Inserting elements into 2D vector

Question

so I\'m creating a class that implements an adjacency list. Currently in my class definition I initialized two vectors:

vector> adjLi         


        

Answer 1

A couple of notes here.

Your loop can be significantly shortened just be using the constructors of your two members:

vector neighbors(1, 0); // set to length 1, value is zero
vector> adjList(numOfValues,neighbors); // "outer" vector is numOfValues long
.                                                   // each row is a *COPY* of neighbor

If you can't do this at construction time (maybe numOfValues isn't known yet), then there's still a better loop phrasing we can use:

// neighbors object can be reused
neighbors.clear(0);
neighbors.push_back(0);
adjList.reserve(numOfValues); // reserving memory ahead of time will prevent allocations
for (int i = 0; i < numOfValues; i++){
    adjList.push_back(neighbors); // push_back is by *COPY*
}

In your example, by using clear and push_back to essentially build the same vector every loop iteration, you are risking an allocation and deallocation each iteration. In practice, most implementations won't do this, but if we can both shorten and potentially make things more efficient, we may as well.

Lastly, if the number of neighbors is relatively small and similar row to row (for instance a finite elements code with tetrahedral elements, where each element connects to ~5 others), then as others have suggested you may be better off with a different structure than vector-of-vector. For instance, a single vector that is logically organized such that a new "row" begins every N elements.