Bit of a lengthy one so those of you who like a challenge (or I'm simply not knowledgeable enough - hopefully it's an easy solution!) read on!
(skip to the actual question part to skip the explanation and what I've tried)
Problem
I have a site that has a dataset that contains an object with multiple objects inside. Each of those objects contains an array, and within that array there are multiple objects. (yes this is painful but its from an API and I need to use this dataset without changing or modifying it.) I am trying to filter the dataset based of the key-value pairs in the final object. However, I have multiple filters being executed at once.
Example of Path before looping which retrieves the key-value pair needed for one hall.
["Hamilton Hall"]["Hire Options"][2].Commercial
After Looping Path of required key-value pair for all halls, not just one (the hall identifier is stored):
[0]["Hire Options"][2].Commercial
Looping allows me to check each hall for a specific key-value pair (kind of like map or forEach, but for an object).
After getting that out of the way back to the question.
How would I go about filtering which of the looped objects are displayed?
What I have Tried
(userInput is defined elsewhere - this happens on a btn click btw)
let results = Object.keys(halls);
for (key of results) {
let weekend = [halls[ `${key}` ][ 'Hire Options' ][4][ 'Weekend function' ]];
if(userInput == weekend) {
outputAll([halls[ `${key}` ]]);
}
}
That filters it fine. However, I run into an issue here. I want to filter by multiple queries, and naturally adding an AND into the if statement doesn't work. I also dont want to have 10 if statements (I have 10+ filters of various data types I need to sort by).
I have recently heard of ternary operators, but do not know enough about them to know if that is the correct thing to do? If so, how? Also had a brief loook at switches, but doesnt seem to look like what I want (correct me if I am wrong.)
Actual Question minus the babble/explanation
Is there a way for me to dynamically modify an if statements conditions? Such as adding or removing conditions of an if statement? Such as if the filter for 'a' is set to off, remove the AND condition for 'a' in the if statement? This would mean that the results would only filter with the active filters.
Any help, comments or 'why haven't you tried this' remark are greatly appreciated!
Thanks!
Just for extra reference, here is the code for retrieving each of the objects from the first object as it loops through them:
(Looping Code)
halls = data[ 'Halls' ];
let results = Object.keys(halls);
for (key of results) {
let arr = [halls[ `${key}` ]];
outputAll(arr);
}
You can use Array.filter on the keys array - you can structure the logic for a match how you like - just make it return true if a match is found and the element needs to be displayed.
let results = Object.keys(halls);
results.filter(key => {
if (userInput == halls[key]['Hire Options'][4]['Weekend function']) {
return true;
}
if (some other condition you want to match) {
return true;
}
return false;
}).forEach(key => outputAll([halls[key]]));
I have an array of Strings in JavaScript. I am trying to develop a function that
Takes a substring as an input.
Searches through the array.
Returns strings from the array close to the substring. The list will be provided as suggestions to the caller.
For example:-
Array contains the below entries.
Hello
What is hello
World
Spacearenotthereinthishello
HELLO
Highway to hell
JavaScript
StackOverflow
I invoke the function as shown below
var result[] = searchFunc('hell');
The result array should contain
Hello
What is hello
Spacearenotthereinthishello
HELLO
Highway to hell
It is possible that the array could contain atleast 100 strings ( or more). I am looking for a scalable solution.
Initially, i figured i should sort and then do a binary search but its cumbersome to do if you wanna pull of all the suggestions from the master array for a particular string input. I am looking for algorithms that can help me achieve a faster search. I am not that worried about insertion timecomplexity in master array.
I did look up multiple stack overflow posts. They do speak about searching a big book for specific strings. None of them talk about returning suggestions from an array for a substring.
Your help is appreciated.
YourArray.filter() will do the job. Quick prototype:
var results = arrayName.filter(function(value) {
return value.toLowerCase().indexOf(searchStr.toLowerCase()) >= 0;
});
Quick Answer (TL;DR)
Use Array.prototype.filter()
Detailed Answer
Context
Javascript
Array filtering
Problem
Scenario: Developer wishes to filter an array for matches on a substring
Solution
// declare array of string fragments
myArrayOfStringFragments = /* TODO: get the content [ ... ] */
// optional step, normalize all strings to lower-case
/* TODO: input normalization with Array.prototype.map() or something similar */
// declare comparison function
function strContainsMatch(slookfor) {
return slookfor.includes("ello");
}
// iterate using Array.prototype.filter()
var filtered = myArrayOfStringFragments.filter(strContainsMatch);
Pitfalls
Data input normalization (are differences based on letter-case significant?)
string comparison based on non-ascii characters
string.includes() method may not be available, instead use string.indexOf()
avoiding false-postive matches based on chosen substring
(e.g., if you are looking for hello then hell will match more than you want, and it will miss Hello if your input is not normalized to lowercase)
See also
How can I only keep items of an array that match a certain condition?
