Related
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 am trying to replicate Python's insert at index functionality in Javascript.
var index = [0,1,2,3,4]
var nums = [0,1,2,2,1]
const target = new Array(nums.length);
for (let i=0; i<index.length; i++) {
target.splice(nums[i], 0, index[i]);
};
console.log(target);
This produces the following output:
[ 0, 4, 1, 3, 2, <5 empty items> ]
However, if I run the following code:
var index = [0,1,2,3,4]
var nums = [0,1,2,2,1]
const target = [] //new Array(nums.length);
for (let i=0; i<index.length; i++) {
target.splice(nums[i], 0, index[i]);
};
console.log(target);
This produces the following output:
[ 0, 4, 1, 3, 2 ]
What is going on? The second output is the one I desire.
new Array(nums.length) will fill the array with 'empty slots' for that number.
const target = [] Will be [ ]
const target = new Array(5) will be [, , , , ]
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Array More info here.
when you initialize array like this :
const target = new Array(index.length);
This code will initiate "target" array with five empty box. in result it contain garbage value and it may affect your value after insertion.
But when you initialize like this :
const target = [];
This code will initiate "target" array without defining the size of it. it means you can insert n number of dynamic value inside this.
('n' denote the size of array)
So always use second option which is good practice to use javascript Array
I get an error when I run this code:
var array = [];
array.push(["one"]:[1,2,3]);
array.push(["two"]:[4,5,6]);
I want my array to look like this in the end:
{"one": [1,2,3], "two": [4,5,6]};
I don't know how to fix this error, I want to use push.
An associative array in JavaScript is an object, so you can't use array.push as that's not valid there. You'd just want: array["one"] = [1,2,3]
var array = {};
array.one = [123, 123];
array.two = [123, 123];
console.log(array)
output
{
one: [
123,
123
],
two: [
123,
123
]
}
You should be opting for something like below. Using push, you will not achieve your desired output.
let obj = {};
const item1 = {
["one"]: [1, 2, 3]
}
const item2 = {
["two"]: [4, 5, 6]
}
obj = {
...obj,
...item1,
...item2
}
The reason you got the error is because you are missing object wrapper notation in your push {}
array.push({["one"]:[1,2,3]});
array.push({["two"]:[4,5,6]});
but as said, this will not give the desired output: {"one": [1,2,3], "two": [4,5,6]};
You must first create the object, assign values into the object, then push it into the array. Refer to this post for more information.
push object into array
Javascript doesn't have Associative Arrays like other languages, but it have Objects, that is similar.
var object = {};
object.one = [1,2,3];
// or if the key name comes from a variable:
var key = "two";
object[key] = [4,5,6];
"one" is an object not an array. remove the parenthesis from there. See below code:
array.push({"one":[1,2,3]});
array.push({"two":[4,5,6]});
I have the following object:
{English: 4, Math: 5, CompSci: 6}
How can I convert it to an array of objects, like:
[{English: 4}, {Math: 5}, {CompSci: 6}]
Can't find the answer anywhere. Thanks!!
Use Array#forEach over Object.keys(YOUR_OBJECT)
var input = {
English: 4,
Math: 5,
CompSci: 6
};
var op = [];
Object.keys(input).forEach(function(key) {
var obj = {};
obj[key] = input[key];
op.push(obj); //push newly created object in `op`array
});
console.log(op);
With newer JS, you could take Object.entries and map single properties.
var object = { English: 4, Math: 5, CompSci: 6 },
array = Object.entries(object).map(([k, v]) => ({ [k]: v }));
console.log(array);
Just loop over each of the keys in the object.
var oldObject = {English: 4, Math: 5, CompSci: 6};
var newArray = [];
// Loop over each key in the object
for (var key in oldObject) {
// Create a temp object
var temp = {};
// Set the key of temp
temp[key] = oldObject[key]
// Push it to the array
newArray.push(temp);
}
console.log(newArray)
you can directly assign object by {} but you must use [] quote for key value if not that will not worked
var obj = {English: 4, Math: 5, CompSci: 6};
var n_obj = [];
for(var i in obj){
n_obj.push({[i]:obj[i]});
}
console.log(n_obj);
You can turn the object into an array of key-value pairs using Object.entries and then map this array to smaller object created using Object.fromEntries from each individual key-value pair (the key part here is the wrapping in another array before passing to fromEntries):
Object.entries(obj).map(e => Object.fromEntries([e]))
The reverse way is similar: We create a big object using Object.fromEntries, and we pass in an array of key-value pairs. This array is created by flat-mapping (i.e. eliminating on extra layer of arrays) the array of objects to an array of key-value pairs we get from calling Object.entries on each small object. The key here is the flat-mapping, without it we would get an array of arrays of key-value pairs because we added that extra layer in the other conversion to separate the properties.
Object.fromEntries(arr.flatMap(o => Object.entries(o)))
You can use JSON.stringify(), String.prototype.match() with RegExp /".*"\:.*(?=,|})/, String.prototype.split() with RegExp /,/, Array.prototype.join() with parameter "},{", JSON.parse()
var obj = {English: 4, Math: 5, CompSci: 6};
var res = JSON.parse("[{"
+ JSON.stringify(obj)
.match(/".*"\:.*(?=,|})/g)[0]
.split(/,/)
.join("},{")
+ "}]");
console.log(res);
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