Need help solving a problem using graphs in C - c

i'm coding a c project for an algorithm class and i really need some help!
Here's the problem:
I have a set of names like this one N = (James,John,Robert,Mary,Patricia,Linda Barbara) wich are stored in an RB tree.
Starting from this set of names a series of couple like those ones are formed:
(James,Mary)
(James,Patricia)
(John,Linda)
(John,Barbara)
(Robert,Linda)
(Robert,Barbara)
Now i need to merge the elements in a way that i can form n subgroups with the constraint that each pairing is respected and the group has the smallest possible cardinality.
With the couples in the example they will form two groups
(James,Mary,Patricia) and (John,Robert,Barbara,Linda).
The task is to return the maximum number of groups formed and the number of males and females in the group with the maximum cardinality.
In this case it would be 2 2 2
I was thinking about building a graph where every name is represented by a vertex and two vertex are in an edge only if they are paired.
I can then use an algorithm (like Kruskal) to find the Minimum spanning tree.Is that right?
The problem is that the graph would not be completely connected.
I also need to find a way to map the names to the edges of the Graph and vice-versa.
Can the edges be indexed by a string?
Every help is really appreciated :)
Thanks in advice!

You don't need to find the minimum spanning tree. That is really for finding the "best" edges in a graph that will still keep the graph connected. In other words, you don't care how John and Robert are connected, just that they are.
You say that the problem is that the graph would not be completely connected, but I think that is actually the point. If you represent graph edges by using the couples as you suggest, then the vertices that are connected form the groups that you are looking for.
In your example, James is connected to Mary and also James is connected to Patricia. No other person connects to any of those three vertices (if they did, you would have another couple that included them), which is why they form a single group of (James, Mary, Patricia). Similarly all of John, Robert, Barbara, and Linda are connected to each other.
Your task is really to form the graph and find all of the connected subgraphs that are disjoint from each other.
While not a full algorithm, I hope that helps get you started.

I think that you can easily solve this with a dfs and connected components. Because every person(node) has a relation with an other one (edge). So you have an outer loop and run an explore function for every node which is unvisited and add the same number for every node explored by the explore function.
e.g
dfs() {
int group 0;
for(int i=0;i<num_nodes;i++) {
if(nodes[i].visited==false){
explore(nodes[i],group);
group++;
}
}
then you simple have to sort the node by the group and then you are ready. if you want to track the path you can use a pre number which indicates which node was explored first, second..etc
(sorry for my bad english)!

The sets of names and pairs of names already form a graph. A data structure with nodes and pointers to other nodes is just another representation, one that you don't necessarily need. Disjoint sets are easier to implement IMO, and their purpose in life is exactly to keep track of sameness as pairs of things are joined together.

Related

How do I get all nodes that do not contain (relate) nodes with certain property in Cypher Neo4j

I have a case where I need to find nodes that do not contain (relate) all the required nodes.
My business logic is as follows:
* A Trajectory contains several Points.
* A Trajectory is complete when it has at least:
* ONE Point START
* ONE Point MIDDLE
* ONE Point FINISH
In the following example, I have 4 Trajectories
http://console.neo4j.org/?id=1fjeyl
One Trajectory is complete and the other three are incomplete.
How can I find all the trajectories that do not contain all the required points?
There's a few ways you can do this.
With this model, one way is you can collect the nodes per trajectory and use list predicates to only include trajectories where any of the required positions are missing:
MATCH (t:Trajectory)-[:CONTAINS]->(p)
WITH t, collect(DISTINCT p.pos) as pointPositions
WHERE size(pointPositions) < 3 OR any(required in ['START', 'END', 'MIDDLE'] WHERE NOT required IN pointPositions)
RETURN t
Note that if you refactor your model such that the position of a point is indicated by the relationship point instead, such as:
(:Trajectory)-[:HAS_START]->(:Point)
(:Trajectory)-[:HAS_END]->(:Point)
(:Trajectory)-[:HAS_MIDDLE]->(:Point)
Then your query will be a bit simpler and efficiency will improve (which will show the most gain when you have lots of trajectories, some with lots of connected nodes).
MATCH (t:Trajectory)
WHERE NOT (t)-[:HAS_START]->() OR NOT (t)-[:HAS_END]->() OR NOT (t)-[:HAS_MIDDLE]->()
RETURN t
With that modeling and this kind of query, we don't even have to expand out from trajectory nodes to get our answer, since a node knows what relationships are connected to it (by type and/or direction) and their count. It's very easy then to determine if certain relationship types are or aren't present.

