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Why I met this problem:
I tried to solve an algorithm problem and I need to return the number which appeared most of the times in an array. Like [5,4,3,2,1,1] should return 1.
And also when two number appear same time as the maximum appearance return the one came first. Like [5,5,2,2,1] return 5 because 5 appear first. I use an object to store the appearance of each number. The key is the number itself.
So When the input is [5,5,2,2,1] my object should be
Object {5: 2, 2: 2, 1: 1} but actually I got Object {1: 1, 2: 2, 5: 2}
So When I use for..in to iterate the object I got 2 returned instead of 5 . So that's why I asked this question.
This problem occurs in Chrome console and I'm not sure if this is a common issue:
When I run the following code
var a = {};
a[0]=1;
a[1]=2;
a[2]=3;
a is: Object {0: 1, 1: 2, 2: 3}
But when I reverse the order of assignment like:
var a = {};
a[2]=3;
a[1]=2;
a[0]=1;
a is also:Object {0: 1, 1: 2, 2: 3}
The numeric property automatic sorted in ascending order.
I tried prefix or postfix the numeric property like
var a = {};
a['p'+0]=1;
a['p'+1]=2;
a['p'+2]=3;
console.log(a);//Object {p0: 1, p1: 2, p2: 3}
And this keep the property order. Is this the best way to solve the problem? And is there anyway to prevent this auto sort behavior? Is this only happen in Chrome V8 JavaScript engine? Thank you in advance!
target = {}
target[' ' + key] = value // numeric key
This can prevent automatic sort of Object numeric property.
You really can't rely on order of an object fields in JavaScript, but I can suggest to use Map (ES6/ES2015 standard) if you need to preserve order of your key, value pair object. See the snippet below:
let myObject = new Map();
myObject.set('z', 33);
myObject.set('1', 100);
myObject.set('b', 3);
for (let [key, value] of myObject) {
console.log(key, value);
}
// z 33
// 1 100
// b 3
You are using a JS object, that by definition does not keep order. Think of it as a key => value map.
You should be using an array, that will keep whatever you insert on the index you inserted it into. Think of it as a list.
Also notice that you did not in fact "reverse the order of the assignment", because you inserted elements on the same index every time.
This is an old topic but it is still worth mentioning as it is hard to find a straight explanation in one-minute googling.
I recently had a coding exercise that finding the first occurrence of the least/most frequent integer in an array, it is pretty much the same as your case.
I encountered the same problem as you, having the numeric keys sorted by ASC in JavaScript object, which is not preserving the original order of elements, which is the default behavior in js.
A better way to solve this in ES6 is to use a new data type called: Map
Map can preserve the original order of elements(pairs), and also have the unique key benefit from object.
let map = new Map()
map.set(4, "first") // Map(1) {4 => "first"}
map.set(1, "second") // Map(2) {4 => "first", 1 => "second"}
map.set(2, "third") // Map(3) {4 => "first", 1 => "second", 2 => "third"}
for(let [key, value] of map) {
console.log(key, value)
}
// 4 "first"
// 1 "second"
// 2 "third"
However, using the object data type can also solve the problem, but we need the help of the input array to get back the original order of elements:
function findMostAndLeast(arr) {
let countsMap = {};
let mostFreq = 0;
let leastFreq = arr.length;
let mostFreqEl, leastFreqEl;
for (let i = 0; i < arr.length; i++) {
let el = arr[i];
// Count each occurrence
if (countsMap[el] === undefined) {
countsMap[el] = 1;
} else {
countsMap[el] += 1;
}
}
// Since the object is sorted by keys by default in JS, have to loop again the original array
for (let i = 0; i < arr.length; i++) {
const el = arr[i];
// find the least frequent
if (leastFreq > countsMap[el]) {
leastFreqEl = Number(el);
leastFreq = countsMap[el];
}
// find the most frequent
if (countsMap[el] > mostFreq) {
mostFreqEl = Number(el);
mostFreq = countsMap[el];
}
}
return {
most_frequent: mostFreqEl,
least_frequent: leastFreqEl
}
}
const testData = [6, 1, 3, 2, 4, 7, 8, 9, 10, 4, 4, 4, 10, 1, 1, 1, 1, 6, 6, 6, 6];
console.log(findMostAndLeast(testData)); // { most_frequent: 6, least_frequent: 3 }, it gets 6, 3 instead of 1, 2
To prevent the automatic sort of numeric keys of Object in Javascript, the best way is to tweak the Object keys a little bit.