Alternate solution would be to create a regex to do the same.
var condition = new RegExp("hell", 'g');
var results = arrayName.filter(function(value) {
return condition.test(value.toLowerCase())
});
I'm trying to write a spell-check script. Let's say I have a keyed list of 10 English words:
var wordList = {
"moas" : "moas",
"moat" : "moat",
"moated" : "moated",
"moating" : "moating",
"moatlike" : "moatlike",
"moats" : "moats",
"mob" : "mob",
"mobbed" : "mobbed",
"mobber" : "mobber",
"mobbers" : "mobbers",
}
and one misspelled word: "motelike" which I want to correct from the word list by finding the most similar word. If the closest word has a similarity above a certain threshold, I'll replace it.
I could put something together that loops through all of the words and checks each letter for a match, but that would be monstrously expensive to process when my dictionary is > 200,000 items). I think there must be a way to target the word's possible matches more efficiently than looping the whole array with the advantage of having a keyed list.
I can't think of how to go about doing this. It seems like it shouldn't be that hard, but I'm drawing a blank on how to get it done. Maybe something with regex involved?
The keyword you're looking for is fuzzy string searching. There are many libraries for that, for example fuzzyset.js. You use it like this:
f = FuzzySet(['moas', 'moat', 'moated', 'moating', 'moatlike', 'moats', 'mob', 'mobbed', 'mobber', 'mobbers']);
f.get('moateb');
// returns [[0.8333333333333334, 'moated']]
// (array of pairs [score, match])
Of course, you can implement it yourself instead of using a library. This wikipedia article is about this problem.
I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;
fbl.child('sell').limit(20).on("value", function(fbdata) {
// handle data display here
}
The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?
Since this answer was written, Firebase has added a feature that allows ordering by any child or by value. So there are now four ways to order data: by key, by value, by priority, or by the value of any named child. See this blog post that introduces the new ordering capabilities.
The basic approaches remain the same though:
1. Add a child property with the inverted timestamp and then order on that.
2. Read the children in ascending order and then invert them on the client.
Firebase supports retrieving child nodes of a collection in two ways:
by name
by priority
What you're getting now is by name, which happens to be chronological. That's no coincidence btw: when you push an item into a collection, the name is generated to ensure the children are ordered in this way. To quote the Firebase documentation for push:
The unique name generated by push() is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted.
The Firebase guide on ordered data has this to say on the topic:
How Data is Ordered
By default, children at a Firebase node are sorted lexicographically by name. Using push() can generate child names that naturally sort chronologically, but many applications require their data to be sorted in other ways. Firebase lets developers specify the ordering of items in a list by specifying a custom priority for each item.
The simplest way to get the behavior you want is to also specify an always-decreasing priority when you add the item:
var ref = new Firebase('https://your.firebaseio.com/sell');
var item = ref.push();
item.setWithPriority(yourObject, 0 - Date.now());
Update
You'll also have to retrieve the children differently:
fbl.child('sell').startAt().limitToLast(20).on('child_added', function(fbdata) {
console.log(fbdata.exportVal());
})
In my test using on('child_added' ensures that the last few children added are returned in reverse chronological order. Using on('value' on the other hand, returns them in the order of their name.
Be sure to read the section "Reading ordered data", which explains the usage of the child_* events to retrieve (ordered) children.
A bin to demonstrate this: http://jsbin.com/nonawe/3/watch?js,console
Since firebase 2.0.x you can use limitLast() to achieve that:
fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) {
// fbdataSnapshot is returned in the ascending order
// you will still need to order these 20 items in
// in a descending order
}
Here's a link to the announcement: More querying capabilities in Firebase
To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:
var fb = new Firebase(URL);
// listen for all changes and update
fb.endAt().limit(100).on('value', update);
// print the output of our array
function update(snap) {
var list = [];
snap.forEach(function(ss) {
var data = ss.val();
data['.priority'] = ss.getPriority();
data['.name'] = ss.name();
list.unshift(data);
});
// print/process the results...
}
Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.
This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.
DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.
