2D vector passing to a function in C++

Question

When I declare a vector like vector<vector<int> >v and try to pass it to a function like below it works:

int change(vector<vector<int> >&v)
{
    v[0][0]=-1;
}
int main()
{
    vector<vector<int> >v;
    int a[]={1,2,3};
    int b[]={4,5,6};
    v.push_back(vector<int>(a,a+3));
    v.push_back(vector<int>(b,b+3));
    change(v);
    cout<<v[0][0];
return 0;
}

But when I declare the vector like vector<int>v[2] and pass it to a function in the same way it's causing an error.

int change(vector<vector<int> >&v)
{
    v[0][0]=-1;
}
int main()
{
    vector<int>v[2];
    int a[]={1,2,3};
    int b[]={4,5,6};
    v[0]=vector<int>(a,a+3);
    v[1]=vector<int>(b,b+3);
    change(v);
    cout<<v[0][0];
return 0;
}

How can I do this and why is this causing an error?

The answers to [your previous question](http://stackoverflow.com/questions/28712364/difference-between-vector-int-v-and-vector-vectorint-v) tell you that `vectorv[2]` is an array of vectors, not a "2D vector". — juanchopanza, Feb 25 '15 at 10:32

Ali Akber · Accepted Answer · 2015-02-25T13:52:46.490

1

In the second example you declared an array of vector.

So you can use either :

int change(vector<int>(&v)[2])

or

int change(vector<int>v[]) // pointer

or

int change(vector<int>*v) // pointer

edited Feb 25 '15 at 13:52

answered Feb 25 '15 at 10:27

Ali Akber

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Those are passing a pointer to vector, not a vector by reference. – juanchopanza Feb 25 '15 at 10:39
both works fine and i think it fullfils the expectation of OP... what's wrong if i pass the vector this way? – Ali Akber Feb 25 '15 at 10:41
What is wrong is you are saying it is passing by reference, but it really is passing a pointer to vector. Any pointer. It can be null, it can be a pointer to a single vector, it can be a pointer to an array of length 1 or 10000. You throw away the type information for no reason. – juanchopanza Feb 25 '15 at 11:20
Your answer still has the same problems. – juanchopanza Feb 25 '15 at 12:34

Guiroux · Answer 2 · 2015-02-25T10:39:37.597

-2

how in hell could this work, you want implicit conversion from vector<int>[] to vector<vector<int>> ? a vector is a object containing things, an array isn't that

note that an array dont need to be passed by reference, so

int change(vector<int> v[])

do the trick

edited Feb 25 '15 at 10:39

answered Feb 25 '15 at 10:32

Guiroux

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4
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2D vector passing to a function in C++

2 Answers2