Flexible algorithm to calculate possibilities of all possible scenarios - javascript

I've been struggling for a little while to find or figure out algorithm.
The task:
Basically, I have an array of probabilities:
var input = [0.1, 0.2, 0.3, 0.1];
Let's name these inputs accordingly to: A, B, C and D.
And I also have a variable "m", which can tell me how many of these things needs to happen in order to get the result.
For example:
var m = 2;
This variable m is telling me that the event will happen if any of those two (or more) probabilities will happen.
So in this case, for event to happen, all possible ways for event to happen is:
ABCD
ABC
ABD
BCD
AB
AC
AD
BC
BD and CD
Now I need to calculate their probabilities, which I already have algorithms to calculate AND and OR (where input is just a probabilities array).
AND:
if (input.length > 0) {
output = 1;
}
for (i = 0; i < input.length; i++) {
output = input[i] * output;
}
OR:
if (input.length > 0) {
output = input[0];
}
for (i = 1; i < input.length; i++) {
output = (output + input[i]) - (output * input[i]);
}
So I am struggling on figuring out how to loop through all possible possibilities... And to have something like:
(A and B and C and D) or (A and B and C) or (A and B and D)... and so on... I hope you get the idea.

Here's a simple non-recursive solution to enumerate all combinations with at least m elements.
range = n => [...Array.from({length: n}).keys()]
mask = xs => b => xs.filter((_, n) => b & (1 << n))
at_least = n => xs => xs.length >= n
//
a = [...'ABCD']
m = 2
result = range(1 << a.length).map(mask(a)).filter(at_least(m))
console.log(result.map(x => x.join('')))
Since JS bit arithmetics is limited to 32 bits, this only works for m < 32.

You could get the combinations of your desired array with a minimum of two by using a recursive function which generates all the possible combinations.
function getC(array, min) {
function iter(i, temp) {
var t = temp.concat(array[i]);
if (i === array.length) return;
iter(i + 1, t);
iter(i + 1, temp);
if (t.length >= min) {
result.push(t);
}
}
var result = [];
iter(0, []);
return result;
}
var input = [0.1, 0.2, 0.3, 0.1];
console.log(getC(input, 2).map(a => a.join(' ')));
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You could go over all combimations of 2 elements (AB, CD etc.) with two nested loops:
for(let i = 0; i < input.length; i++) {
for(let j = i + 1; j < input.length; j++) {
// possible combination: i and j
for(let k = j; k < input.length; k++) {
// possible combination: i, j, k
// and so on
}
}
}
for at least m elements that can be generalized with a nested generator that generates an array of indices ([0, 1], [0, 2], [1, 2], [0, 1, 2]):
function* combinations(length, m = 1, start = 0) {
// Base Case: If there is only one index left, yield that:
if(start === length - 1) {
yield [length - 1];
return;
}
// Otherwise go over all left indices
for(let i = start; i < length; i++) {
// And get all further combinations, for 0 that will be [1, 2], [1] and [2]
for(const nested of combinations(length, m - 1, i + 1)) {
// Yield the nested path, e.g. [0, 1], [0, 1, 2] and [0, 2]
yield [i, ...nested];
}
// If the minimum length is already reached yield the index itself
if(m <= 1) yield [i];
}
}
Now for every combination, we just have to multiply the probabilities and add them up:
let result = 0;
for(const combination of combimations(input.length, m))
result += combination.reduce((prev, i) => prev * input[i], 1);

