CODEWITHSUNDEEP
c++vectorlinked-list
sgmm
sgmm

Reputation: 590

How do you create a vector of linked lists in C++?

I declared a vector of linked list (and the linked lists hold int values). Then I tried to push a value onto the linked list (i is just a position in the vector):

vector< list<int> > linkedListVector;
adjacencyVector[i].push_back(s);

The problem is that I run into a segmentation fault when I run the above command and I don't know why. I looked up the C++ documentation and my formatting looks correct. Any ideas?

Upvotes: 1

Views: 11092

Answers (4)

D&#225;niel S&#225;ndor
D&#225;niel S&#225;ndor

Reputation: 1294

Then I tried to push a value onto the linked list.

You tried to access an object, which didn't exist. The adjacencyVector[i] list does not exist as adjacencyVector is an empty vector if you did not put an element into it.

The following example will make some details clear.

#include <iostream>
#include <vector>
#include <list>

int main()
{
    std::vector<std::list<int> > v; // This statement creates an empty vector of lists of integers.
    std::list<int> l; // This statement creates an empty list of integers.
    for(int i=0; i<10; ++i){
        l.push_back(i);
        v.push_back(l);
    }
    for(unsigned int i=0; i<v.size(); ++i){ // You can access elements calling the operator[].
        for(std::list<int>::iterator it=v[i].begin(); it!=v[i].end(); ++it){
            std::cout<<*it<<' '; // You can access elements calling the operator[] , you need an iterator.
        }
        std::cout<<std::endl;
    }
    //std::cout<<*(v[v.size()].begin())<<std::endl; // It causes segmentation fault, because v[10] does not exist,
                                                  // 10 is out of range of valid indexes.
}

Vectors and lists are quit similar. Inserting and deleting are more effective if you use list, but accessing an element is usually more effective if you use vector.

Upvotes: 0

user3003064
user3003064

Reputation:

If you are using a for loop then you probably already know what the max size will be so you can just use the resize method like this:

adjacencyVector.resize(MAXSIZE)

Upvotes: 0

woolstar
woolstar

Reputation: 5083

The vector starts out with a size of zero, and you must create the vector element before pushing onto the list at [i].

If your i progresses in a normal fashion, you could push_back() onto the vector first, then onto the list in the vector. If not you could add something a bit ugly such as:

if ( adjacencyVector.size() <= i ) { adjacencyVector.resize(i + 1) ; }
adjacencyVector[i].push_back(s);

Upvotes: 2

Remy Lebeau
Remy Lebeau

Reputation: 595896

You have to add an instance of list<int> to the vector before you can then access a specific list<int> by index, then you can call push_back() on the list<int>. So either push_back() a list<int> into the vector:

vector< list<int> > adjacencyVector;

list<int> l;
adjacencyVector.push_back(l);
...
adjacencyVector[0].push_back(s);

Or call the vector's resize() method to add multiple lists at one time:

vector< list<int> > adjacencyVector;

adjacencyVector.resize(number of lists);
...
adjacencyVector[index].push_back(s);

Upvotes: 2

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