I have the following data(example) -
1 - "Value1A"
1 - "Value1B"
1 - "Value1C"
2 - "Value2A"
2 - "Value2B"
I'm using Multimaps for the above data, such that the key 1, has 3 values(Value1A, Value1B, Value1C) and key 2 has 2 values(Value2A, Value2B).
When I try to retrieve all the values for a given key using the get function, it works. But I want to get the key given the value. i.e. if I have "Value1C", I want to use this to get its key 1, from the Multimap. Is this possible, if so how and if not what other than Multimap can I use to achieve this result.
Thanks for the help
https://www.npmjs.com/package/multimap
It is not possible to do this with a single operation, You will need to choose beetween use some extra memory or consume CPU resource.
Use more memory
In this case you need to store the data in a reverse mapping. So you will have another map to store as "Value1C" -> 1. This solution can cause consistency issues, since all the operations will need to be updated in both map. The original one and the reverse one.
The example for this code is basic:
//insert
map.set(1, "Value1C");
reverseMap.set("Value1C", 1);
//search
console.log(map.get(reverseMap.get("Value1C")));
Use more CPU
In this cause you will need to do a search throught all the values, this will be an O(n) complexity. It is not good if your list is too big, even worst in a single thread environment like Node.js.
Check the code example below:
function findValueInMultiMap(map, value, callback){
map.forEachEntry(function (entry, key) {
for(var e in entry){
if(entry[e]==value){
callback(map.get(key));
}
}
});
}
findValueInMultiMao(map, 'Value1C', function(values){
console.log(values);
});
Related
Take a windowed virtual list with the capability of loading an arbitrary range of rows at any point in the list, such as in this following example.
The virtual list provides a callback that is called anytime the user scrolls to some rows that have not been fetched from the backend yet, and provides the start and stop indexes, so that, in an offset based pagination endpoint, I can fetch the required items without fetching any unnecessary data.
const loadMoreItems = (startIndex, stopIndex) => {
fetch(`/items?offset=${startIndex}&limit=${stopIndex - startIndex}`);
}
I'd like to replace my offset based pagination with a cursor based one, but I can't figure out how to reproduce the above logic with it.
The main issue is that I feel like I will need to download all the items before startIndex in order to receive the cursor needed to fetch the items between startIndex and stopIndex.
What's the correct way to approach this?
After some investigation I found what seems to be the way MongoDB approaches the problem:
https://docs.mongodb.com/manual/reference/method/cursor.skip/#mongodb-method-cursor.skip
Obviously he same approach can be adopted by any other backend implementation.
They provide a skip method that allows to skip an arbitrary amount of items after the provided cursor.
This means my sample endpoint would look like the following:
/items?cursor=${cursor}&skip=${skip}&limit=${stopIndex - startIndex}
I then need to figure out the cursor and the skip values.
The following code could work to find the closest available cursor, given I store them together with the items:
// Limit our search only to items before startIndex
const fragment = items.slice(0, startIndex);
// Find the closest cursor index
const cursorIndex = fragment.length - 1 - fragment.reverse().findIndex(item => item.cursor != null);
// Get the cursor
const cursor = items[cursorIndex];
And of course, I also have a way to know the skip value:
const skip = items.length - 1 - cursorIndex;
I'm using a MongoDB mapReduce to code a ranking feed algorithm, it almost works but the latest thing to implement is the pagination. The map reduce supports the results limitation but how could I implement the offset (skipping) based e.g. on the latest viewed _id of the results, knowing that I'm using mongoose?
This is the procedure I wrote:
o = {};
o.map = function() {
//log10(likes+comments) / elapsed hours from the post creation
emit(Math.log(this.likes + this.comments + 1) / Math.LN10 / Math.abs((now - this.createdAt) / 6e7 + 1), this);
};
o.reduce = function(key, values) {
//sort the values, when they have the same score
values.sort(function(a, b) {
a.createdAt - b.createdAt;
});
//serialize the values, because mongoose does not support multiple returned values
return JSON.stringify(values);
};
o.scope = {now: new Date()};
o.limit = 15;
Posts.mapReduce(o, function(err, results) {
if (err) return console.log(err);
console.log(results);
});
Also, if the mapReduce it's not the way to go, do you suggest other on how to implement something like this?
What you need is a page delimiter which is not the id of the latest viewed as you say, but your sorting property. In this case, it seems to be the formula Math.log(this.likes + this.comments + 1) / Math.LN10 / Math.abs((now - this.createdAt) / 6e7 + 1).
So, in your mapReduce query needs to hold a where value of that formula above. Or specifically, 'formula >= . And also it needs to hold the value of createdAt at the last page, since you don't sort by that. (Assuming createdAt is unique). So yourqueryof mapReduce would saywhere: theFormulaExpression, createdAt: { $lt: lastCreatedAt }`
If you do allow multiple identical createdAt values, you have to play a little outside of the database itself.
So you just search by formula.
Ideally, that gives you one element with exactly that value, and the next ones sorted after that. So in reply to the module caller, remove this first element off the array (and make sure you actually ask for more results then you need because of this).
