CODEWITHSUNDEEP
c++stlvector
user1354510
user1354510

Reputation: 151

how to store vector of vector by index

Hi I have to store the below mentioned information in a vector of vectors format...

 vector <vector <int>> ph;
 vector<int> p, q;
 p.push_back(1);
 p.push_back(2);
 p.push_back(3);

 q.push_back(10);
 q.push_back(20);
 q.push_back(30);
 q.push_back(40);

Now instead of using:

 ph.push_back(p);
 ph.push_back(q);

I want to use:

 ph.at(0)=p
 ph.at(1)=q

(This is the error that I am getting when I am using this: terminate called after throwing an instance of 'std::out_of_range' what(): vector::_M_range_check)

The reason why I want to store it this way is....later I want to access the elements of a particular vector identified by its index i.e. 1 or 0.

For example I wish to access ph[0].size() i.e the size of p...identified by its index. That is, I want to perform the same operation as we are able to do in the case of simple arrays in c++ i.e. store the data in that array at a particular index and access the data from a particular index.

Upvotes: 0

Views: 5443

Answers (3)

juanchopanza
juanchopanza

Reputation: 227558

You are getting the error because when you instantiate the vector of vectors, it has size 0, and the at() method is allowed to do bound checks and raise an exception.

If you want to access by index like that then you are better off with an std::map or std::unordered_map. That makes the index independent of the order of insertion:

std::map<int, std::vector<int>> MapOfVectors;
std::vector<int> p, q;
// fill vectors
MapOfVectors[1] = q;
MapOfVectors[0] = p;

Otherwise, you would have to make sure your vector of vectors is large enough to insert elements by index:

// instantiate vector with size 2. You can insert p and q by index. (0, 1) only
vector<vector<int>> ph(2);

Or, resize it after creation:

vector<vector<int>> ph;
ph.resize(2);

Upvotes: 2

Tony Delroy
Tony Delroy

Reputation: 106236

I want to use:

ph.at(0)=p  ph.at(1)=q

That attempts to access indices past the end of the array, hence the exception.

The reason why I want to store it this way is....later I want to access the elements of a particular vector identified by its index i.e. 1 or 0.

For example I wish to access ph[0].size() i.e the size of p...identified by its index. That is, I want to perform the same operation as we are able to do in the case of simple arrays in c++ i.e. store the data in that array at a particular index and access the data from a particular index.

You can achieve this by first filling the outer vector with empty inner vectors:

vector<vector<int> > ph(2);

Here, the 2 says to construct the outer vector to contain 2 default-constructed elements (themselves vector<int>).

Alternatively, you can use:

vector<vector<int> > ph;
ph.resize(2);

After you've done that, it will be safe to access the elements to store non-default-constructed values. That means the following won't throw:

ph.at(0)=p;  ph.at(1)=q;

Upvotes: 0

Blastfurnace
Blastfurnace

Reputation: 18652

When you define vector<vector<int>> ph the vector ph is empty. The .at() member function does bounds checking and correctly throws an exception when you try to write to non-existing elements.

You could do ph.resize(2) so that indices 0-1 would then be valid or just use push_back like you do currently.

Upvotes: 0

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