Related

Maintain the duplicates in the sum of array integers

I wrote a program to:
Print the new array of elements
Print the sum of all elements (or integers)
Actually, I got it right, however, the little problem is, I want to maintain all the duplicates (still within the range of four largest elements). Here's what I mean:
Take an array of numbers: [4,5,-2,3,1,2,6,6]
The four largest numbers are 4,5,6,6. And their sum is 4+5+6+6=21
What the code is doing (not good):
Instead of getting "6,6,5,4" as (described above), the code is printing "6,5,4,3" with the sum as 18.
ALSO, when there are only four elements [with or without duplicates] as in [1,1,1,-5],
let it just add ALL elements. You guessed it, the sum of all elements is -2
How do I order the program to print the necessary duplicate(s) to make the four largest integers?
Here's my code...
var arr = new Array(4,5,-2,3,1,2,6,6);
// var arr = new Array(1,1,1,-5);
// var largArr = new Array();
function largest() {
largArr = Array(0, 0, 0, 0)
for (i = 0; i < arr.length; i++) {
if (arr[i] > largArr[0]) {
largArr[0] = arr[i];
}
}
for (i = 0; i < arr.length; i++) {
if (arr[i] > largArr[1] && arr[i] < largArr[0]) {
largArr[1] = arr[i];
}
}
for (i = 0; i < arr.length; i++) {
if (arr[i] > largArr[2] && arr[i] < largArr[1]) {
largArr[2] = arr[i];
}
}
for (i = 0; i < arr.length; i++) {
if (arr[i] > largArr[3] && arr[i] < largArr[2]) {
largArr[3] = arr[i];
}
}
console.log(largArr[0], largArr[1], largArr[2], largArr[3]);
console.log(largArr[0] + largArr[1] + largArr[2] + largArr[3]);
}
largest();
I believe there is a genius out there who can help me solve this :)
You could get the top four and filter the original array.
function get4Largest(array) {
const top4 = [...array].sort((a, b) => b - a).slice(0, 4);
return array.filter(v => {
if (top4.includes(v)) {
top4.splice(top4.indexOf(v), 1);
return true;
}
});
}
console.log(get4Largest([4, 5, -2, 3, 1, 2, 6, 6]));
A different approach by keeping indices.
function get4Largest(array) {
return array
.map((v, i) => [v, i])
.sort(([a], [b]) => b - a)
.slice(0, 4)
.sort(([, a], [, b]) => a - b)
.map(([v]) => v);
}
console.log(get4Largest([4, 5, -2, 3, 1, 2, 6, 6]));
If you want sum of largest four numbers then you can easily do with sort, slice and reduce:
numbers.sort((a, b) => b - a).slice(0, 4).reduce((acc, curr) => acc + curr, 0)
const numbers = [4, 5, -2, 3, 1, 2, 6, 6];
const result = numbers
.sort((a, b) => b - a)
.slice(0, 4)
.reduce((acc, curr) => acc + curr, 0);
console.log(result);
You can use a reduce, sort and slice methods of an array like so:
function sumMaxValues (maxLength, values) {
return values
.sort((v1, v2) => v1 > v2 ? -1 : 1)
.slice(0, maxLength)
.reduce((sum, v) => sum + v, 0)
}
console.log(
sumMaxValues(4, [4, 5, -2, 3, 1, 2, 6, 6, 10]),
)
Edit: I fixed, a bug that #gog pointed out. The root cause of a problem was that sort when invoked without a compareFn then "the array elements are converted to strings, then sorted according to each character's Unicode code point value."(sort docs)
If for some reason you want to have a classical type of solution, that avoids modern javascript methods, here's one
const arr = Array(4, 5, -2, 3, 1, 2, 6, 6);
//const arr = Array(1, 1, 1, -5);
function largest(){
const largArr = Array(-1/0, -1/0, -1/0, -1/0);
for(let i = 0; i < arr.length; i++){
for(let j = 0; j < largArr.length; j++){
if(arr[i] > largArr[j]){
for(let k = largArr.length - 2; k >= j; k--){
largArr[k + 1] = largArr[k];
}
largArr[j] = arr[i];
break;
}
}
}
let sum = 0;
for(let j = 0; j < largArr.length; j++){
if(largArr[j] === -1/0){
largArr[j] = 0;
}
sum += largArr[j];
}
console.log(largArr, sum);
}
largest();
-1/0 stands for minus infinity (there can be no smaller number); you may also use Number.NEGATIVE_INFINITY for it. If it's too exotic for your needs, replace -1/0 with any number you are certain is less than any possible number in the array (that however, cannot be zero, since you allow negative numbers also).

Is there a way to add elements with .splice() during a loop in javascript?

