C++ Dynamic Arrays Creation

Question

I am prompted with the question as follows

1. Write a program that prints dynamic arrays.
2. The program creates an int 1D dynamic array of 3 elements and a float 2D dynamic array of 3 ROWS and 3 COLS.
3. Initialize both arrays with random values.
4. Both arrays will be printed separately in two separate functions
5. void print_2d_array(float**);
6. void print_1d_array(int*);"

I have created a code that will not produce any output. I am guessing the issue is in the initialization of the arrays, but I cannot figure it out. How do I get it to display the randomly generated numbers?

#include <iostream>
#include <iomanip>

using namespace std;

void print_2d_array(float**);
void print_1d_array(int*);

int main() {
    srand(time(NULL));

    int* arr[3];
    float** arr_two[3][3];

    for (int i = 0; i < 3; i++)
        *arr[i] = rand() % 100;
    
    for (int j = 0; j < 3; j++)
        for (int k = 0; k < 3; k++)
            **arr_two[j][k] = rand() % 100;

    print_1d_array(*arr);
    print_2d_array(**arr_two);
}

void print_2d_array(float** arr_two) {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++)
            cout << arr_two[i][j];
    }
    cout << endl;
}

void print_1d_array(int* arr) {
    for (int i = 0; i < 3; i++)
        cout << arr[i];
}

Answer 1

I want to post my code because this was a nightmare for me with a lot of testing

This would help with a 1Dimensional array for dynamic array assignment

I can't find all the stack overflow comments that helped me reach this conclusion of code so, this is how I give back. Hopefully this helps make sense of it.

I read a lot of stack questions saying this isn't possible and use a vector, if I find them I intend to post this there as well.

#include <iostream>
#include <array>
using namespace std;

// Reference // https://www.learncpp.com/cpp-tutorial/dynamically-allocating-arrays/ // https://www.youtube.com/watch?v=FHhcSncuHEI

int main()
{

 int size;
 int number = 0;
 int count  = 0;
 cout << "enter size for array: ";
 cin  >> size;
 int *array;
 int *tempArray;
 tempArray = new int[size];
 array     = new int[size];

 // can also be declared
 // int *array = new int[size];

 while(number != -1)
 {

  cout << " enter values for array: "; 
  cin  >> number;

  if(number == -1)
  {
      break;
  }

  if(count == size)
  {
      delete [] tempArray;
      size = size + 1;
      tempArray = new int[size];
    
     for(int i=0; i < count; i++)
     {
         tempArray[i] = array[i];
     }
         delete [] array;
         array = new int[size];                 // re-use array pointer
         
     for(int i=0; i < count; i++)
     {
         array[i] = tempArray[i];
     }     
  }
 
 array[count] = number;
 count++;
  
}

//   if(number == -1)
//     {
//     for(int i=0; i < count; i++)
//       {
//     cout << &array[i];
//       }
//     }

cout << endl;

// proves the array is tracking in memory to the same places.
// if it goes out of bounds of memory, the cout will just
// print the memory address of the last memory address block
// of the dynamic array, the assignment out of bounds does not
// cause significant change.

// array[0] = 1;
// array[1] = 2;
// array[2] = 3;
// array[3] = 4;

// cout << &array[0];
// cout << &array[1];
// cout << &array[2];
// cout << &array[3];

// cout << sizeof(array)/sizeof(array[0]);

// cout << endl;

// cout << array[0];
// cout << array[1];
// cout << array[2];
// cout << array[3];
// cout << array[4];
// cout << array[5];

// cout << endl;

for(int i=0; i < size; i++)
{
cout <<  "array element " << i << ": " << array[i] << endl;
}

return 0;

}

https://onlinegdb.com/YKF9skF2Ql

Answer 2

You never allocate memory for the 1D int array, nor for the float 2D array. Your declarations are also wrong, for example float** arr_two[3][3] is an array of size 3 of arrays of size 3 of pointers to pointer to float, clearly not what you want.

Your code should look more like this:

int *arr = new int[3]; // pointer to int will hold 3 ints

float **arr_two = new float *[3]; // array of 3 pointers to float

for (int i = 0; i < 3; i++)
{
    arr_two[i] = new float[3]; // each pointer will hold 3 floats
}

for (int i = 0; i < 3; i++)
    arr[i] = rand() % 100; // indexing is the same as if it was an array[size]

for (int j = 0; j < 3; j++) // indexing is the same as if it was an array[size][size]
    for (int k = 0; k < 3; k++)
        arr_two[j][k] = rand() % 100;

print_1d_array(arr); // you pass the name of the pointers, no dereference needed
print_2d_array(arr_two);

You should also print some spaces and newlines in your print functions, otherwise this will look like a very large single value.

Also, don't forget to free the memory later, when you no longer need the data:

delete[] arr;

for (int i = 0; i < 3; i++)
{
    delete[] arr_two[i];
}

delete[] arr_two;

I should also mention that in modern C++ very rarely we see the usage of raw pointers (except maybe in SO questions). Alternatives are:

• Smart pointers, take a look into that when you are done understanding pointers and arrays, here is a good starting point: What is a smart pointer and when should I use one?.
• Or preferably using STL containers, in this case, std::vector would be the right tool.

Also note that your random values are all int, so you won't see float values in the float arrays. If you want randomly generate fractional values, you need a new approach, take a look here: Random float number generation