Using a 2D array as an argument in recursive function

Question

I'm trying to recursively call a function with a 2d array as an argument.

My compile error is

knapsack.cpp:76:55: error: cannot convert ‘int (*)[(((sizetype)(((ssizetype)(knapsack_capacity + 1)) + -1)) + 1)]’ to ‘int**’ for argument ‘1’ to ‘void backtrack(int**, int, int, std::vector)’ backtrack(T, items_amount, knapsack_capacity, weights);

My function is declared like this:

void backtrack (int **T, int item, int weight, vector<int> weights)
{
        if (T[item][weight] == T[item-1][weight]) {
                cout << item << ", ";
                backtrack(T, item-1, weight-weights[item-1], weights);
        }
        else if (item <= 0 && weight <= 0) {
                // Dont do anything
        }
        else {
                backtrack(T, item-1, weight, weights);
        }
}

And being called like this:

backtrack(T, items_amount, knapsack_capacity, weights);

I looked in a million different ways of combining [] and *, but no success. Any ideas?

vsoftco · Accepted Answer · 2015-05-01 23:43:41Z

You are probably passing an array, like int arr[10][20]; to your function. Such an array does not decay to int**, but to int (*)[20] (i.e., pointer to array-of-20-int) . So, either change your function signature so that you specify the second dimension, or, instead of passing an array int[10][20], pass a pointer-to-pointer int** (for which you have to allocate memory etc). Example of using both approaches:

#include <cstddef>

void f(int (*arr)[20]){}
void g(int**){}

int main()
{
    // using a pointer to array-of-20-ints
    int arr[10][20];
    f(arr);

    // 10 x 20 dynamic array, allocate
    int** ptr = new int*[10]; // allocate memory for the pointers to rows
    for(std::size_t i = 0; i < 10; ++i)
        ptr[i] = new int[20]; // allocate each row

    // process
    g(ptr);

    // deallocate
    for(std::size_t i = 0; i < 10; ++i)
        delete[] ptr[i]; // delete each row
    delete[] ptr; // delete the memory allocated for the pointers to the rows
}

But, really, you should use a std::vector<std::vector<int>> instead.

Collectives™ on Stack Overflow

Using a 2D array as an argument in recursive function

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related