I have an array like this:
[0, 24.646213151927437, 27.737256235827665, 29.08326530612245]
I want to fill every space between the elements which are greater than 4 with random numbers. Like this:
var arr = [0, 24.646213151927437, 27.737256235827665, 29.08326530612245];
var length = arr.length;
for (var j = 0; j < length;) {
var spacebetween = arr[j + 1] - arr[j];
if (spacebetween > 4) {
// fill the space until the sum of numbers is greater than space between
var m = Math.random() * (3 - 1 + 1);
for (k = m; k < spacebetween;) {
arr.splice(j, 0, parseFloat(k));
m = Math.random() * (3 - 1 + 1);
k = k + m;
}
}
j++;
}
console.log(arr);
The problem is with splice(), the index is shifting and I have no idea how to solve this. The new numbers should stay in ascending order within the array.
Instead of looping through the array (with a changing length), you can use a reduce to loop through the original array and push updates to the accumulator
var arr = [0, 24, 27, 29];
var newArr = arr.reduce((accumulator, cur, i) => {
var needTempArr = arr[i + 1] && (arr[i + 1] - cur);
// if need temp array, generate it; otherwise, empty array
var tempArr = needTempArr
? Array(needTempArr).fill(0).map(() => Math.random())
: [];
return accumulator.concat(cur, tempArr);
}, []);
Is there a way to add elements with .splice() during a loop in javascript?
Yes, the solution is to simply start from the end and decrement, the new entries will no longer disturb the process:
changed with no gap greater than 4
let arr = [ 0, 24.646213151927437, 27.737256235827665, 29.08326530612245 ]
for (let j=arr.length; --j;) // inside loop j start to (arr.length-1) to value==1 ( value zéro mean false)
{
let vRef = arr[j-1]
, vMin = arr[j] -4
;
while (vMin > vRef)
{
vMin = (Math.random()*4) + vMin
arr.splice( j, 0, vMin )
vMin -= 4
}
}
console.log( JSON.stringify( arr,0,2 ))
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Calculating Josephus Permutations efficiently in Javascript