We can insert an "e" in front of every key name to avoid lexicographical sorting of keys and to get the proper output slice the "e", by using the following code;
object_1 = {
"3": 11,
"2": 12,
"1": 13
}
let automaticSortedKeys = Object.keys(object_1);
console.log(automaticSortedKeys) //["1", "2", "3"]
object_2 = {
"e3": 11,
"e2": 12,
"e1": 13
}
let rawObjectKeys = Object.keys(object_2);
console.log(rawObjectKeys) //["e3", "e2", "e1"]
let properKeys = rawObjectKeys.map(function(element){
return element.slice(1)
});
console.log(properKeys) //["3", "2", "1"]
instead of generating an object like {5: 2, 2: 2, 1: 1}
generate an array to the effect of
[
{key: 5, val: 2},
{key: 2, val: 2},
{key: 1, val: 1}
]
or... keep track of the sort order in a separate value or key
I've stumbled with this issue with our normalised array which keyed with Ids> After did my research, I found out there's no way to fix using the object keys because by default the Javascript is sorting any object key with number when you iterate it.
The solution I've done and it worked for me is to put a 'sortIndex' field and used that to sort the list.
The simplest and the best way to preserve the order of the keys in the array obtained by Object.keys() is to manipulate the Object keys a little bit.
insert a "_" in front of every key name. then run the following code!
myObject = {
_a: 1,
_1: 2,
_2: 3
}
const myObjectRawKeysArray = Object.keys(myObject);
console.log(myObjectRawKeysArray)
//["_a", "_1", "_2"]
const myDesiredKeysArray = myObjectRawKeysArray.map(rawKey => {return rawKey.slice(1)});
console.log(myDesiredKeysArray)
//["a", "1", "2"]
You get the desired order in the array with just a few lines of code. hApPy CoDiNg :)
I came across this same problem, and after search a lot about that, i found out that the solution to prevent this behavior is make key as string.
Like that:
{"a": 2, "b": 2}
you can use Map() in javascript ES6 which will keep the order of the keys insertion.
just trying to solve your problem in an alternative solution, recently like to practise leetcode-like question
function solution(arr) {
const obj = {};
const record = {
value: null,
count: 0
};
for (let i = 0; i < arr.length; i++) {
let current = arr[i];
if (!obj[current]) {
obj[current] = 0;
}
obj[current]++;
if (obj[current] > record.count) {
record.value = current;
record.count = obj[current];
}
}
console.log("mode number: ", record.value);
console.log("mode number count: ", record.count);
}
simply do that while you're working with a numeric array index
data = {}
data[key] = value
I know what is a for... in loop (it iterates over the keys), but I have heard about for... of for the first time (it iterates over values).
I am confused about for... of loop.
var arr = [3, 5, 7];
arr.foo = "hello";
for (var i in arr) {
console.log(i); // logs "0", "1", "2", "foo"
}
for (var i of arr) {
console.log(i); // logs "3", "5", "7"
// it doesn't log "3", "5", "7", "hello"
}
I understand that for... of iterates over property values. Then why doesn't it log "3", "5", "7", "hello" instead of "3", "5", "7"?
Unlike for... in loop, which iterates over each key ("0", "1", "2", "foo") and also iterates over the foo key, the for... of does not iterate over the value of foo property, i.e., "hello". Why it is like that?
Here I console for... of loop. It should log "3", "5", "7","hello" but it logs "3", "5", "7". Why?
Example Link
for in loops over enumerable property names of an object.
for of (new in ES6) does use an object-specific iterator and loops over the values generated by that.
In your example, the array iterator does yield all the values in the array (ignoring non-index properties).
I found a complete answer at Iterators and Generators (Although it is for TypeScript, this is the same for JavaScript too)
Both for..of and for..in statements iterate over lists; the values
iterated on are different though, for..in returns a list of keys on
the object being iterated, whereas for..of returns a list of values
of the numeric properties of the object being iterated.
Here is an example that demonstrates this distinction:
let list = [4, 5, 6];
for (let i in list) {
console.log(i); // "0", "1", "2",
}
for (let i of list) {
console.log(i); // "4", "5", "6"
}
Another distinction is that for..in operates on any object; it serves
as a way to inspect properties on this object. for..of on the other
hand, is mainly interested in values of iterable objects. Built-in
objects like Map and Set implement Symbol.iterator property allowing
access to stored values.
let pets = new Set(["Cat", "Dog", "Hamster"]);
pets["species"] = "mammals";
for (let pet in pets) {
console.log(pet); // "species"
}
for (let pet of pets) {
console.log(pet); // "Cat", "Dog", "Hamster"
}
Difference for..in and for..of:
Both for..in and for..of are looping constructs which are used to iterate over data structures. The only difference between them is the entities
they iterate over:
for..in iterates over all enumerable property keys of an object
for..of iterates over the values of an iterable object. Examples of iterable objects are arrays, strings, and NodeLists.