effective way to store and access an existing list of videogame characters with counters and synnergies with C++

I'm attempting to make a DotA drafting companion application in C++
I'll give you a little run down of how it will work. In DotA there are two teams of 5 players. No character can be picked twice in a single match, meaning teams cannot share a character. It is important to keep in mind both your team's and opponent's picks. So what this will do is suggest characters for you to pick based on your and your opponent's teams.
To clarify, the existing list is not some sort of database, just a menagerie of suggestions on various websites, forums, and videos. I would be organizing the list myself, and so my question is ultimately what format should I create this list in?
So for instance a popular character is Phatom Assassin who is strong with characters who can reduce armor, and weak against characters who can do lots of damage.
So a hero class might look like this
class DotaHero
{
Name = "Phantom Assassin";
vector<DotaHero> Counters{"Lina", "Lion"};
vector<DotaHero> Friends{"Templar Assassin", "Shadow Demon"};
CImg<unsigned char> src("PA.jpg");
}
My program would allow you to input the first few heroes and then display a list of characters as a suggestion.
Should I...
Create a class for each and every hero in the game
Stream from a text file with each hero delineated by counters/friends
Create some sort of database to store each hero's counters/friends in
Please feel free to give any suggestions!
A very difficult question.
The question of what you store and how your express relationships between characters is going to define fundamentally how your tool works and how effective it is.
Of the top of my head I would create 2 weighted graphs,
1.) friends.
2.) counters.
Each node represents a hero and each edge of the graph would correspond to how strong of a counter or how strong of a friend you consider that hero. As heros are picks, find the highest weighted neighbor node to all of the current picked hero, and this would be your highest weighted suggestion. That would be the naive approach based only on the next pick (not always the best, a good way to get into trouble). Add layers and create weightings for 2nd order neighbors and you can start doing non deterministic analysis of the data set to get more sophisticated results.
Just a jumping off point. It would be a truly fascinating problem.