While training in code-wars I came across a challenge about Joseph-permutations, I tried solving it on paper first and latter translate it to code.
The problem is as follows:
"Create a function that returns a Josephus permutation, taking as parameters the initial array/list of items to be permuted as if they were in a circle and counted out every k places until none remained."
My main idea was:
Have an auxiliary array to keep the response
Use two iterators:
i: to keep track of current index in the given array
k: to keep track of the step of the permutation
Initialize i at 0 and k at 1
When the original array has only one element left:
Push element to output array
Whenever i is not the last index of the array:
If k = step:
Take the element out of the original array, push it into the output array and finally replace k = 1
If k != step:
Increment i and k
When i is the last index of the original array (and the array has more than one element):
If k = step:
Take the element out of the original array, push it into the output array, replace k = 1 and set i = 0
If k != step:
Set i = 0 and increment k
function josephus(items,step){
var output = [];
var i = 0;
var k = 1;
if( items == [] ) {
return [];
}
while (items.length != 1) {
if (k == step && i == items.length - 1) {
output.push(items[i]);
items.splice(i, 1);
i = 0;
k = 1;
} else if (k == step && i != items.length - 1) {
output.push(items[i]);
items.splice(i, 1);
k = 1
} else if (k < step && i == items.length - 1) {
k++;
i=0;
} else if (k < step && i != items.length - 1) {
k++;
i++;
}
}
output.push(items[0]);
return output;
}
This works but it's not efficient, when I run it on the run sample tests I get that 5 of the sample tests have passed but it also includes an STDERR: Execution Timed Out (12000 ms).
The sample tests are the following ones:
Test.assertSimilar(josephus([1,2,3,4,5,6,7,8,9,10],1),[1,2,3,4,5,6,7,8,9,10])
Test.assertSimilar(josephus([1,2,3,4,5,6,7,8,9,10],2),[2, 4, 6, 8, 10, 3, 7, 1, 9, 5])
Test.assertSimilar(josephus(["C","o","d","e","W","a","r","s"],4),['e', 's', 'W', 'o', 'C', 'd', 'r', 'a'])
Test.assertSimilar(josephus([1,2,3,4,5,6,7],3),[3, 6, 2, 7, 5, 1, 4])
Test.assertSimilar(josephus([],3),[])
My question is, how could I make this more efficient?
Is it the algorithm that I'm using that's wrong or is it the implementation?
A comment mentioned two things:
push() is very slow, which was one of my possibilities (wrong data structure)
suggested looking at recursion (which goes more into my doubt about the algorithm). I'm not really seeing how to make this recursive though.
Thanks in advance for your help!
There's a recurrence, which could be memoized. (This seems to pass the Codewars tests.)
function g(n, k, i, memo){
if (memo.hasOwnProperty([n, k, i]))
return memo[[n, k, i]];
if (i == 1)
return memo[[n, k, i]] = (k - 1) % n;
return memo[[n, k, i]] =
(k + g(n - 1, k, i - 1, memo)) % n;
}
function f(A, k){
let n = A.length;
let result = new Array(n);
let memo = {};
for (let i=1; i<=n; i++)
result[i - 1] = A[ g(n, k, i, memo) ];
return result;
}
let str = '';
str += JSON.stringify(f([1,2,3,4,5,6,7,8,9,10],1)) + '\n';
//[1,2,3,4,5,6,7,8,9,10])
str += JSON.stringify(f([1,2,3,4,5,6,7,8,9,10],2)) + '\n';
//[2, 4, 6, 8, 10, 3, 7, 1, 9, 5])
str += JSON.stringify(f(["C","o","d","e","W","a","r","s"],4)) + '\n';
//,['e', 's', 'W', 'o', 'C', 'd', 'r', 'a'])
str += JSON.stringify(f([1,2,3,4,5,6,7],3)) + '\n';
//,[3, 6, 2, 7, 5, 1, 4])
str += JSON.stringify(f([],3))
//,[])
console.log(str);
To explain the recurrence, the first element removed (when i = 1) is clearly (k - 1) mod n (zero-indexed). Now consider finding g(n, k, i). The first element that gets removed is (k - 1) mod n and then we start at the kth position. So the problem is then to find the (i - 1)th element removed after removing the element at (k - 1) mod n and starting at k, which is (k + g(n - 1, k, i - 1)) mod n.
have you tried implementing the functional approach?
from wikipedia:
function getSafePosition(n) {
// find value of L for the equation
valueOfL = n - highestOneBit(n);
safePosition = 2 * valueOfL + 1;
return safePosition;
}
function highestOneBit(i) {
i |= (i >> 1);
i |= (i >> 2);
i |= (i >> 4);
i |= (i >> 8);
i |= (i >> 16);
return i - (i >> 1);
}
this should run in O(n)
You could shift the leading bit to the end.
const josephus = (x) => parseInt(x.toString(2).substr(1) + 1, 2);

Find the smallest sum of all indexes of unique number pairs summing to a given number

