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Finding total number of maximum number in array fail only for big data

I doing some Hackerrank challange to improve my problem solving skills, so one of the challanges was about finding the total maximum numbers from an array of numbers. For example if we have 3 2 1 3 1 3 it should return 3
This is what I did :
function birthdayCakeCandles(ar) {
let total= 0
let sortedArray = ar.sort((cur,next)=>{
return cur<next
})
ar.map(item => {
if(item===sortedArray[0]) {
total ++;
}
})
return total
}
So I sorted the given array and then map through the array and check how many of the numbers are equal to the maximum number in that array and count the total.
This will pass 8/9 test cases, one of the test cases, have a array with length of 100000 and for this one it failed, this is the given data for this test case.
Really can't get it why it fails in this test, is it possible that this happened because of JavaScript which is always synchronous and single-threaded?
I tried to use Promise and async await, but hackerrank will consider the first return as the output ( Which is the Promise itself ) and it not use the resolve value as a output, so can't really test this.
Is it something wrong with my logic?

The sorting approach is too slow (O(n log n) time complexity). For algorithmic challenges on HR, it's unlikely that features somewhat particular to your language choice like promises/async are going to rescue you.
You can do this in one pass using an object to keep track of how many times you've "seen" each number and the array's maximum number, then simply index into the object to get your answer:
function birthdayCakeCandles(ar) {
let best = -Infinity;
const seen = {};
for (let i = 0; i < ar.length; i++) {
if (ar[i] > best) {
best = ar[i];
}
seen[ar[i]] = ++seen[ar[i]] || 1;
}
return seen[best];
}
Time and space complexity: O(n).
Edit:
This answer is even better, with constant space (here it is in JS):
function birthdayCakeCandles(ar) {
let best = -Infinity;
let count = 0;
for (const n of ar) {
if (n > best) {
best = n;
count = 1;
}
else if (n === best) {
count++;
}
}
return count;
}

In your case, the build in function sort is using the resource heavily. Maybe that's the reason it is failing for a space/time complexity.
BTW, This problem can be solved easily using a for loop. The idea is
Pseudocode
var maxNum = -999999; // put here the highest limit of int or what ever data type
int count = 0;
for(x in arr)
{
if (x > maxNum)
{
maxNum = x;
count = 1;
}
if(x==maxNum) count ++;
}
Here count will be the output.
The full code is
function birthdayCakeCandles(ar) {
var maxNum = -1;
var count = 0;
for(var i=0; i< ar.length; i++){
var x = ar[i];
if(x<maxNum) continue;
if(x>maxNum){
maxNum = x;
count = 1;
}
else{
count++;
}
}
return count;
}

Javascript - For loop vs Linked List vs ES6 Set to find two matching integers

I have prepared 2 Javascript functions to find matching integer pairs that add up to a sum and returns a boolean.
The first function uses a binary search like that:
function find2PairsBySumLog(arr, sum) {
for (var i = 0; i < arr.length; i++) {
for (var x = i + 1; x < arr.length; x++) {
if (arr[i] + arr[x] == sum) {
return true;
}
}
}
return false;
}
For the second function I implemented my own singly Linked List, in where I add the complementary integer to the sum and search for the value in the Linked List. If value is found in the Linked List we know there is a match.
function find2PairsBySumLin(arr, sum) {
var complementList = new LinkedList();
for (var i = 0; i < arr.length; i++) {
if (complementList.find(arr[i])) {
return true;
} else {
complementList.add(sum - arr[i]);
}
}
return false;
}
When I run both functions I clearly see that the Linked List search executes ~75% faster
var arr = [9,2,4,1,3,2,2,8,1,1,6,1,2,8,7,8,2,9];
console.time('For loop search');
console.log(find2PairsBySumLog(arr, 18));
console.timeEnd(‘For loop search’);
console.time('Linked List search');
console.log(find2PairsBySumLin(arr, 18));
console.timeEnd('Linked List search');
true
For loop search: 4.590ms
true
Linked List search: 0.709ms
Here my question: Is the Linked List approach a real linear search? After all I loop through all the nodes, while my outer loop iterates through the initial array.
Here is my LinkedList search function:
LinkedList.prototype.find = function(data) {
var headNode = this.head;
if(headNode === null) {
return false;
}
while(headNode !== null) {
if(headNode.data === data) {
return true;
} else {
headNode = headNode.next;
}
}
return false;
}
UPDATE:
It was a good idea to go back and have another think of the problem based the comments so far.
Thanks to #nem035 comment on small datasets, I ran another test but this time with 100,000 integers between 1 and 8. I assigned 9 to the first and last position and searched for 18 to make sure the entire array will be searched.
I also included the relatively new ES6 Set function for comparison thanks to #Oriol.
Btw #Oriol and #Deepak you are right. The first function is not a binary search but rather a O(n*n) search, which has no logarithmic complexity.
It turns out my Linked List implementation was the slowest of all searches. I ran 10 iterations for each function individually. Here the result:
For loop search: 24.36 ms (avg)
Linked List search: 64328.98 ms (avg)
Set search: 35.63 ms (avg)
Here the same test for a dataset of 10,000,000 integers:
For loop search: 30.78 ms (avg)
Set search: 1557.98 ms (avg)
Summary:
So it seems the Linked List is really fast for smaller dataset up to ~1,000, while ES6 Set is great for larger datasets.
Nevertheless the For loop is the clear winner in all tests.
All 3 methods will scale linearly with the amount of data.
Please note: ES6 Set is not backward compatible with old browsers in case this operation has to be done client side.