Need algorithm for fast storage and retrieval (search) of sets and subsets

I need a way of storing sets of arbitrary size for fast query later on.
I'll be needing to query the resulting data structure for subsets or sets that are already stored.
===
Later edit: To clarify, an accepted answer to this question would be a link to a study that proposes a solution to this problem. I'm not expecting for people to develop the algorithm themselves.
I've been looking over the tuple clustering algorithm found here, but it's not exactly what I want since from what I understand it 'clusters' the tuples into more simple, discrete/aproximate forms and loses the original tuples.
Now, an even simpler example:
[alpha, beta, gamma, delta] [alpha, epsilon, delta] [gamma, niu, omega] [omega, beta]
Query:
[alpha, delta]
Result:
[alpha, beta, gama, delta] [alpha, epsilon, delta]
So the set elements are just that, unique, unrelated elements. Forget about types and values. The elements can be tested among them for equality and that's it. I'm looking for an established algorithm (which probably has a name and a scientific paper on it) more than just creating one now, on the spot.
==
Original examples:
For example, say the database contains these sets
[A1, B1, C1, D1], [A2, B2, C1], [A3, D3], [A1, D3, C1]
If I use [A1, C1] as a query, these two sets should be returned as a result:
[A1, B1, C1, D1], [A1, D3, C1]
Example 2:
Database:
[Gasoline amount: 5L, Distance to Berlin: 240km, car paint: red]
[Distance to Berlin: 240km, car paint: blue, number of car seats: 2]
[number of car seats: 2, Gasoline amount: 2L]
Query:
[Distance to berlin: 240km]
Result
[Gasoline amount: 5L, Distance to Berlin: 240km, car paint: red]
[Distance to Berlin: 240km, car paint: blue, number of car seats: 2]
There can be an unlimited number of 'fields' such as Gasoline amount. A solution would probably involve the database grouping and linking sets having common states (such as Gasoline amount: 240) in such a way that the query is as efficient as possible.
What algorithms are there for such needs?
I am hoping there is already an established solution to this problem instead of just trying to find my own on the spot, which might not be as efficient as one tested and improved upon by other people over time.
Clarifications:
If it helps answer the question, I'm intending on using them for storing states:
Simple example:
[Has milk, Doesn't have eggs, Has Sugar]
I'm thinking such a requirement might require graphs or multidimensional arrays, but I'm not sure
Conclusion
I've implemented the two algorithms proposed in the answers, that is Set-Trie and Inverted Index and did some rudimentary profiling on them. Illustrated below is the duration of a query for a given set for each algorithm. Both algorithms worked on the same randomly generated data set consisting of sets of integers. The algorithms seem equivalent (or almost) performance wise:
I'm confident that I can now contribute to the solution. One possible quite efficient way is a:
Trie invented by Frankling Mark Liang
Such a special tree is used for example in spell checking or autocompletion and that actually comes close to your desired behavior, especially allowing to search for subsets quite conveniently.
The difference in your case is that you're not interested in the order of your attributes/features. For your case a Set-Trie was invented by Iztok Savnik.
What is a Set-Tree? A tree where each node except the root contains a single attribute value (number) and a marker (bool) if at this node there is a data entry. Each subtree contains only attributes whose values are larger than the attribute value of the parent node. The root of the Set-Tree is empty. The search key is the path from the root to a certain node of the tree. The search result is the set of paths from the root to all nodes containing a marker that you reach when you go down the tree and up the search key simultaneously (see below).
But first a drawing by me:
The attributes are {1,2,3,4,5} which can be anything really but we just enumerate them and therefore naturally obtain an order. The data is {{1,2,4}, {1,3}, {1,4}, {2,3,5}, {2,4}} which in the picture is the set of paths from the root to any circle. The circles are the markers for the data in the picture.
Please note that the right subtree from root does not contain attribute 1 at all. That's the clue.
Searching including subsets Say you want to search for attributes 4 and 1. First you order them, the search key is {1,4}. Now startin from root you go simultaneously up the search key and down the tree. This means you take the first attribute in the key (1) and go through all child nodes whose attribute is smaller or equal to 1. There is only one, namely 1. Inside you take the next attribute in the key (4) and visit all child nodes whose attribute value is smaller than 4, that are all. You continue until there is nothing left to do and collect all circles (data entries) that have the attribute value exactly 4 (or the last attribute in the key). These are {1,2,4} and {1,4} but not {1,3} (no 4) or {2,4} (no 1).
Insertion Is very easy. Go down the tree and store a data entry at the appropriate position. For example data entry {2.5} would be stored as child of {2}.