Example:
let arr = ['el1', 'el2', 'el3'];
arr.addedProp = 'arrProp';
// elKey are the property keys
for (let elKey in arr) {
console.log(elKey);
}
// elValue are the property values
for (let elValue of arr) {
console.log(elValue)
}
In this example we can observe that the for..in loop iterates over the keys of the object, which is an array object in this example. The keys are 0, 1, 2 (which correspond to the array elements) and addedProp. This is how the arr array object looks in chrome devtools:
You see that our for..in loop does nothing more than simply iterating over these keys.
The for..of loop in our example iterates over the values of a data structure. The values in this specific example are 'el1', 'el2', 'el3'. The values which an iterable data structure will return using for..of is dependent on the type of iterable object. For example an array will return the values of all the array elements whereas a string returns every individual character of the string.
For...in loop
The for...in loop improves upon the weaknesses of the for loop by eliminating the counting logic and exit condition.
Example:
const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const index in digits) {
console.log(digits[index]);
}
But, you still have to deal with the issue of using an index to access the values of the array, and that stinks; it almost makes it more confusing than before.
Also, the for...in loop can get you into big trouble when you need to add an extra method to an array (or another object). Because for...in loops loop over all enumerable properties, this means if you add any additional properties to the array's prototype, then those properties will also appear in the loop.
Array.prototype.decimalfy = function() {
for (let i = 0; i < this.length; i++) {
this[i] = this[i].toFixed(2);
}
};
const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const index in digits) {
console.log(digits[index]);
}
Prints:
0
1
2
3
4
5
6
7
8
9
function() {
for (let i = 0; i < this.length; i++) {
this[i] = this[i].toFixed(2);
}
}
This is why for...in loops are discouraged when looping over arrays.
NOTE: The forEach loop is another type of for loop in JavaScript.
However, forEach() is actually an array method, so it can only be used
exclusively with arrays. There is also no way to stop or break a
forEach loop. If you need that type of behavior in your loop, you’ll
have to use a basic for loop.
For...of loop
The for...of loop is used to loop over any type of data that is iterable.
Example:
const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const digit of digits) {
console.log(digit);
}
Prints:
0
1
2
3
4
5
6
7
8
9
This makes the for...of loop the most concise version of all the for loops.
But wait, there’s more! The for...of loop also has some additional benefits that fix the weaknesses of the for and for...in loops.
You can stop or break a for...of loop at anytime.
const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
for (const digit of digits) {
if (digit % 2 === 0) {
continue;
}
console.log(digit);
}
Prints:
1
3
5
7
9
And you don’t have to worry about adding new properties to objects. The for...of loop will only loop over the values in the object.
Here is a useful mnemonic for remembering the difference between for...in Loop and for...of Loop.
"index in, object of"
for...in Loop => iterates over the index in the array.
for...of Loop => iterates over the object of objects.
for of is used to iterate over iterables and for in is used to iterate over object properties
Here's a trick to remember:
for of is not for objects (so it's for iterables)
for in is not for iterables (so it's for objects)
Another trick:
for in returns object indices (keys) while for of returns values
//for in, iterates keys in an object and indexes in an array
let obj={a:1, b:2}
for( const key in obj)
console.log(obj[key]); //would print 1 and 2
console.log(key); //would print a and b
let arr = [10, 11, 12, 13];
for (const item in arr)
console.log(item); //would print 0 1 2 3
//for of, iterates values in an array or any iterable
let arr = [10, 11, 12, 13];
for (const item of arr )
console.log(item); //would print 10 11 12 13
Another difference between the two loops, which nobody has mentioned before:
Destructuring for...in is deprecated. Use for...of instead.
Source
So if we want to use destructuring in a loop, for get both index and value of each array element, we should to use the for...of loop with the Array method entries():
for (const [idx, el] of arr.entries()) {
console.log( idx + ': ' + el );
}
The for...in statement iterates over the enumerable properties of an object, in an arbitrary order.
Enumerable properties are those properties whose internal [[Enumerable]] flag is set to true, hence if there is any enumerable property in the prototype chain, the for...in loop will iterate on those as well.
The for...of statement iterates over data that iterable object defines to be iterated over.
Example:
Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};
let iterable = [3, 5, 7];
for (let i in iterable) {
console.log(i); // logs: 0, 1, 2, "arrCustom", "objCustom"
}
for (let i in iterable) {
if (iterable.hasOwnProperty(i)) {
console.log(i); // logs: 0, 1, 2,
}
}
for (let i of iterable) {
console.log(i); // logs: 3, 5, 7
}
Like earlier, you can skip adding hasOwnProperty in for...of loops.
Short answer: for...in loops over keys, while for...of loops over values.
for (let x in ['a', 'b', 'c', 'd'] {
console.log(x);
}
// Output
0
1
2
3
for (let x of ['a', 'b', 'c', 'd'] {
console.log(x);
}
// Output
a
b
c
d
The for-in statement iterates over the enumerable properties of an object, in arbitrary order.