I want to loop through an array and then add each value to each other (except itself + itself) and if the sum of the two values that were looped through equals the second argument in my function, and the pair of values hasn't been encountered before, then remember their indices and, at the end, return the full sum of all remembered indices.
In other words, the problem statement is: given an array A of integers and a second value s that is a desired sum, find all pairs of values from array A at indexes i, j such that i < j and A[i] + A[j] = s, and return the sum of all indexes of these pairs, with the following restriction:
don't reuse value pairs, i.e. if two index pairs i, j and k, l satisfying the above conditions are found and if A[i] == A[k] and A[j] == A[l] or A[i] == A[l] and A[j] == A[k], then ignore the pair with the higher index sum.
Example
For example, functionName([1, 4, 2, 3, 0, 5], 7) should return 11 because values 4, 2, 3 and 5 can be paired with each other to equal 7 and the 11 comes from adding the indices of them to get to 11 where:
4 + 3 = 7
5 + 2 = 7
4 [index: 1]
2 [index: 2]
3 [index: 3]
5 [index: 5]
1 + 2 + 3 + 5 = 11
Example #2
functionName([1, 3, 2, 4], 4) would only equal 1, because only the first two elements can be paired to equal 4, and the first element has an index of 0 and the second 1
1 + 3 = 4
1 [index: 0]
3 [index: 1]
0 + 1 = 1
This is what I have so far:
function functionName(arr, arg) {
var newArr = [];
for(var i = 0; i < arr.length; i++){
for(var j = i + 1; j < arr.length; j++) {
if((arr[i] + arr[j]) === arg ) {
newArr.push(i , j);
}
}
}
if(newArr.length === 0) {
return console.log(0);
}
return console.log(newArr.reduce(function(a,b){return a + b}));
}
functionName([1, 4, 2, 3, 0, 5], 7);
The problem I have is that it all works but I have the issue that once it finds a pair that equals the second argument, then it's not supposed to use the same value pairs again but mine does, e.g.:
if the array is [1,1,1] and the second argument is 2, the loop will go through and find the answer but it continues to search after it finds the sum and I only want it to use the pair [1, 1] once, so if it finds a pair like this at indexes [0, 1] then it should not include any other pair that contains the value 1.
I was thinking that i could remove the rest of the values that are the same if more than 2 are found using filter leaving me with only 2 of the same value if there is in an array thus not having to worry about the loop finding a 1 + 1 twice but is this the best way to go about doing it?
I'm still new to this but looking forward to your comments
PS I'm planning on doing this using pure JavaScript and no libraries
Link to a JS fiddle that might make things easier to see what I have.
https://jsfiddle.net/ToreanJoel/xmumv3qt/
This is more complicated than it initially looks. In fact, making a loop inside a loop causes the algorithm to have quadratic time complexity with regard to the size of the array. In other words, for large arrays of numbers, it will take a very long time to complete.
Another way to handle this problem is to notice that you actually have to use each unique value in the array only once (or twice, if s is even and you have two s/2 values somewhere in the array). Otherwise, you would have non-unique pairs. This works because if you need pairs of numbers x and y such that x + y = s, if you know x, then y is determined -- it must be equal s - x.
So you can actually solve the problem in linear time complexity (to be fair, it's sometimes n*log(n) if all values in A are unique, because we have to sort them once).
The steps of the algorithm are as follows:
Make a map whose keys are values in array A, and values are sorted lists of indexes these values appear at in A.
Move through all unique values in A (you collected them when you solved step 1) in ascending order. For each such value:
Assume it's the lower value of the searched pair of values.
Calculate the higher value (it's equal to s - lower)
Check if the higher value also existed in A (you're doing it in constant time thanks to the map created in step 1).
If it does, add the lowest indexes of both the lower and the higher value to the result.
Return the result.
Here's the full code:
function findSumOfUniquePairs(numbers, sum) {
// First, make a map from values to lists of indexes with this value:
var indexesByValue = {},
values = [];
numbers.forEach(function (value, index) {