Don't use this. Use a set.
function find2PairsBySum(arr, sum) {
var set = new Set();
for(var num of arr) {
if (set.has(num)) return true;
set.add(sum - num);
}
return false;
}
That's all. Both add and has are guaranteed to be sublinear (probably constant) in average.

You can optimize this substantially, by pre-sorting the array and then using a real binary search.
// Find an element in a sorted array.
function includesBinary(arr, elt) {
if (!arr.length) return false;
const middle = Math.floor(arr.length / 2);
switch (Math.sign(elt - arr[middle])) {
case -1: return includesBinary(arr.slice(0, middle - 1), elt);
case 0: return true;
case +1: return includesBinary(arr.slice(middle + 1), elt);
}
}
// Given an array, pre-sort and return a function to detect pairs adding up to a sum.
function makeFinder(arr) {
arr = arr.slice().sort((a, b) => a - b);
return function(sum) {
for (let i = 0; i < arr.length; i++) {
const remaining = sum - arr[i];
if (remaining < 0) return false;
if (includesBinary(arr, remaining)) return true;
}
return false;
};
}
// Test data: 100 random elements between 0 and 99.
const arr = Array.from(Array(100), _ => Math.floor(Math.random() * 100));
const finder = makeFinder(arr);
console.time('test');
for (let i = 0; i < 1000; i++) finder(100);
console.timeEnd('test');
According to this rough benchmark, one lookup into an array of 100 elements costs a few microseconds.
Rewriting includesBinary to avoid recursion would probably provide a further performance win.

first of all find2PairsBySumLog function is not a binary search, it's a kind of brute force method which parses all the elements of array and it's worst case time complexity should be O(n*n), and the second function is a linear search that' why you are getting the second method to run fastly, for the first function i.e. find2PairsBySumLog what you can do is initialize binary HashMap and check for every pair of integers in array kind of like you are doing in the second function probably like
bool isPairsPresent(int arr[], int arr_size, int sum)
{
int i, temp;
bool binMap[MAX] = {0};
for (i = 0; i < arr_size; i++)
{
temp = sum - arr[i];
if (temp >= 0 && binMap[temp] == 1)
return true;
binMap[arr[i]] = 1;
}
}

Get a number of elements from a specified starting index

Let's say I have a simple array like this:
var myArr = [0,1,2,3,4,5,6,7,8,9]
I'd like to extract a number of elements, starting from a specific index, like this:
myArr.getElementsFromIndex(index, numberOfElements)
where, unlike .slice(), if we hit the last index, elements from the start of the array should be returned instead (so that the total number of elements returned will always be respected). Either pure javascript or a library like underscore/lodash can be used.
Examples:
myArr.getElementsFromIndex(3, 5)
should return[3,4,5,6,7]
and
myArr.getElementsFromIndex(8, 5)
should return [8,9,0,1,2]

Use the below code
var myArr = [0,1,2,3,4,5,6,7,8,9];
function getElementsFromIndex(startIndex, num) {
var elems = [];
for(var iter = 0; iter<num; iter++) {
if(startIndex >= myArr.length) {
while(startIndex >= myArr.length) {
startIndex -= myArr.length;
}
}
elems.push(myArr[startIndex]);
startIndex++;
}
return(elems);
}

Array#slice takes a start and end index (not a start index and a number of elements).
Array#splice does what you want, except for the wrapping around (but also modifies the original array).
You can write a wrapper function using slice (which will not modify the original array):
function getElementsFromIndex(arr, start, numElements) {
if(start + numElements > arr.length) {
var endOfArr = arr.slice(start, arr.length);
var elementsFound = arr.length - start;
var restElements = getElementsFromIndex(arr, 0, numElements - elementsFound);
return endOfArr.concat(restElements);
}
return arr.slice(start, start + numElements);
}
This function returns what you require (see example), and even wraps around multiple times, if needed.
If you want to tie the function to arrays, in order to use it as you propose (ie. myArr.getElementsFromIndex(start, numElements)), you can add it to Array's prototype. You might want to look up arguments for/against modifying prototypes of built-in types, though.
Array.prototype.getElementsFromIndex = function(start, numElements) {
if(start + numElements > this.length) {
var endOfArr = this.slice(start, this.length);
var elementsFound = this.length - start;
return endOfArr.concat(this.getElementsFromIndex(0, numElements - elementsFound));
}
return this.slice(start, start + numElements);
};
See example of the last one here.