Add attributes dynamically Is naturally ready, you could immediately insert {1,4,6}. It would come below {1,4} of course.
I hope you understand what I want to say about Set-Tries. In the paper by Iztok Savnik it's explained in much more detail. They probably are very efficient.
I don't know if you still want to store the data in a database. I think this would complicate things further and I don't know what is the best to do then.
How about having an inverse index built of hashes?
Suppose you have your values int A, char B, bool C of different types. With std::hash (or any other hash function) you can create numeric hash values size_t Ah, Bh, Ch.
Then you define a map that maps an index to a vector of pointers to the tuples
std::map<size_t,std::vector<TupleStruct*> > mymap;
or, if you can use global indices, just
std::map<size_t,std::vector<size_t> > mymap;
For retrieval by queries X and Y, you need to
get hash value of the queries Xh and Yh
get the corresponding "sets" out of mymap
intersect the sets mymap[Xh] and mymap[Yh]
If I understand your needs correctly, you need a multi-state storing data structure, with retrievals on combinations of these states.
If the states are binary (as in your examples: Has milk/doesn't have milk, has sugar/doesn't have sugar) or could be converted to binary(by possibly adding more states) then you have a lightning speed algorithm for your purpose: Bitmap Indices
Bitmap indices can do such comparisons in memory and there literally is nothing in comparison on speed with these (ANDing bits is what computers can really do the fastest).
http://en.wikipedia.org/wiki/Bitmap_index
Here's the link to the original work on this simple but amazing data structure: http://www.sciencedirect.com/science/article/pii/0306457385901086
Almost all SQL databases supoort Bitmap Indexing and there are several possible optimizations for it as well(by compression etc.):
MS SQL: http://technet.microsoft.com/en-us/library/bb522541(v=sql.105).aspx
Oracle: http://www.orafaq.com/wiki/Bitmap_index
Edit:
Apparently the original research work on bitmap indices is no longer available for free public access.
Links to recent literature on this subject:
Bitmap Index Design Choices and Their Performance
Implications
Bitmap Index Design and Evaluation
Compressing Bitmap Indexes for Faster Search Operations
This problem is known in the literature as subset query. It is equivalent to the "partial match" problem (e.g.: find all words in a dictionary matching A??PL? where ? is a "don't care" character).
One of the earliest results in this area is from this paper by Ron Rivest from 19761. This2 is a more recent paper from 2002. Hopefully, this will be enough of a starting point to do a more in-depth literature search.
Rivest, Ronald L. "Partial-match retrieval algorithms." SIAM Journal on Computing 5.1 (1976): 19-50.
Charikar, Moses, Piotr Indyk, and Rina Panigrahy. "New algorithms for subset query, partial match, orthogonal range searching, and related problems." Automata, Languages and Programming. Springer Berlin Heidelberg, 2002. 451-462.
This seems like a custom made problem for a graph database. You make a node for each set or subset, and a node for each element of a set, and then you link the nodes with a relationship Contains. E.g.:
Now you put all the elements A,B,C,D,E in an index/hash table, so you can find a node in constant time in the graph. Typical performance for a query [A,B,C] will be the order of the smallest node, multiplied by the size of a typical set. E.g. to find {A,B,C] I find the order of A is one, so I look at all the sets A is in, S1, and then I check that it has all of BC, since the order of S1 is 4, I have to do a total of 4 comparisons.
A prebuilt graph database like Neo4j comes with a query language, and will give good performance. I would imagine, provided that the typical orders of your database is not large, that its performance is far superior to the algorithms based on set representations.
Hashing is usually an efficient technique for storage and retrieval of multidimensional data. Problem is here that the number of attributes is variable and potentially very large, right? I googled it a bit and found Feature Hashing on Wikipedia. The idea is basically the following:
Construct a hash of fixed length from each data entry (aka feature vector)
The length of the hash must be much smaller than the number of available features. The length is important for the performance.
On the wikipedia page there is an implementation in pseudocode (create hash for each feature contained in entry, then increase feature-vector-hash at this index position (modulo length) by one) and links to other implementations.
Also here on SO is a question about feature hashing and amongst others a reference to a scientific paper about Feature Hashing for Large Scale Multitask Learning.
I cannot give a complete solution but you didn't want one. I'm quite convinced this is a good approach. You'll have to play around with the length of the hash as well as with different hashing functions (bloom filter being another keyword) to optimize the speed for your special case. Also there might still be even more efficient approaches if for example retrieval speed is more important than storage (balanced trees maybe?).