The loop will iterate over all enumerable properties of the object itself and those the object inherits from its constructor's prototype
You can think of it as "for in" basically iterates and list out all the keys.
var str = 'abc';
var arrForOf = [];
var arrForIn = [];
for(value of str){
arrForOf.push(value);
}
for(value in str){
arrForIn.push(value);
}
console.log(arrForOf);
// ["a", "b", "c"]
console.log(arrForIn);
// ["0", "1", "2", "formatUnicorn", "truncate", "splitOnLast", "contains"]
There are some already defined data types which allows us to iterate over them easily e.g Array, Map, String Objects
Normal for in iterates over the iterator and in response provides us with the keys that are in the order of insertion as shown in below example.
const numbers = [1,2,3,4,5];
for(let number in number) {
console.log(number);
}
// result: 0, 1, 2, 3, 4
Now if we try same with for of, then in response it provides us with the values not the keys. e.g
const numbers = [1,2,3,4,5];
for(let numbers of numbers) {
console.log(number);
}
// result: 1, 2, 3, 4, 5
So looking at both of the iterators we can easily differentiate the difference between both of them.
Note:- For of only works with the Symbol.iterator
So if we try to iterate over normal object, then it will give us an error e.g-
const Room = {
area: 1000,
height: 7,
floor: 2
}
for(let prop in Room) {
console.log(prop);
}
// Result area, height, floor
for(let prop of Room) {
console.log(prop);
}
Room is not iterable
Now for iterating over we need to define an ES6 Symbol.iterator e.g
const Room= {
area: 1000, height: 7, floor: 2,
[Symbol.iterator]: function* (){
yield this.area;
yield this.height;
yield this.floors;
}
}
for(let prop of Room) {
console.log(prop);
}
//Result 1000, 7, 2
This is the difference between For in and For of. Hope that it might clear the difference.
The for-in loop
for-in loop is used to traverse through enumerable properties of a collection, in an arbitrary order. A collection is a container type object whose items can be using an index or a key.
var myObject = {a: 1, b: 2, c: 3};
var myArray = [1, 2, 3];
var myString = "123";
console.log( myObject[ 'a' ], myArray[ 1 ], myString[ 2 ] );
for-in loop extracts the enumerable properties (keys) of a collection all at once and iterates over it one at a time. An enumerable property is the property of a collection that can appear in for-in loop.
By default, all properties of an Array and Object appear in for-in loop. However, we can use Object.defineProperty method to manually configure the properties of a collection.
var myObject = {a: 1, b: 2, c: 3};
var myArray = [1, 2, 3];
Object.defineProperty( myObject, 'd', { value: 4, enumerable: false } );
Object.defineProperty( myArray, 3, { value: 4, enumerable: false } );
for( var i in myObject ){ console.log( 'myObject:i =>', i ); }
for( var i in myArray ){ console.log( 'myArray:i =>', i ); }
In the above example, the property d of the myObject and the index 3 of myArray does not appear in for-in loop because they are configured with enumerable: false.
There are few issues with for-in loops. In the case of Arrays, for-in loop will also consider methods added on the array using myArray.someMethod = f syntax, however, myArray.length remains 4.
The for-of loop
It is a misconception that for-of loop iterate over the values of a collection. for-of loop iterates over an Iterable object. An iterable is an object that has the method with the name Symbol.iterator directly on it one on one of its prototypes.
Symbol.iterator method should return an Iterator. An iterator is an object which has a next method. This method when called return value and done properties.
When we iterate an iterable object using for-of loop, the Symbol.iterator the method will be called once get an iterator object. For every iteration of for-of loop, next method of this iterator object will be called until done returned by the next() call returns false. The value received by the for-of loop for every iteration if the value property returned by the next() call.
var myObject = { a: 1, b: 2, c: 3, d: 4 };
// make `myObject` iterable by adding `Symbol.iterator` function directlty on it
myObject[ Symbol.iterator ] = function(){
console.log( `LOG: called 'Symbol.iterator' method` );
var _myObject = this; // `this` points to `myObject`
// return an iterator object
return {
keys: Object.keys( _myObject ),
current: 0,
next: function() {
console.log( `LOG: called 'next' method: index ${ this.current }` );
if( this.current === this.keys.length ){
return { done: true, value: null }; // Here, `value` is ignored by `for-of` loop
} else {
return { done: false, value: _myObject[ this.keys[ this.current++ ] ] };
}
}
};
}
// use `for-of` loop on `myObject` iterable
for( let value of myObject ) {
console.log( 'myObject: value => ', value );
}
The for-of loop is new in ES6 and so are the Iterable and Iterables. The Array constructor type has Symbol.iterator method on its prototype. The Object constructor sadly doesn't have it but Object.keys(), Object.values() and Object.entries() methods return an iterable (you can use console.dir(obj) to check prototype methods). The benefit of the for-of loop is that any object can be made iterable, even your custom Dog and Animal classes.
