Array data being corrupted after passing pointer, C

Question

I'm dereferencing a pointer to my array struct in my function and printing it within the function. This works correctly, however, as soon as I return the pointer out of the function it prints incorrectly. I did some research on similar questions, but I just cannot seem to find my exact problem.

correct print: 11 5 1 2 3 4 5 10 20 30 40 incorrect: 11 5 1 2 5024 96 0 0 20 30 40

(Problem areas are in commented in CAPS)

[arraytools.cpp]

#include "arraytools.h"
#include <iostream>

using namespace std;

void DisplayArray (short int* a)
{
    short int size = a[0];
    int i;
    for (i=0; i<size; i++)
    {
        cout<< a[i] << " ";
    }
    cout<<endl;
}

short int* ConcatArray (short int* a1, short int* a2)
{
    short int size = a1[0] + a2[0] + 1;  //size of newarray
    short int *ptr;          //pointer for newarray
    short int newarray[size];  //initializing new array with given new size
    newarray[0] = size;    //making first object in new array the size of it

    int i,j;    
    for (i=0; i<a1[0]; i++)  //loop to store first array objects to newarray
    {
        newarray[i+1] = a1[i];
    }

    int lastpoint = a1[0] + 1;  //marks the point to start adding the second array to newarray
    for (j=0; j<a2[0]; j++)  //loop to store second array objects to newarray
    {
        newarray[lastpoint] = a2[j];
        lastpoint++;

    }

    ptr = &newarray[0];  //assigning new array to pointer
    DisplayArray(ptr);  //PRINTS CORRECTLY HERE
    return ptr;
}

[main.cpp]

#include "arraytools.h"
#include <iostream>
using namespace std;

int main()
{
    char choice = 'y';  //user defined later in program
    while (choice == 'y') // for repeating process
    {
        //declaring two arrays of short int
        short int arr1[] = {5,1,2,3,4};
        short int arr2[] = {5, 10, 20, 30, 40};

        //pointers to refer to declared arrays
        short int* nptr, *ar1, *ar2;

        ar1 =arr1;
        ar2 =arr2;

        DisplayArray(ar1);
        DisplayArray(ar2);

        nptr = ConcatArray(ar1, ar2); //RECIEVES RETURNED POINTER
        DisplayArray(nptr);  //PRINTS INCORRECTLY

        cout<<"Run process again? y/n: "; //loop exit condition
    cin >> choice;
    }
   return 0;
}

Answer 1

This line is root of problem:

short int newarray[size];

It allocates array in stack, and then you return address of that, even though it becomes invalid as soon as you return from that function. Try this:

short *newarray = new short [size];

Now it goes to heap. Of course you should also delete that with delete[] operator when you no longer need it, like maybe in main after printing:

delete[] nptr;