Passing a 2D vector to a function in CPP

Question

I am trying to pass a 2D vector to a function and trying to modify a value and then print it. For that I have written the following code.

#include<bits/stdc++.h>
using namespace std;


void fun(vector < vector<int>  > &arr)
{
    arr[2][2]=100;
    cout<<"DOne";
}


int main()
{
    int n=8;
   vector < vector<int>  > arr(n*n);

    

    fun(arr);

    cout<< arr[2][2];

    return 0;

}

But this is not working and no output is being printed. Can someone please tell what is happening and how to achieve the objective that I am looking for. I havent found any proper help in the internet. Thank you.

`vector < vector > arr(n*n);` create a vecotr of `n * n` elements, where each element is an *empty* `vector`. It doesn't create a "matrix" or "2D" vector. — Some programmer dude, Sep 19 '21 at 18:48
After initialization, `arr` is a `vector` containing `64` `vector` objects, each of which is default-initialized. It is therefore undefined behavior to try to access `arr[2][2]` in `func`, because `arr[2]` doesn't have at least three elements. — Nathan Pierson, Sep 19 '21 at 18:49
@TedLyngmo What if we want 3D vector? Then how will the syntax change? — Normal_Coder, Sep 19 '21 at 19:04
On another note, *please* don't use that [``](https://stackoverflow.com/questions/31816095/why-should-i-not-include-bits-stdc-h) header file. [`using namespace std;`](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) is a bad habit, but might be acceptable in smaller examples like this. That header file is not a bad habit, it's plain wrong to use. — Some programmer dude, Sep 20 '21 at 04:20

score 0 · Answer 1 · answered Sep 19 '21 at 20:36

You are creating a vector of n*n empty vectors, not a 2D matrix. One way I got around this is making it a vector of pointers to vectors, and then creating vectors for the pointers to point to, like this:

#include<bits/stdc++.h>
using namespace std;


void fun(vector <vector<int>*> &arr)
{
    (*arr[2])[2]=100;
    cout<<"DOne";
}


int main()
{
    int n=8;
   vector < vector<int>*  > arr(n);
   for(auto i = arr.begin(); i != arr.end(); i++){
       *i = new vector<int>(n); //Fill the vector with vectors, which will be added to the heap.
   }

    

    fun(arr);

    cout<< (*arr[2])[2];

    return 0;

}

This worked for me as expected, printing DOne100. You may notice, however, that I needed to access the elements in a special way. arr[i][j] would not work in this case, since a vector and an array are not really the same. Instead, you have to use (*arr[i])[j]. If you want to make it a vector to vectors, rather than a vector of pointers to vectors, you will need to give each of the vectors in your vector the desired size, which would look something like this:

std::vector<std::vector<int>> arr(n);
for(auto i = arr.begin(); i != arr.end(); i++){
        i->resize(n); //Give each vector a size of n
}

Accessing each element is as easy as arr[i][j]. I tested this out with your implementation of fun(arr) and it gave the expected result.

Passing a 2D vector to a function in CPP

1 Answers1