CODEWITHSUNDEEP
c++arrayspointerssrand
Q_A
Q_A

Reputation: 451

C++ - Initializing a 2D array with pointer notation

I'm working on 2D array in C++. I'm having trouble connecting the concepts of arrays and pointers. I know that they are related in terms of memory allocation and accessing elements. For example

int *arr;
int num = arr + 1*sizeof(int);

is the same as

int arr[];
int num = arr[1];

I'm trying to find the same connection between 2D arrays and pointers Here's my code

void printGrid(int **arr) {
 for (int i = 0; i < 10; i++) {
  for (int j = 0; j < 10; j++) {
   cout << setw(3);
   cout << arr[i][j] << " ";
  }
  cout << endl;
 }
}

int main() {
 int **grid;
 srand(time(NULL));
 for (int i = 0; i < 10; i++) {
  for (int j = 0; j < 10; j++) {
   grid[i][j] = rand() % 11;
  }
 }
 printGrid(grid);
}

When I compile this, it compiles. When I then try to run it, I get a segfault. Could someone please point out the error in my code?

Thanks SO

Upvotes: 2

Views: 3059

Answers (3)

AJNeufeld
AJNeufeld

Reputation: 8695

grid is an uninitialized pointer. Attempting to store anything through that pointer will result in undefined behaviour, such as segmentation failure.

Your code would need to look like:

 grid = new (int*) [10];
 for (int i = 0; i < 10; i++) {
     grid[i] = new int[10];
     for (int j = 0; j < 10; j++) {
         grid[i][j] = rand() % 11;
     }
}

(And you should delete the memory you allocated when you are finished)

for (int i = 0; i < 10; i++) {
    delete[] grid[i];
}
delete[] grid;

Upvotes: 1

Giorgi Moniava
Giorgi Moniava

Reputation: 28654

int **grid;
 srand(time(NULL));
 for (int i = 0; i < 10; i++) {
  for (int j = 0; j < 10; j++) {
   grid[i][j] = rand() % 11;
  }
 }

Where is the part which should allocate memory for the dynamic array? And possibly also for its elements? To remedy this you could have done

// Allocating memory for array and elements
int **grid = new int*[10];
for (int i = 0; i < 10; i++) {
  grid[i] = new int[10];
}
// Now fill the array as you had in your code
//
...
// Deletion part
for (int i = 0; i < 10; i++) {
  delete[] grid[i];
}
delete[] grid;

Also,

I'm working on 2D array in C++. I'm having trouble connecting the concepts of arrays and pointers. I know that they are related in terms of memory allocation and accessing elements. For example

int *arr; int num = arr + 1*sizeof(int);

is the same as

int arr[]; int num = arr[1];

No they are not the same. This would be same though:

int x[] = {0, 2, 3};
int *arr = x;
int num = *(arr + 1); //Look up pointer arithmetic; same as num=arr[1];

Upvotes: 3

Sel&#231;uk Cihan
Sel&#231;uk Cihan

Reputation: 2034

One big difference between int ** ptrptr and int arr[X][Y] is that ptrptr is a pointer to an int pointer, so it can hold a variable length of int pointers, each of which can represent different sized arrays like:

ptrptr (ptrptr points to the beginning of three different sized arrays) ▼ address_1: 0 1 2 3 address_2: 4 5 address_3: 6 7 8

For the arr variable, the array occuppies contiguous memory, and thus it can be visualized as a rectangle, so think of it like each row must have the same number of elements.

Upvotes: 1

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