Erasing duplicates from nested map (k-core algorithm) - c++

Question

i am trying to finish a homework and came with an error.
What i am trying to do is: There is an unordered map with keys containing values as unordered_set which may have keys inside that "may have the same names as keys of unordered outer map".
To be spesific, i am trying to implement a k-core algorithm, the count and k numbers are there for it. I am trying to erase unordered_map keys (lets call P) that have unordered_sets(containing neighbors, call N) which have lower than k sizes.
But since i delete the N entry from the P node, i also need to find the N entry from the outer map and find P and delete it there.
The following code is what i came up with so far (there is more but i don't want plagiarism or anything). During the debugging i noticed that somewhat cout<<nodes.size() never executes. I don't know what is the problem. Probably the inner while infinitely loops, and it is related with how iterators function with erase etc. but i am not sure. Hope it is clear.
EDIT : I have updated the code according to the suggestion, but there is a new problem: for k=1 the amount of nodes are right. But after k=2,3,4.. there is infinite loop of outermost loop. And the size stays constant (after taking out nodes with k=1)
while(nodes.size()!=0)
{
int count = 0;
while(stillLeft){
stillLeft = 0;
it = nodes.begin();
while(it != nodes.end()){
if(it->second.size()<=k){
stillLeft = 1;
++count;
for ( innerit = it->second.begin(); innerit != it->second.end();innerit++ ){
nodes.find(*innerit)->second.erase(it->first);
}
it = nodes.erase(it);
}
else{
++it;
}
}
}
cout << nodes.size() << endl;
kCoreNumbers.push_back(count);
++k;
}

Answer 1

You don't need to loop till while(nodes.size()!=0) i.e. till nodes is empty. After running the k-core algorithm, you might be left with nodes whose vertexes have a degree greater than k. You only need to loop till there are no more nodes that have a degree less than k.
Your initial code was in the right direction but with the problem of deleting a node in the map while traversing it. I solved it by just adding the vertices to be deleted to a vector while iterating the graph and then after each graph traversal just deleted all the nodes makred for deletion. Here's my version of code, based off yours with a few variable name changes to reflect what they are:
void runKCore(size_t k) {
bool nodesDeleted;
do {
// To keep track of how many times the loop needs to run
nodesDeleted = false;
// To keep track of the nodes to be deleted after one pass
std::vector<int> markDeleted;
for(auto const& node: graph) {
auto vertex = node.first;
auto const& neighbours = node.second;
// Pick vertex if it has less than k neighbours
if(neighbours.size() < k) {
for(auto neighbour: neighbours) {
// Get the neighbour's list of vertices
auto& neighBoursEdges = graph.at(neighbour);
// Remove vertex from this list
neighBoursEdges.erase(vertex);
}
// Mark this node as deleted
markDeleted.push_back(vertex);
}
}
// Update nodesDeleted if any nodes were marked for deletion
nodesDeleted = markDeleted.size() > 0;
for(auto node: markDeleted) {
// Erase the node and it's neighbours
graph.erase(node);
}
}while(nodesDeleted);
}
Here is the complete program.
For an initial graph
translating to initial graph
8: 6 5
7: 6 3 4
6: 7 3 8 5 4 2
5: 8 6 2 1
4: 7 3 6 2
3: 7 6 4 2
2: 6 3 4 5 1 0
1: 5 2 0
0: 2 1
the output of the program after running the k-core algo for k = 3 output is:
7: 6 3 4
6: 7 3 4 2
4: 7 3 6 2
3: 7 6 4 2
2: 6 3 4