std;
int n = 4;
int count = 0;
void append(int *a, int k)
{
if(count == n)
{
n *= 2;
int *b = new int[n];
for(int i = 0; i < count; ++i)
{
b[i] = a[i];
}
a = b;
}
a[count] = k;
++count;
}
void display(int *a)
{
for(int i = 0; i < count; ++i)
cout << a[i] << endl;
}
int main()
{
int *array = new int[n];
char t = 'y';
int num;
while(t != 'n')
{
cout << "Enter Value: ";
cin >> num;
append(array, num);
cout << "Do you want to enter more values?(y/n): ";
cin >> t;
}
cout << "The values entered are:\n";
display(array);
return 0;
}
After giving the following values for entering into the array:
1
2
3
4
5
6
7
8
8
9
10
11
12
13
15
I am getting the following output(using display() function)
1
2
3
4
0 [(This value depends on the compiler) #Erroneous Output]
6
7
8
1 [(This value depends on the compiler) #Erroneous Output]
10
11
12
13
14
15
I do not know why I am getting random values for the elements entered just after the array is updated(size is doubled), and how I should fix it.

Answer 1

Vectors, in essence are just dynamically allocated arrays. My question here is why not just use a vector with 4 slots initially. std::vector<int>(4); will do the trick although if you are adamant on remaking the vector class, research the topic of dynamic arrays and create a c++ template class based off them. According to my understanding .append() deallocates all elements in the vector, then reallocates them with the extra element at the end.
There are many articles online created by people trying to accomplish similar tasks, just do a little bit of digging around:
http://www.gamedev.net/topic/528822-c-writing-my-own-vector-class/
https://codereview.stackexchange.com/questions/50975/creating-a-custom-vector-class