There is really no way but seems we have the recyclerview we can have this
query=mCommentsReference.orderByChild("date_added");
query.keepSynced(true);
// Initialize Views
mRecyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
mManager = new LinearLayoutManager(getContext());
// mManager.setReverseLayout(false);
mManager.setReverseLayout(true);
mManager.setStackFromEnd(true);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(mManager);
I have a date variable (long) and wanted to keep the newest items on top of the list. So what I did was:
Add a new long field 'dateInverse'
Add a new method called 'getDateInverse', which just returns: Long.MAX_VALUE - date;
Create my query with: .orderByChild("dateInverse")
Presto! :p
You are searching limitTolast(Int x) .This will give you the last "x" higher elements of your database (they are in ascending order) but they are the "x" higher elements
if you got in your database {10,300,150,240,2,24,220}
this method:
myFirebaseRef.orderByChild("highScore").limitToLast(4)
will retrive you : {150,220,240,300}
In Android there is a way to actually reverse the data in an Arraylist of objects through the Adapter. In my case I could not use the LayoutManager to reverse the results in descending order since I was using a horizontal Recyclerview to display the data. Setting the following parameters to the recyclerview messed up my UI experience:
llManager.setReverseLayout(true);
llManager.setStackFromEnd(true);
The only working way I found around this was through the BindViewHolder method of the RecyclerView adapter:
#Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, int position) {
final SuperPost superPost = superList.get(getItemCount() - position - 1);
}
Hope this answer will help all the devs out there who are struggling with this issue in Firebase.
Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.
This will get the last 10 items of the list
FBRef.child("childName")
.limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example
This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().
LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.
this.FBRef
.child("childName")
.endAt(null, lastThreadId) // Get this from the previous step
.limitToLast(loadMoreLimit+2)
You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.
I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.
list = snapObjectAsArray; // The list is an array from snapObject
lastItemId = key; // get the first key of the list
if (list.length < loadMoreLimit+1) {
lastItemId = false;
}
if (list.length > loadMoreLimit+1) {
list.pop();
}
if (list.length > loadMoreLimit) {
list.shift();
}
// Return the list.reverse() and lastItemId
// If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
}
I'm using ReactFire for easy Firebase integration.
Basically, it helps me storing the datas into the component state, as an array. Then, all I have to use is the reverse() function (read more)
Here is how I achieve this :
import React, { Component, PropTypes } from 'react';
import ReactMixin from 'react-mixin';
import ReactFireMixin from 'reactfire';
import Firebase from '../../../utils/firebaseUtils'; // Firebase.initializeApp(config);
#ReactMixin.decorate(ReactFireMixin)
export default class Add extends Component {
constructor(args) {
super(args);
this.state = {
articles: []
};
}
componentWillMount() {
let ref = Firebase.database().ref('articles').orderByChild('insertDate').limitToLast(10);
this.bindAsArray(ref, 'articles'); // bind retrieved data to this.state.articles
}
render() {
return (
<div>
{
this.state.articles.reverse().map(function(article) {
return <div>{article.title}</div>
})
}
</div>
);
}
}
There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:
var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
offsetRef.on("value", function(snap) {
var offset = snap.val();
var estimatedServerTimeMs = new Date().getTime() + offset;
});
To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so
Setting
const timestamp = new Date().getTime();
const data = {
name: 'John Doe',
city: 'New York',
sort_key: timestamp * -1 // Gets the negative value of the timestamp
}
Getting
const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function
This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster
I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.
.reverse {
display: flex;
flex-direction: column-reverse;
}
myarray.reverse(); or this.myitems = items.map(item => item).reverse();
I did this by prepend.
query.orderByChild('sell').limitToLast(4).on("value", function(snapshot){
snapshot.forEach(function (childSnapshot) {
// PREPEND
});
});
Someone has pointed out that there are 2 ways to do this:
Manipulate the data client-side
Make a query that will order the data
The easiest way that I have found to do this is to use option 1, but through a LinkedList. I just append each of the objects to the front of the stack. It is flexible enough to still allow the list to be used in a ListView or RecyclerView. This way even though they come in order oldest to newest, you can still view, or retrieve, newest to oldest.
You can add a column named orderColumn where you save time as
Long refrenceTime = "large future time";
Long currentTime = "currentTime";
Long order = refrenceTime - currentTime;
now save Long order in column named orderColumn and when you retrieve data
as orderBy(orderColumn) you will get what you need.
just use reverse() on the array , suppose if you are storing the values to an array items[] then do a this.items.reverse()
ref.subscribe(snapshots => {
this.loading.dismiss();
this.items = [];
snapshots.forEach(snapshot => {
this.items.push(snapshot);
});
**this.items.reverse();**
},
For me it was limitToLast that worked. I also found out that limitLast is NOT a function:)
const query = messagesRef.orderBy('createdAt', 'asc').limitToLast(25);
The above is what worked for me.