The easy way to make an object iterable is by implementing ES6 Generator instead of custom iterator implementation.
Unlike for-in, for-of loop can wait for an async task to complete in each iteration. This is achieved using await keyword after for statement documentation.
Another great thing about for-of loop is that it has Unicode support. According to ES6 specifications, strings are stored with UTF-16 encoding. Hence, each character can take either 16-bit or 32-bit. Traditionally, strings were stored with UCS-2 encoding which has supports for characters that can be stored within 16 bits only.
Hence, String.length returns number of 16-bit blocks in a string. Modern characters like an Emoji character takes 32 bits. Hence, this character would return length of 2. for-in loop iterates over 16-bit blocks and returns the wrong index. However, for-of loop iterates over the individual character based on UTF-16 specifications.
var emoji = "😊🤣";
console.log( 'emoji.length', emoji.length );
for( var index in emoji ){ console.log( 'for-in: emoji.character', emoji[index] ); }
for( var character of emoji ){ console.log( 'for-of: emoji.character', character ); }
for...of loop works only with iterable objects. In JavaScript, iterables are objects which can be looped over.
String, Array, TypedArray, Map, and Set are all built-in iterables, because each of their prototype objects implements an ##iterator method. So, for...of loop works on the mentioned object types.
Object in JavaScript is not iterable by default. So, for...of loop does not work on objects.
In simple words, for...of works with strings and arrays but not with objects.
for...in works with those properties whose enumerable flag is set to true.
Enumerable flag for properties created via simple assignment or property initializer are by default true.
Enumerable flag for properties created via Object.defineProperty are by default false.
Here is a more detailed post with examples: https://dev.to/swastikyadav/difference-between-forof-and-forin-loop-in-javascript-j2o
A see a lot of good answers, but I decide to put my 5 cents just to have good example:
For in loop
iterates over all enumerable props
let nodes = document.documentElement.childNodes;
for (var key in nodes) {
console.log( key );
}
For of loop
iterates over all iterable values
let nodes = document.documentElement.childNodes;
for (var node of nodes) {
console.log( node.toString() );
}
When I first started out learning the for in and of loop, I was confused with my output too, but with a couple of research and understanding you can think of the individual loop like the following :
The
for...in loop returns the indexes of the individual property and has no effect of impact on the property's value, it loops and returns information on the property and not the value.
E.g
let profile = {
name : "Naphtali",
age : 24,
favCar : "Mustang",
favDrink : "Baileys"
}
The above code is just creating an object called profile, we'll use it for both our examples, so, don't be confused when you see the profile object on an example, just know it was created.
So now let us use the for...in loop below
for(let myIndex in profile){
console.log(`The index of my object property is ${myIndex}`)
}
// Outputs :
The index of my object property is 0
The index of my object property is 1
The index of my object property is 2
The index of my object property is 3
Now Reason for the output being that we have Four(4) properties in our profile object and indexing as we all know starts from 0...n, so, we get the index of properties 0,1,2,3 since we are working with the for..in loop.
for...of loop* can return either the property, value or both, Let's take a look at how.
In javaScript, we can't loop through objects normally as we would on arrays, so, there are a few elements we can use to access either of our choices from an object.
Object.keys(object-name-goes-here) >>> Returns the keys or properties of an object.
Object.values(object-name-goes-here) >>> Returns the values of an object.
Object.entries(object-name-goes-here) >>> Returns both the keys and values of an object.
Below are examples of their usage, pay attention to Object.entries() :
Step One: Convert the object to get either its key, value, or both.
Step Two: loop through.
// Getting the keys/property
Step One: let myKeys = ***Object.keys(profile)***
Step Two: for(let keys of myKeys){
console.log(`The key of my object property is ${keys}`)
}
// Getting the values of the property
Step One: let myValues = ***Object.values(profile)***
Step Two : for(let values of myValues){
console.log(`The value of my object property is ${values}`)
}
When using Object.entries() have it that you are calling two entries on the object, i.e the keys and values. You can call both by either of the entry. Example Below.
Step One: Convert the object to entries, using ***Object.entries(object-name)***
Step Two: **Destructure** the ***entries object which carries the keys and values***
like so **[keys, values]**, by so doing, you have access to either or both content.
// Getting the keys/property
Step One: let myKeysEntry = ***Object.entries(profile)***
Step Two: for(let [keys, values] of myKeysEntry){
console.log(`The key of my object property is ${keys}`)
}
// Getting the values of the property
Step One: let myValuesEntry = ***Object.entries(profile)***
Step Two : for(let [keys, values] of myValuesEntry){
console.log(`The value of my object property is ${values}`)
}
// Getting both keys and values
Step One: let myBothEntry = ***Object.entries(profile)***
Step Two : for(let [keys, values] of myBothEntry){
console.log(`The keys of my object is ${keys} and its value
is ${values}`)
}
Make comments on unclear parts section(s).
