Using dynamic memory allocation to add elements into an array - c++

Question

I'm supposed to write some code to ask the user to enter an array (1.3 4 5.2 16.3 9.99 7.21 4.5 7.43 11.21 12.5).
After that, I create a new array with a bigger size (double the size), copy all the elements from the old array to the new one, then ask the user to continue to enter 5 more elements to the new array: 1.5 4.5 9.5 16.5 7.5 11.5, and then print out the final array (15 elements).
Here is my code:
#include "stdafx.h"
#include <iostream>
#include <string>
using namespace std;
double* read_data(int& size)
{
int max = 10;
double* a = new double[max]; // allocated on heap
size = 0;
cout << "Enter the array: " << endl;
while (cin >> a[size])
{
size++;
}
if (size >= max)
{
double* temp = new double[max * 2];
for (int i = 0; i < size; i++)
{
temp[i] = a[i];
}
delete[] a;
a = temp;
max = max * 2;
}
return a;
}
int main ()
{
int input1, input2, input3, input4, input5;
int size = 0;
double* arr = read_data(size);
cout << "Please enter 5 more elements: " << endl;
cin >> input1 >> input2 >> input3 >> input4 >> input5;
arr[10] = input1;
arr[11] = input2;
arr[12] = input3;
arr[13] = input4;
arr[14] = input5;
cout << "The final array is: " << endl;
for (int i = 0; i < 15; i++)
{
cout << arr[i];
}
system("pause");
return 0;
}
It doesn't let me enter 5 more elements and I don't know why.
Please help.

Answer 1

You are waiting for the input stream to enter a "false" state, that happens most visibly when there is an end of file character inputted to the stream. So while inputting code to the terminal if you type in a -d you should see the code move onto the segment after the while loop.
To get away from this problem you will have to specify a limit to the array you are getting from the user so as to not overwrite memory out of bounds.
So change your code to this
#include <iostream>
#include <string>
using namespace std;
double* read_data(int& size)
{
int max = 10;
double* a = new double[max]; // allocated on heap
size = 0;
cout << "Enter the array: " << endl;
while (size < 10 && cin >> a[size])
{
size++;
}
if (size >= max)
{
double* temp = new double[max * 2];
for (int i = 0; i < size; i++)
{
temp[i] = a[i];
}
delete[] a;
a = temp;
max = max * 2;
}
return a;
}
int main ()
{
int input1, input2, input3, input4, input5;
int size = 0;
double* arr = read_data(size);
cout << "Please enter 5 more elements: " << endl;
cin >> input1 >> input2 >> input3 >> input4 >> input5;
arr[10] = input1;
arr[11] = input2;
arr[12] = input3;
arr[13] = input4;
arr[14] = input5;
cout << "The final array is: " << endl;
for (int i = 0; i < 15; i++)
{
cout << arr[i] << ' ';
} cout << endl;
system("pause");
return 0;
}
Notice how I added in the check for the size variable before the "cin" statement, this is referred to as short circuiting (https://en.wikipedia.org/wiki/Short-circuit_evaluation). I do not consider this good style myself but I have included it here to show you how input statements are used in while loops and can cause bad behavior if you don't use it properly.
Also I added in a cout << endl; to flush the stream buffer so that the output does go to the screen before the pause
I just read what you said in the comments and I suggest the following code for the purpose to quit when a 'q' to exit.
#include <iostream>
#include <string>
using namespace std;
double* read_data(int& size)
{
int max = 10;
double* a = new double[max]; // allocated on heap
size = 0;
cout << "Enter the array: " << endl;
char character;
while (size < 10 && cin >> character)
{
if (character == 'q') break;
a[size] = character - '0';
size++;
}
if (size >= max)
{
double* temp = new double[max * 2];
for (int i = 0; i < size; i++)
{
temp[i] = a[i];
}
delete[] a;
a = temp;
max = max * 2;
}
return a;
}
int main ()
{
int size = 0;
double* arr = read_data(size);
cout << "Please enter 5 more elements: " << endl;
for (int i = size; i < size + 5; ++i) {
cin >> arr[i];
}
// add 5 to the size
size += 5;
cout << "The final array is: " << endl;
for (int i = 0; i < size; i++)
{
cout << arr[i] << ' ';
} cout << endl;
system("pause");
return 0;
}