Now, since you allow for multiple similar values, you need another identifying prop, say, object id or created_at. Your consumer (caller of this module) will have to provide both (last value of the score, createdAt of the last object). Say you have a page split exactly in the middle - one or more objects is on the previous page, another set on the next
. You'd have to not simply remove the top value (because that same score is already served on the previous page), but possibly several of them from the top.
Then it goes really crazy, because potentially your whole page was already served - compare the _ids, look for the first one after the one your module caller has provided you with. Or look into the data and determine how many matching values like that are there, try to get at least as many more values from mapReduce then you have on your actual page size.
Aside from that, I would do this with aggregation instead, it should be much more preformant.
suppose i have the following data in my database:
[1,2],[2,1],[1,3],[3,1]...
were the numbers represent the a and b values of the formula a*x+b
what i now want is a query that returns the difference to a given point x,y.
for example: the point [2,6] is given. i want my query to return
[1,2] = -2 (1*2+2=4 4-6=-2)
[2,1] = -1 (2*2+1=5 5-6=-1)
[1,3] = -1 (1*2+3=5 4-6=-1)
[3,1] = 1 (3*2+1=7 7-6=-1)
I know how to do this in SQL but the data is already in a couchdb. I'm quite new to the NoSQL world and was wondering if something like this would be possible in couchdb.
what you can do is to use the standard MapReduce functionality of CouchDB.
Map is function you put in a view, which finds your data. You can have various criteria how to locate the docs you need. Next, if you specify so in the query with reduce=true, a reduce function is executed on each document that matched the map condition. You can use JavaScript to perform various operations on the document's values.
In your case, the map can look something like this:
function(doc) {
if(doc.a && doc.b) {
emit(doc._id,[doc.a, doc.b]);
}
}
then, the reduce gets called, like this:
function(keys, values, rereduce) {
var res;
//do something with values...
return res;
}
In your case keys will be list of document ID's and values will be the array of your a & b fields.
When you call the MapReduce (depending what method you use to access the DB), you should specify reduce=true.
Good resources on MapReduce (and on Views, Sorting and List funtions) are:
http://guide.couchdb.org/draft/views.html
http://www.slideshare.net/okurow/couchdb-mapreduce-13321353
Another way to go is to use a list function on the Map result, if you want to output the result in HTML. A good reason to use List function is that you can pass arguments to it with querystring, in your case it may be the point for which you want to calculate distances.
For detailed description on List functions, have a look here:
http://guide.couchdb.org/draft/transforming.html
Hope this helps.
I'm attempting to get a key value pair out of couch db. The key is the player id, and the value is how many games exist where it's their turn. I have a map method that successfully gets a list of playerID,gameID where the playerID is who's turn it is for the gameID. My reduce function is a simple length call.
function(keys, values){
return values.length;
}
When I run this from Futon, it runs fine. I get the sample output:
5,11
6,3
However, when I call it from Divan (C# lib for couchdb), I get the result
null, 14
My guess is it's merging these into one item through a rereduce. Is there a way to disable rereduce?
Thanks.
-Nick
No, you can't disable rereduce. However, the difference here is that Futon is adding group=true when calling your view but Divan is not, which explains the different results.
You should replace your reduce function with "_count" which correctly handles both the reduce and re-reduce cases. Your function returns the length of the values array, which is only correct for the reduce case. A correct solution in javascript would look like this;
function(keys, values, rereduce) {
if (rereduce) {
return sum(values);
} else {
return values.length
}
}
In the reduce call, then the values array contains whatever you emitted as the value, one entry for each emit. Since you're counting, you don't care what that value is, only how many of them there were. In the re-reduce call, the values array contains values previously emitted by a reduce call. Here the length of the values array is completely irrelevant, instead you want the sum of the lengths of previous reduce phases.
I have working on clustering algorithm. I decided to use hashmap to store the points because thinking that i can use as clusterID and as the point. I do a dfs fashion search to identify nearest and my calculation related work and all the looping on data take place outside of the method that I identify the clusters.
Also the intention of this clustering is that, if a point belongs to a same cluster its id remain the same. What I want to find out is that once i enter value in the hash map how can increase the index for the next value (Key would be same) with out using loop.
Here is how my method looks like, I took up some content of the algorithm out of since it really not relevant to the question.
public void dfsNearest(double point) {
double aPointInCluster = point;
if(!cluster.contains(aPointInCluster)) {
...
this.setNumOfClusters(this.getNumOfClusters() + 1);
mapOfCluster.put(this.getNumOfClusters(), aPointInCluster);
//after this i want to increase the index so no override happens
}
...
if(newNeighbor != 0.0) {
cluster.add(newNeighbor);
mapOfCluster.put(this.getNumOfClusters(), newNeighbor);
//want to increase the index....
...
if (!visitedMap.containsKey(newNeighbor)) {
dfsNearest(newNeighbor);
}
}
...
}
Thanks for any suggestions, also please let me know if rest of the code is necessary to make a good decision. Just wanted to keep it simple.