Everybody did explain why this problem occurs, but it's still very easy to forget about it and then scratching your head why you got wrong results. Especially when you're working on big sets of data when the results seem to be fine at first glance.
Using Object.entries you ensure to go trough all properties:
var arr = [3, 5, 7];
arr.foo = "hello";
for ( var [key, val] of Object.entries( arr ) ) {
console.log( val );
}
/* Result:
3
5
7
hello
*/
In simple terms forIN iterates over the KEYS IN the array(index)/object(key),
whereas forOF iterates over the VALUES OF the array(value).
Im trying to remove a entery from the array, but after the loop, it leaves an comma seperator where the removed result is.
Q: How can i remove the comma from the array?
wanted output:
after loop
[{ number: 10, count: 1 }, { number: 5, count: 1 } ]
My current code:
console.log("before loop:");
console.log(bNumbersCount);
for(var key in bNumbersCount) {
var card = bNumbersCount[key];
if (card['count'] != x) {
delete bNumbersCount[key];
}
}
console.log("after loop");
console.log(bNumbersCount);
console log result:
before loop:
[ { number: 2, count: 3 },
{ number: 10, count: 1 },
{ number: 5, count: 1 } ]
after loop
[ , { number: 10, count: 1 }, { number: 5, count: 1 } ]
This isn't how you work with arrays in JavaScript.
Don't use for-in on a JavaScript array unless you know what you're doing and have a specific reason; instead, use any of the several other ways outlined in this answer.
Don't use delete on an array entry unless you know what you're doing and want to create a sparse array.
You probably wanted to loop through the array and actually remove entries. Two ways to do that:
Create a new array with only the entries you don't want to remove, via filter:
aNumbersCount = aNumbersCount.filter(function(entry) {
return card.count == x;
});
Use splice to modify your existing array in place, in which case you'll probably want to loop backward so the array indexes don't change on you:
for (var n = aNumbersCount.length - 1; n >= 0; --n) {
if (aNumbersCount[n].count != x) {
aNumbersCount.splice(n, 1); // Removes one entry at index n
}
}
Side note: card['count'] can more simply be written card.count. You only need brackets notation and a string when the name of the property comes from an expression (such as getting a value from a variable) or if the name contains characters that make it an invalid IdentifierName (such as spaces).
Iterate backwards and use splice for deleting.
var array = [ { number: 2, count: 3 }, { number: 10, count: 1 }, { number: 5, count: 1 } ],
i = array.length,
x = 3;
while (i--) {
if (array[i].count === x) {
array.splice(i, 1);
}
}
console.log(array);
You could use filter for that:
bNumbersCount = bNumbersCount.filter(card => card.count === x);
var bNumbersCount = [ { number: 2, count: 3 },
{ number: 10, count: 1 },
{ number: 5, count: 1 } ],
x = 1;
bNumbersCount = bNumbersCount.filter(card => card['count'] === x);
console.log(bNumbersCount);
The delete method does not shift the array elements to fill up the property (index) you deleted. You could use splice for that: that will shift the elements into place. But then you need to take care how you loop, as you can find your self skipping the value that followed just after the deleted/spliced value.
All this becomes more simple with .filter().
NB: Be aware that with .filter you actually create a new array. This means that if you had passed that array as function argument, the caller will not see the change. The function should then return that array to make it available to the caller.
Why I met this problem:
I tried to solve an algorithm problem and I need to return the number which appeared most of the times in an array. Like [5,4,3,2,1,1] should return 1.
And also when two number appear same time as the maximum appearance return the one came first. Like [5,5,2,2,1] return 5 because 5 appear first. I use an object to store the appearance of each number. The key is the number itself.
So When the input is [5,5,2,2,1] my object should be
Object {5: 2, 2: 2, 1: 1} but actually I got Object {1: 1, 2: 2, 5: 2}
So When I use for..in to iterate the object I got 2 returned instead of 5 . So that's why I asked this question.
This problem occurs in Chrome console and I'm not sure if this is a common issue:
When I run the following code
var a = {};
a[0]=1;
a[1]=2;
a[2]=3;
a is: Object {0: 1, 1: 2, 2: 3}
But when I reverse the order of assignment like:
var a = {};
a[2]=3;
a[1]=2;
a[0]=1;
a is also:Object {0: 1, 1: 2, 2: 3}
The numeric property automatic sorted in ascending order.
I tried prefix or postfix the numeric property like
var a = {};
a['p'+0]=1;
a['p'+1]=2;
a['p'+2]=3;
console.log(a);//Object {p0: 1, p1: 2, p2: 3}
And this keep the property order. Is this the best way to solve the problem? And is there anyway to prevent this auto sort behavior? Is this only happen in Chrome V8 JavaScript engine? Thank you in advance!
target = {}
target[' ' + key] = value // numeric key
This can prevent automatic sort of Object numeric property.