Add this to your js code:
Array.prototype.getElementsFromIndex = function (start, len) {
var newArray = [],
origArray = this,
i = start;
while (newArray.length < len) {
newArray.push(origArray[i++]);
if (i >= origArray.length)
i = 0;
}
return newArray;
}
You can use it exactly the way you wanted:
var myArr = [0,1,2,3,4,5,6,7,8,9];
alert(myArr.getElementsFromIndex(8, 5));
JSFIDDLE DEMO: http://jsfiddle.net/x6oy0krL/1
Maybe some people will say that it is not right to extend objects like Array, documentElement and so on, but the result here is as the OP wanted.
I want to say that the original array will not be modified, too.

Just concatenate the array to itself, then use slice:
function sliceWrap(arr, start, num) {
return arr.concat(arr).slice(start, start+num);
}
Another approach, which wraps around:
function sliceWrap2(arr, start, num) {
var result = [], i, end = start+num, len = arr.length;
for (i=start; i<end; i++) {
result.push(arr[i % len]);
}
return result;
}

Knockout: More confusion with indexOf returning -1

Background
I'm attempting to check the existence of a value in array A in a second array, B. Each value is an observable number. Each observable number is contained in an observable array. The comparison always returns -1, which is known to be incorrect (insofar as values in A and B overlap). Therefore, there's something wrong with my logic or syntax, but I have not been able to figure out where.
JSBin (full project): http://jsbin.com/fehoq/190/edit
JS
//set up my two arrays that will be compared
this.scores = ko.observableArray();
//lowest is given values from another method that splices from scores
this.lowest = ko.observableArray();
//computes and returns mean of array less values in lowest
this.mean = (function(scores,i) {
var m = 0;
var count = 0;
ko.utils.arrayForEach(_this.scores(), function(score) {
if (!isNaN(parseFloat(score()))) {
//check values
console.log(score());
// always returns -1
console.log(_this.lowest.indexOf(score()));
//this returns an error, 'not a function'
console.log(_this.lowest()[i]());
//this returns undefined
console.log(_this.lowest()[i]);
//only do math if score() isn't in lowest
// again, always returns -1, so not a good check
if (_this.lowest.indexOf(score())<0) {
m += parseFloat(score());
count += 1;
}
}
});
// rest of the math
if (count) {
m = m / count;
return m.toFixed(2);
} else {
return 'N/A';
}
});
Update
#Major Byte noted that mean() is calculated before anything gets pushed to lowest, hence why I get undefined. If this is true, then what might be the best way to ensure that mean() will update based on changes to lowest?

You really just could use a computed for the mean
this.mean = ko.computed(
function() {
var sum = 0;
var count = 0;
var n = 0;
for(n;n < _this.scores().length;n++)
{
var score = _this.scores()[n];
if (_this.lowest.indexOf(score)<0) {
sum += parseFloat(score());
count++;
}
}
if (count > 0) {
sum = sum / count;
return sum.toFixed(2);
} else {
return 'N/A';
}
});
this will trigger when you add to lower(), scores() and change scores().
obligatory jsfiddle.
Update:
Forgot to mention that I change something crucial as well. From you original code:
this.dropLowestScores = function() {
ko.utils.arrayForEach(_this.students(), function(student){
var comparator = function(a,b){
if(a()<b()){
return 1;
} else if(a() > b()){
return -1;
} else {
return 0;
}
};
var tmp = student.scores().sort(comparator).slice(0);
student.lowest = ko.observableArray(tmp.splice((tmp.length-2),tmp.length-1));
});
};
apart from moving the comparator outside of dropLowestScores function, I changed the line:
student.lowest = ko.observableArray(tmp.splice((tmp.length-2),tmp.length-1));
to
student.lowest(tmp.splice((tmp.length-2),tmp.length-1));
student.lowest is an observable array, no need to define it as an observableArray again, in fact that actually breaks the computed mean. (The correction for Drop Lowest Scores as per my previous comment is left out here).

Return a new number that is unlike the current numbers inside the array

I have this problem about arrays that I can't seem to solve. What I'm trying to do is to return numbers that are not inside the array. Return can only be done if the new value is not inside the array, else it's going to increment the value (to make sure that there is no space).
My code goes like:
function create_number(number) {
var array = [1,2,3,6,7,8,9];
for (var i=0;i<array.length;i++) {
if (array[i] == number) {
return number;
} else {
// create a new number that is not inside the array, and return it.
}
// If not just do the loop again.
// If the loop is over, then just create a valid number
// that is not found inside the array.
}
}

var array = [1,2,3,6,7,8,9];
var number = 0;
while (true) {
if (array.indexOf(++number) == -1) {
array.push(number);
return number;
}
}
but you need to persist array somewhere
PS: Array.prototype.indexOf shim for ancient browsers (credits to #Lochemage)
PPS: the solution above is O(N^2), just for fun here is O(N) one (it requires the array to be sorted initially):
var array = [1,2,3,6,7,8,9];
var number = 1;
while (true) {
if (array[number - 1] != number) {
array.splice(number - 1, 0, number);
return number;
}
++number;
}