### Help with dynamic arrays

Just trying to fill a dynamically allocated array with values then I want to print out the values using pointer method:

 ``12345678910111213141516171819202122232425262728293031323334353637`` ``````#include using namespace std; long * extend_arr(long int arr[], long int length, long int val) { long * array2 = new long [length + 1]; for (int J = 0; J < length; ++J) array2[J] = arr[J]; array2[length] = val; delete[] arr; arr = NULL; return array2; } void FillMyArrayWithPrimes(long arr[], int length ) { for (int P = 3; P < 100; P += 2) if (isPrime(arr, P)) { arr = extend_arr(arr, length, P); ++length; } for (;*arr > 0; ++arr) cout << *arr << " "; } int main() { long *Block = new long [1]; *Block = 2; FillMyArrayWithPrimes(Block, 1); return 0; }``````

When this runs, I get an array with random numbers in it. For example, just trying to print the first value in *Block gives me random numbers each time. Can anyone tell me what is wrong with this as to why it is not holding the right values?

The extend_arr works perfectly fine, because when I try to access the values in the array using indexes (arr[0], arr[1], etc) it shows the right output, but using pointers does not. How can I make it work?

Output:
 `2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 134561 23 29 640 112`

As you can see, the primes end at 97 but it just keeps printing more

EDIT:
It now prints the correct values in the function if I set the last value in the array to 0. `arr[length] = 0;`

Now if I wanted to print the values in the array within main, why is that not working?

EDIT2:
Nvm, I just changed the void function to return a pointer to an array and problem solved!
Last edited on
C-style arrays doesn't store information about their length. What happens is you getting your pointer in `for (;*arr > 0; ++arr)` point to area of memory outside an array, where some random data is stored. You should manually check if your pointer is in array, like that: `for (;arr < (arr + length); ++arr)`
Figured that was the problem, thanks anyway for the reply
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