Accessing a dynamically created multi dimensional array with pointers arithmetic

Question

I would like to understand how does it works to scan a dynamically allocated two-dimensional array using pointers arithmetics?

I can't figurate why in my example pointer arithmetic isn't returning the same results as using array indexing.

Here is my implementation:

    #include <iostream>
    using namespace std;

void init_matrix(int ** m, int size);
void print_matrix(int ** m, int size);
void print_matrix_pointers(int ** m, int size);

int main (){
    srand (time(NULL));
    int size = 3;
    int ** dynamic_matrix = new int * [size];
    for (int i = 0; i < 10; i++) {
        dynamic_matrix[i] = new int [size];
    }
    init_matrix(dynamic_matrix, size);
    
    cout << "Dynamic matrix accessed using square brackets ([i][j]): " << endl;
    print_matrix(dynamic_matrix, size);

    cout << "Dynamic matrix accessed using pointers arithmetics: " << endl;
    print_matrix_pointers(dynamic_matrix, size);

    return 0;

}

void init_matrix(int ** m, int size) {
    for (int i = 0; i < size; i++){
        for (int j = 0; j < size; j++){
            m[i][j] = rand()%10;
        }
    }
}

void print_matrix(int ** m, int size){
    for (int i = 0; i < size; i++){
        for (int j = 0; j < size; j++){
            cout << m[i][j] << " ";
        }
        cout << endl;
    }
}


void print_matrix_pointers(int ** m, int size){
    for (int i = 0; i < size; i++){
        for (int j = 0; j < size; j++){
            cout << *(*m + (i * size) + j) << " "; //
        }
        cout << endl;
    }
    cout << endl;
}

For instance if size was 3 I would get this output.

Dynamic matrix accessed using array indexing ([i][j]):

3 3 4

9 5 9

4 9 4

Dynamic matrix accessed using pointers arithmetics:

3 3 4

32735 9 5

9 32735 4

int size = 3; but for (int i = 0; i < 10; i++) { thats undefined behavior. Output of the code could be anything. — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Feb 11, 2022 at 10:54
please dont fix errors in the quesiton. The question is for the broken code. Corrected code goes to answers — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Feb 11, 2022 at 10:58
If you use std::vector and appropriate functions and statements (like range for loops), you can avoid many problems. — Some programmer dude
– Some programmer dude, Commented Feb 11, 2022 at 10:59
Could you please clarify the question to explain that you are trying to understand why your pointer arithmetic example is not returning the same results as your array indexing? — AndrewF
– AndrewF, Commented Feb 12, 2022 at 2:46

Some programmer dude · Accepted Answer · 2022-02-11 10:56:08Z

2

With *(*m + (i * size) + j) you treat m as a contiguous array of values, which it isn't. It's more like a jagged array. You need to treat it like the array of pointers it really is, like *(*(m + i) + j).

answered Feb 11, 2022 at 10:56

Some programmer dude

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Comments

463035818_is_not_an_ai · Accepted Answer · 2022-02-11 11:00:27Z

Two typos in your code:

int size = 3;
int ** dynamic_matrix = new int * [size];
for (int i = 0; i < 10; i++) {
    dynamic_matrix[i] = new int [size];
}

size is 3 but you write 10 elements. That is undefined behavior.

Then you are advancing the pointer wrongly.

void print_matrix_pointers(int ** m, int size){
    for (int i = 0; i < size; i++){
        for (int j = 0; j < size; j++){
            cout << *( *(m + i) + j) << " "; //
        }
        cout << endl;
    }
    cout << endl;
}

m points to an array of pointers to int. You have to increment m by i to get to the i-th row. Then you want to access the jth element of the array pointed to by m[i], hence you have to first dereference m+i then add j to get the address of jth element.

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