Vector of a Vector troubles

Question

Basic Problem Can you please help me understand how to use a vector of a vector. Take for example vector< vector<int> > help. I do not understand if it is a vector of ints who each are a vector of ints or if it is a vector of a vector of ints? I also don't understand how to utilize it.

Example Code

vector< vector<int> > test[500];
test[0].emplace_back(1);
cout << test[0][0];
test[50].emplace_back(4);
cout << " " <<test[50][0];

-console-
1 50 //this is not what happens btw, but it is the desired results

Disclaimer I have spent the better part of a morning testing and googling this. Please help :) I did my hw. I can't find any documentation of vectors of a vector. Also I have all the correct libraries and I am using namespace std. I am a noob and i understand that namespaces are bad practice, but its very convient for me right now.

Basically what I want is a set size of a vector filled with each pt being a vector of int. I would rather not go the way of a separate class. Is a vector of a vector of int, the right thing to be looking into?

Thank you :)

You don't have a vector of vectors. You have an array of vectors of vectors. — juanchopanza, May 02 '13 at 17:57
so if i did vector test[500]; I would have an array that contains 500 vectors of int? — Krtko, May 02 '13 at 18:00

score 2 · Answer 1 · answered May 02 '13 at 18:00

test is an array of 500 vectors of vectors of int. The second line of your example should not even compile here, as you are calling std::vector< std::vector<int> >::emplace_back(), which expects an argument compatible with std::vector<int>, and you have provided an int. To clarify:

test is a std::vector< std::vector<int> >[500].
test[0] is a std::vector< std::vector<int> >.
test[0][0] is a std::vector<int>.
test[0][0][0] is an int.

(Pedantic C++ developers will note that the latter three are actually references, but I'm omitting that from the type for clarity.)

score 2 · Accepted Answer · answered May 02 '13 at 18:01

2

This is a vector of int:

std::vector<int> v;

this is a vector of vectors of int:

std::vector<std::vector<int>> v2;

this is an array of vectors of vectors of ints, which is what you have:

std::vector<std::vector<int>> test[500];

each element of that array is an std::vector<std::vector<int>>. So test[0] is one of those.

If you want a vector of 500 default constructed vectors of int, you need

std::vector<std::vector<int>> test(500);

answered May 02 '13 at 18:01

juanchopanza

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now googling difference between an array and a vector. Awesome thank you. – Krtko May 02 '13 at 18:05
for future reference, http://stackoverflow.com/questions/6632971/what-is-the-difference-between-stdarray-and-stdvector-when-do-you-use-one-o – Krtko May 02 '13 at 18:06
i can't vote u up cuz i no have rep, I would otherwize thank you u for your speedy and concise response – Krtko May 02 '13 at 18:10

score 2 · Answer 3 · answered May 02 '13 at 18:02

A vector is just a resizable array.

To declare a vector of int (an array of int), just do:

std::vector<int> vec;

To declare a array in which individual elements are vectors, you do:

std::vector< std::vector<int> > vecarr;

To set the initial size of the vector, you do:

std::vector<int> vec(500); not std::vector<int> vec[500], because this creates an array of 500 std::vectors. Similarly, std::vector< std::vector<int> > vec[500]; creates a array of 500 vector of vectors.

To skip writing std:: you can say using namespace std before all this to tell that you're using the std namespace.

score 1 · Answer 4 · answered May 02 '13 at 18:00

What you have there is an array of vector-of-vector which I believe since you're accessing the data with two indexes is not what you wanted.

I believe you may have just typo-ed your constructor initialization:

vector< vector<int> > test(500);   // Note () instead of [] here.

This creates a vector-of-vectors, with 500 inner vectors pre-created for you. Then the rest of your code should just work!

Vector of a Vector troubles

4 Answers4