C++ trying to swap values in a vector

Question

This is my swap function:

template <typename t>
void swap (t& x, t& y)
{
    t temp = x;
    x = y;
    y = temp;
    return;
}

And this is my function (on a side note v stores strings) call to swap values but whenever I try to call using values in a vector I get an error. I'm not sure what I'm doing wrong.

swap(v[position], v[nextposition]); //creates errors

Everest

Do you have somewhere using namespace std in your code? Because you could have a name conflict. – Rothstein

BTW You don't need that return; statement at the end of your function – Cyrstalcyrus

My best guess is that you are implementing a quadratic sort algorithm, and you have an off-by-one error, thus trying to access the vector out of bounds. It would really help to see the actual error message. Is it a compile-time error or a runtime error? – Chadbourne

Why aren't you just using std::swap? – Electrodynamometer

Answer 1

I think what you are looking for is iter_swap which you can find also in <algorithm>.
all you need to do is just pass two iterators each pointing at one of the elements you want to exchange.
since you have the position of the two elements, you can do something like this:

// assuming your vector is called v
iter_swap(v.begin() + position, v.begin() + next_position);
// position, next_position are the indices of the elements you want to swap

Crosscheck

I had this same issue, and there are a lot of answers recommending std::swap (like the highest voted one here does) but std::swap does not work (directly) for swapping the contents of two elements of a vector. Your answer, with std::iter_swap does. – Kaenel

@Kaenel Could you please clarify what you mean by "std::swap does not work (directly) for swapping the contents of two elements of a vector"? – Facelifting

@ChrisCulter Its been awhile since I made that comment (a little over two years), all I can recall is that std::swap did not give me the behavior I wanted (or expected) but std::iter_swap did. – Kaenel

Note that std::iter_swap(it1, it2) is equivalent to std::swap(*it1, *it2). – Determination

Answer 2

Both proposed possibilities (std::swap and std::iter_swap) work, they just have a slightly different syntax. Let's swap a vector's first and second element, v[0] and v[1].

We can swap based on the objects contents:

std::swap(v[0],v[1]);

Or swap based on the underlying iterator:

std::iter_swap(v.begin(),v.begin()+1);

Try it:

int main() {
  int arr[] = {1,2,3,4,5,6,7,8,9};
  std::vector<int> * v = new std::vector<int>(arr, arr + sizeof(arr) / sizeof(arr[0]));
  // put one of the above swap lines here
  // ..
  for (std::vector<int>::iterator i=v->begin(); i!=v->end(); i++)
    std::cout << *i << " ";
  std::cout << std::endl;
}

Both times you get the first two elements swapped:

2 1 3 4 5 6 7 8 9

Contributory

std::swap() does not work for iterators, consider using std::iter_swap() instead as it was suggested in another answer on this page. – Scheming

@Scheming As suggested by Superfly: Note that std::iter_swap(it1, it2) is equivalent to std::swap(*it1, *it2). So std::swap can work just fine. – Maypole

Answer 3

There is a std::swap in <algorithm>

Glynis

...for swapping out the contents of 2 separate vectors! – Jopa

Answer 4

after passing the vector by reference

swap(vector[position],vector[otherPosition]);

will produce the expected result.

Checkoff

Answer 5

1. Using std::swap by including the <algorithm> library to swap values by references / smart pointers,
   e.g. std::swap(v[0], v[1])
   Note: v[i] is a reference

2. Using std::iter_swap from the same library,
   e.g. std::iter_swap(v.begin(), v.begin() + v.size() - 1)

3. Using lvalue references and rvalue tuple by including the <tuple> library
   e.g. std::tie(v[0], v[1]) = std::make_tuple(v[1], v[0])
   Note: constexpr since C++14

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