You really can't rely on order of an object fields in JavaScript, but I can suggest to use Map (ES6/ES2015 standard) if you need to preserve order of your key, value pair object. See the snippet below:
let myObject = new Map();
myObject.set('z', 33);
myObject.set('1', 100);
myObject.set('b', 3);
for (let [key, value] of myObject) {
console.log(key, value);
}
// z 33
// 1 100
// b 3
You are using a JS object, that by definition does not keep order. Think of it as a key => value map.
You should be using an array, that will keep whatever you insert on the index you inserted it into. Think of it as a list.
Also notice that you did not in fact "reverse the order of the assignment", because you inserted elements on the same index every time.
This is an old topic but it is still worth mentioning as it is hard to find a straight explanation in one-minute googling.
I recently had a coding exercise that finding the first occurrence of the least/most frequent integer in an array, it is pretty much the same as your case.
I encountered the same problem as you, having the numeric keys sorted by ASC in JavaScript object, which is not preserving the original order of elements, which is the default behavior in js.
A better way to solve this in ES6 is to use a new data type called: Map
Map can preserve the original order of elements(pairs), and also have the unique key benefit from object.
let map = new Map()
map.set(4, "first") // Map(1) {4 => "first"}
map.set(1, "second") // Map(2) {4 => "first", 1 => "second"}
map.set(2, "third") // Map(3) {4 => "first", 1 => "second", 2 => "third"}
for(let [key, value] of map) {
console.log(key, value)
}
// 4 "first"
// 1 "second"
// 2 "third"
However, using the object data type can also solve the problem, but we need the help of the input array to get back the original order of elements:
function findMostAndLeast(arr) {
let countsMap = {};
let mostFreq = 0;
let leastFreq = arr.length;
let mostFreqEl, leastFreqEl;
for (let i = 0; i < arr.length; i++) {
let el = arr[i];
// Count each occurrence
if (countsMap[el] === undefined) {
countsMap[el] = 1;
} else {
countsMap[el] += 1;
}
}
// Since the object is sorted by keys by default in JS, have to loop again the original array
for (let i = 0; i < arr.length; i++) {
const el = arr[i];
// find the least frequent
if (leastFreq > countsMap[el]) {
leastFreqEl = Number(el);
leastFreq = countsMap[el];
}
// find the most frequent
if (countsMap[el] > mostFreq) {
mostFreqEl = Number(el);
mostFreq = countsMap[el];
}
}
return {
most_frequent: mostFreqEl,
least_frequent: leastFreqEl
}
}
const testData = [6, 1, 3, 2, 4, 7, 8, 9, 10, 4, 4, 4, 10, 1, 1, 1, 1, 6, 6, 6, 6];
console.log(findMostAndLeast(testData)); // { most_frequent: 6, least_frequent: 3 }, it gets 6, 3 instead of 1, 2
To prevent the automatic sort of numeric keys of Object in Javascript, the best way is to tweak the Object keys a little bit.
We can insert an "e" in front of every key name to avoid lexicographical sorting of keys and to get the proper output slice the "e", by using the following code;
object_1 = {
"3": 11,
"2": 12,
"1": 13
}
let automaticSortedKeys = Object.keys(object_1);
console.log(automaticSortedKeys) //["1", "2", "3"]
object_2 = {
"e3": 11,
"e2": 12,
"e1": 13
}
let rawObjectKeys = Object.keys(object_2);
console.log(rawObjectKeys) //["e3", "e2", "e1"]
let properKeys = rawObjectKeys.map(function(element){
return element.slice(1)
});
console.log(properKeys) //["3", "2", "1"]
instead of generating an object like {5: 2, 2: 2, 1: 1}
generate an array to the effect of
[
{key: 5, val: 2},
{key: 2, val: 2},
{key: 1, val: 1}
]
or... keep track of the sort order in a separate value or key
I've stumbled with this issue with our normalised array which keyed with Ids> After did my research, I found out there's no way to fix using the object keys because by default the Javascript is sorting any object key with number when you iterate it.
The solution I've done and it worked for me is to put a 'sortIndex' field and used that to sort the list.
The simplest and the best way to preserve the order of the keys in the array obtained by Object.keys() is to manipulate the Object keys a little bit.
insert a "_" in front of every key name. then run the following code!
myObject = {
_a: 1,
_1: 2,
_2: 3
}
const myObjectRawKeysArray = Object.keys(myObject);
console.log(myObjectRawKeysArray)
//["_a", "_1", "_2"]
const myDesiredKeysArray = myObjectRawKeysArray.map(rawKey => {return rawKey.slice(1)});
console.log(myDesiredKeysArray)
//["a", "1", "2"]
You get the desired order in the array with just a few lines of code. hApPy CoDiNg :)
I came across this same problem, and after search a lot about that, i found out that the solution to prevent this behavior is make key as string.