Enumerating Graphs with Self-Loops

Brendan McKay has already done the work for finding all non-isomorphic graphs of n variables that can be found here (under Simple Graphs): http://cs.anu.edu.au/~bdm/data/graphs.html
I believe this was done using polya enumeration, which I understand the basics of. I would like to expand on this, and allow self loops in these graphs. So, i'd like to find all non-ismorphic graphs of n variables, including self loops. This will be directly used for another part of my code and provide a massive optimization. I'm just not quite sure how to go about it.
To be clear, Brendan Mckay's files give all non ismorphic graphs, ie in edge notation,
1-2 1-3
is a graph with an edge between vertex 1 and 2, and 1 and 3. I want this list to also include self loops, ie:
1-2 1-3 1-1
or
1-2 1-3 1-1 2-2
I want the minimum number of graphs, so all non-ismorphic ones. How can I go about finding them, hopefully using the data Brendan McKay has available for simple graphs?
First of all, you should observe that if two graphs are not isomorphic, then these graphs with some additional self loops are not isomorphic.
If you need this during programming and size of graphs is small, I would generate for each non iso graph all possible self loop graphs.
Easiest thing to do is to add additional node, and every node with self loop will be connected with given node. (instead of having loop) Using nauty you can check if any two are isomorphic. You can additionally speed up things if you observe that if two loop encoded versions are iso, then they have to have same number of connections with "special" node.