PRINT in reverse order
Let's think outside the box... If your information will be printed directly into user's screen (without any content that needs to be modified in a consecutive order, like a sum or something), simply print from bottom to top.
So, instead of inserting each new block of content to the end of the print space (A += B), add that block to the beginning (A = B+A).
If you'll include the elements as a consecutive ordered list, the DOM can put the numbers for you if you insert each element as a List Item (<li>) inside an Ordered Lists (<ol>).
This way you save space from your database, avoiding unnecesary reversed data.
Can you suggest me an algorithm for filtering out data.
I am using javascript and trying to write out a filter function which filters an array of data.I have an array of data and an array of filters, so in order to apply each filter on every data, I have written 2 for loops
foreach(data)
{
foreach(filter)
{
check data with filter
}
}
this is not the proper code, but in short that what my function does, the problem is this takes a huge amount of time, can someone suggest a better method.
I am using the Mootools library and the array of data is JSON array
Details of data and Filter
Data is JSON array of lets say user, so it will be
data = [{"name" : "first", "email" : "first#first", "age" : "20"}.
{"name" : "second", "email" : "second#second", "age" : "21"}
{"name" : "third", "email" : "third#third", "age" : "22"}]
Array of filters is basically self define class for different fields of data
alFilter[0] = filterName;
alFilter[1] = filterEmail;
alFilter[2] = filterAge;
So when I enter the first for loop, I get a single JSON opbject (first row) in the above case.
When I enter the second for loop (filters loop) I have a filter class which extracts the exact field on which the current filter would work and check the filter with the appropriate field of the data.
So in my example
foreach(data)
{
foreach(filter)
{
//loop one - filter name
// loop two - filter email
// loop three - filter age
}
}
when the second loop ends i set a flag denoting if the data has been filtered or not and depending on it the data is displayed.
You're going to have to give us some more detail about the exact structure of your data and filters to really be able to help you out. Are the filters being used to select a subset of data, or to modify the data? What are the filters doing?
That said, there are a few general suggestions:
Do less work. Is there some way you can limit the amount of data you're working on? Some pre-filter that can run quickly and cut it down before you do your main loop?
Break out of the inner loop as soon as possible. If one of the filters rejects a datum, then break out of the inner loop and move on to the next datum. If this is possible, then you should also try to make the most selective filters come first. (This is assuming that your filters are being used to reject items out of the list, rather than modify them)
Check for redundancy in the computation the filters perform. If each of them performs some complicated calculations that share some subroutines, then perhaps memoization or dynamic programming may be used to avoid redundant computation.
Really, it all boils down to the first point, do less work, at all three levels of your code. Can you do less work by limiting the items in the outer loop? Do less work by stopping after a particular filter and doing the most selective filters first? Do less work by not doing any redundant computation inside of each filter?
That's pretty much how you should do it. The trick is to optimize that "check data with filter"-part. You need to traverse all your data and check against all your filters - you'll not going to get any faster than that.
Avoid string comparisons, use data models as native as possible, try to reduce the data set on each pass with filter, etc.
Without further knowledge, it's hard to optimize this for you.
You should sort the application of your filters, so that two things are optimized: expensive checks should come last, and checks that eliminate a lot of data should come first. Then, you should make sure that checking is cut short as soon as an "out" result occurs.
If your filters are looking for specific values, a range, or start of a text then jOrder (http://github.com/danstocker/jorder) will fit your problem.
All you need to do is create a jOrder table like this:
var table = jOrder(data)
.index('name', ['name'], { grouped: true, ordered: true })
.index('email', ['email'])
.index('age', ['age'], { grouped: true, ordered: true, type: jOrder.number });
And then call table.where() to filter the table.
When you're looking for exact matches:
filtered = table.where([{name: 'first'}, {name: 'second'}]);
When you're looking for a certain range of one field:
filtered = table.where([{age: {lower: 20, upper: 21}}], {mode: jOrder.range});
Or, when you're looking for values starting with a given string:
filtered = table.where([{name: 'fir'}], {mode: jOrder.startof});
Filtering will be magnitudes faster this way than with nested loops.
Supposing that a filter removes the data if it doesn't match, I suggest, that you switch the two loops like so:
foreach(filter) {
foreach(data) {
check data with filter
}
}
By doing so, the second filter doesn't have to work all data, but only the data that passed the first filter, and so on. Of course the tips above (like doing expensive checks last) are still true and should additionally be considered.