Like that:
{"a": 2, "b": 2}
you can use Map() in javascript ES6 which will keep the order of the keys insertion.
just trying to solve your problem in an alternative solution, recently like to practise leetcode-like question
function solution(arr) {
const obj = {};
const record = {
value: null,
count: 0
};
for (let i = 0; i < arr.length; i++) {
let current = arr[i];
if (!obj[current]) {
obj[current] = 0;
}
obj[current]++;
if (obj[current] > record.count) {
record.value = current;
record.count = obj[current];
}
}
console.log("mode number: ", record.value);
console.log("mode number count: ", record.count);
}
simply do that while you're working with a numeric array index
data = {}
data[key] = value
I have an array of objects with duplicates and I'm trying to get a unique listing, where uniqueness is defined by a subset of the properties of the object. For example,
{a:"1",b:"1",c:"2"}
And I want to ignore c in the uniqueness comparison.
I can do something like
_.uniq(myArray,function(element) { return element.a + "_" + element+b});
I was hoping I could do
_.uniq(myArray,function(element) { return {a:element.a, b:element.b} });
But that doesn't work. Is there something like that I can do, or do I need to create a comparable representation of the object if I'm comparing multiple properties?
Use Lodash's uniqWith method:
_.uniqWith(array, [comparator])
This method is like _.uniq except that it accepts comparator which is invoked to compare elements of array. The order of result values is determined by the order they occur in the array. The comparator is invoked with two arguments: (arrVal, othVal).
When the comparator returns true, the items are considered duplicates and only the first occurrence will be included in the new array.
Example:
I have a list of locations with latitude and longitude coordinates -- some of which are identical -- and I want to see the list of locations with unique coordinates:
const locations = [
{
name: "Office 1",
latitude: -30,
longitude: -30
},
{
name: "Office 2",
latitude: -30,
longitude: 10
},
{
name: "Office 3",
latitude: -30,
longitude: 10
}
];
const uniqueLocations = _.uniqWith(
locations,
(locationA, locationB) =>
locationA.latitude === locationB.latitude &&
locationA.longitude === locationB.longitude
);
// Result has Office 1 and Office 2
There doesn't seem to be a straightforward way to do this, unfortunately. Short of writing your own function for this, you'll need to return something that can be directly compared for equality (as in your first example).
One method would be to just .join() the properties you need:
_.uniqBy(myArray, function(elem) { return [elem.a, elem.b].join(); });
Alternatively, you can use _.pick or _.omit to remove whatever you don't need. From there, you could use _.values with a .join(), or even just JSON.stringify:
_.uniqBy(myArray, function(elem) {
return JSON.stringify(_.pick(elem, ['a', 'b']));
});
Keep in mind that objects are not deterministic as far as property order goes, so you may want to just stick to the explicit array approach.
P.S. Replace uniqBy with uniq for Lodash < 4
Here there's the correct answer
javascript - lodash - create a unique list based on multiple attributes.
FYI var result = _.uniqBy(list, v => [v.id, v.sequence].join());
I do think that the join() approach is still the simplest. Despite concerns raised in the previous solution, I think choosing the right separator is the key to avoiding the identified pitfalls (with different value sets returning the same joined value). Keep in mind, the separator need not be a single character, it can be any string that you are confident will not occur naturally in the data itself. I do this all the time and am fond of using '~!$~' as my separator. It can also include special characters like \t\r\n etc.
If the data contained is truly that unpredictable, perhaps the max length is known and you could simply pad each element to its max length before joining.
There is a hint in #voithos and #Danail combined answer. How I solved this was to add a unique key on the objects in my array.
Starting Sample Data
const animalArray = [
{ a: 4, b: 'cat', d: 'generic' },
{ a: 5, b: 'cat', d: 'generic' },
{ a: 4, b: 'dog', d: 'generic' },
{ a: 4, b: 'cat', d: 'generic' },
];
In the example above, I want the array to be unique by a and b but right now I have two objects that have a: 4 and b: 'cat'. By combining a + b into a string I can get a unique key to check by.
{ a: 4, b: 'cat', d: 'generic', id: `${a}-${b}` }. // id is now '4-cat'
Note: You obviously need to map over the data or do this during creation of the object as you cannot reference properties of an object within the same object.
Now the comparison is simple...
_.uniqBy(animalArray, 'id');
The resulting array will be length of 3 it will have removed the last duplicate.
late to the party but I found this in lodash docs.
var objects = [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }, { 'x': 1, 'y': 2 }];
_.uniqWith(objects, _.isEqual);
// => [{ 'x': 1, 'y': 2 }, { 'x': 2, 'y': 1 }]