Suggestions of the easiest algorithms for some Graph operations

The deadline for this project is closing in very quickly and I don't have much time to deal with what it's left. So, instead of looking for the best (and probably more complicated/time consuming) algorithms, I'm looking for the easiest algorithms to implement a few operations on a Graph structure.
The operations I'll need to do is as follows:
List all users in the graph network given a distance X
List all users in the graph network given a distance X and the type of relation
Calculate the shortest path between 2 users on the graph network given a type of relation
Calculate the maximum distance between 2 users on the graph network
Calculate the most distant connected users on the graph network
A few notes about my Graph implementation:
The edge node has 2 properties, one is of type char and another int. They represent the type of relation and weight, respectively.
The Graph is implemented with linked lists, for both the vertices and edges. I mean, each vertex points to the next one and each vertex also points to the head of a different linked list, the edges for that specific vertex.
What I know about what I need to do:
I don't know if this is the easiest as I said above, but for the shortest path between 2 users, I believe the Dijkstra algorithm is what people seem to recommend pretty often so I think I'm going with that.
I've been searching and searching and I'm finding it hard to implement this algorithm, does anyone know of any tutorial or something easy to understand so I can implement this algorithm myself? If possible, with C source code examples, it would help a lot. I see many examples with math notations but that just confuses me even more.
Do you think it would help if I "converted" the graph to an adjacency matrix to represent the links weight and relation type? Would it be easier to perform the algorithm on that instead of the linked lists? I could easily implement a function to do that conversion when needed. I'm saying this because I got the feeling it would be easier after reading a couple of pages about the subject, but I could be wrong.
I don't have any ideas about the other 4 operations, suggestions?
List all users in the graph network given a distance X
A distance X from what? from a starting node or a distance X between themselves? Can you give an example? This may or may not be as simple as doing a BF search or running Dijkstra.
Assuming you start at a certain node and want to list all nodes that have distances X to the starting node, just run BFS from the starting node. When you are about to insert a new node in the queue, check if the distance from the starting node to the node you want to insert the new node from + the weight of the edge from the node you want to insert the new node from to the new node is <= X. If it's strictly lower, insert the new node and if it is equal just print the new node (and only insert it if you can also have 0 as an edge weight).
List all users in the graph network given a distance X and the type of relation
See above. Just factor in the type of relation into the BFS: if the type of the parent is different than that of the node you are trying to insert into the queue, don't insert it.
Calculate the shortest path between 2 users on the graph network given a type of relation
The algorithm depends on a number of factors:
How often will you need to calculate this?
How many nodes do you have?
Since you want easy, the easiest are Roy-Floyd and Dijkstra's.
Using Roy-Floyd is cubic in the number of nodes, so inefficient. Only use this if you can afford to run it once and then answer each query in O(1). Use this if you can afford to keep an adjacency matrix in memory.
Dijkstra's is quadratic in the number of nodes if you want to keep it simple, but you'll have to run it each time you want to calculate the distance between two nodes. If you want to use Dijkstra's, use an adjacency list.
Here are C implementations: Roy-Floyd and Dijkstra_1, Dijkstra_2. You can find a lot on google with "<algorithm name> c implementation".
Edit: Roy-Floyd is out of the question for 18 000 nodes, as is an adjacency matrix. It would take way too much time to build and way too much memory. Your best bet is to either use Dijkstra's algorithm for each query, but preferably implementing Dijkstra using a heap - in the links I provided, use a heap to find the minimum. If you run the classical Dijkstra on each query, that could also take a very long time.
Another option is to use the Bellman-Ford algorithm on each query, which will give you O(Nodes*Edges) runtime per query. However, this is a big overestimate IF you don't implement it as Wikipedia tells you to. Instead, use a queue similar to the one used in BFS. Whenever a node updates its distance from the source, insert that node back into the queue. This will be very fast in practice, and will also work for negative weights. I suggest you use either this or the Dijkstra with heap, since classical Dijkstra might take a long time on 18 000 nodes.
Calculate the maximum distance between 2 users on the graph network
The simplest way is to use backtracking: try all possibilities and keep the longest path found. This is NP-complete, so polynomial solutions don't exist.
This is really bad if you have 18 000 nodes, I don't know any algorithm (simple or otherwise) that will work reasonably fast for so many nodes. Consider approximating it using greedy algorithms. Or maybe your graph has certain properties that you could take advantage of. For example, is it a DAG (Directed Acyclic Graph)?
Calculate the most distant connected users on the graph network
Meaning you want to find the diameter of the graph. The simplest way to do this is to find the distances between each two nodes (all pairs shortest paths - either run Roy-Floyd or Dijkstra between each two nodes and pick the two with the maximum distance).
Again, this is very hard to do fast with your number of nodes and edges. I'm afraid you're out of luck on these last two questions, unless your graph has special properties that can be exploited.
Do you think it would help if I "converted" the graph to an adjacency matrix to represent the links weight and relation type? Would it be easier to perform the algorithm on that instead of the linked lists? I could easily implement a function to do that conversion when needed. I'm saying this because I got the feeling it would be easier after reading a couple of pages about the subject, but I could be wrong.
No, adjacency matrix and Roy-Floyd are a very bad idea unless your application targets supercomputers.
This assumes O(E log V) is an acceptable running time, if you're doing something online, this might not be, and it would require some higher powered machinery.
List all users in the graph network given a distance X
Djikstra's algorithm is good for this, for one time use. You can save the result for future use, with a linear scan through all the vertices (or better yet, sort and binary search).
List all users in the graph network given a distance X and the type of relation
Might be nearly the same as above -- just use some function where the weight would be infinity if it is not of the correct relation.
Calculate the shortest path between 2 users on the graph network given a type of relation
Same as above, essentially, just determine early if you match the two users. (Alternatively, you can "meet in the middle", and terminate early if you find someone on both shortest path spanning tree)
Calculate the maximum distance between 2 users on the graph network
Longest path is an NP-complete problem.
Calculate the most distant connected users on the graph network
This is the diameter of the graph, which you can read about on Math World.
As for the adjacency list vs adjacency matrix question, it depends on how densely populated your graph is. Also, if you want to cache results, then the matrix might be the way to go.
The simplest algorithm to compute shortest path between two nodes is Floyd-Warshall. It's just triple-nested for loops; that's it.
It computes ALL-pairs shortest path in O(N^3), so it may do more work than necessary, and will take a while if N is huge.

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