var indexes = indexesByValue[value];
if (!indexes) {
indexes = indexesByValue[value] = [];
values.push(value);
}
indexes.push(index);
});
values.sort();
var result = 0;
for (var i = 0, maxI = values.length; i < maxI; ++i) {
var lowerValue = values[i],
higherValue = sum - lowerValue;
if (lowerValue > higherValue) {
// We don't have to check symmetrical situations, so let's quit early:
break;
}
var lowerValueIndexes = indexesByValue[lowerValue];
if (lowerValue === higherValue) {
if (lowerValueIndexes.length >= 2) {
result += lowerValueIndexes[0] + lowerValueIndexes[1];
}
} else {
var higherValueIndexes = indexesByValue[higherValue];
if (higherValueIndexes) {
result += lowerValueIndexes[0] + higherValueIndexes[0];
}
}
}
return result;
}
document.write(findSumOfUniquePairs([1, 4, 2, 3, 0, 5], 7) + '<br>'); // 11;
document.write(findSumOfUniquePairs([1, 3, 2, 4], 4) + '<br>'); // 1
document.write(findSumOfUniquePairs([1, 1, 1], 2) + '<br>'); // 1
document.write(findSumOfUniquePairs([1, 1, 1, 1], 2) + '<br>'); // 1
document.write(findSumOfUniquePairs([1, 2, 3, 1, 2, 3, 1], 4) + '<br>'); // 7
document.write(findSumOfUniquePairs([5, 5, 1, 1, 1], 6) + '<br>'); // 2
document.write(findSumOfUniquePairs([0, 5, 0, 5, 1, 1, 1], 6) + '<br>'); // 5
This works, but it mucks up the initial array.
function functionName(arr, arg) {
var newArr = [];
for(var i = 0; i < arr.length; i++){
for(var j = i + 1; j < arr.length; j++) {
if((arr[i] + arr[j]) === arg ) {
newArr.push(i , j);
arr[i] = null;
arr[j] = null;
}
}
}
if(newArr.length === 0) {
return console.log(0);
}
return console.log(newArr.reduce(function(a,b){return a + b}));
}
Solution with loops with restart, if a sum is found. the found summands are stored in usedNumbers and later sorted and used to get the index for summing the index.
The sorting and the last index provides the correct start position for the Array.prototype.indexOf.
Edit:
what about [1,1,1,1], 2 ... should that be 6 or 1? – Jaromanda X 21
#JaromandaX that should be 1, after the pair is found with the values then it shouldn't look for a pair with the same values again – Torean
This version takes care of the requirement.
function f(array, sum) {
var arrayCopy = array.slice(0),
usedNumbers = [],
index = 0,
indexA = 0,
indexB,
a, b;
while (indexA < arrayCopy.length) {
indexB = indexA + 1;
while (indexB < arrayCopy.length) {
a = arrayCopy[indexA];
b = arrayCopy[indexB];
if (a + b === sum) {
usedNumbers.push(a, b);
arrayCopy = arrayCopy.filter(function (i) { return a !== i && b !== i; });
indexA--; // correction to keep the index
break;
}
indexB++;
}
indexA++;
}
return usedNumbers.sort().reduce(function (r, a, i) {
index = array.indexOf(a, i === 0 || a !== usedNumbers[i - 1] ? 0 : index + 1);
return r + index;
}, 0);
}
document.write(f([1, 4, 2, 3, 0, 5], 7) + '<br>'); // 11
document.write(f([1, 1, 1], 2) + '<br>'); // 1
document.write(f([5, 5, 1, 1, 1], 6) + '<br>'); // 2
document.write(f([0, 5, 0, 5, 1, 1, 1], 6) + '<br>'); // 5
document.write(f([1, 1, 1, 1], 2) + '<br>'); // 1
The solution below is very compact. It avoids unnecessary checks and loops only through the relevant elements. You can check the working codepen here:
http://codepen.io/PiotrBerebecki/pen/RRGaBZ.
function pairwise(arr, arg) {
var sum = 0;
for (var i=0; i<arr.length-1; i++) {
for (var j=i+1; j<arr.length; j++) {
if (arr[i] <= arg && arr[j] <= arg && arr[i] + arr[j] == arg) {
sum += i+j;
arr[i] = arr[j] = NaN;
}
}
}
return sum;
}
console.log( pairwise([1, 1, 0, 2], 2) ) // should return 6
Under the hood:
Start looping from the element with index (i) = 0.
Add a second loop only for the elements which are later in the array. Their index j is always higher than i as we are adding 1 to i.
If both elements (numbers) are less than or equal to to the arg, check if their sum equals to the arg. This avoids checking the sum if either of the numbers are greater than the arg.
If the pair has been found then change their values to NaN to avoid further checks and duplication.
This solution should have a time complexity of 0(n) or linear
Much faster than two nested for-loops. This function will give you the two indices that add up to the target number. It can easily be modified to solve any other configuration of this problem.
var twoSum = function(nums, target) {
const hash = {}
for(let i = 0; i < nums.length; i++) {
hash[nums[i]] = i
}
for(let j = 0; j < nums.length; j++) {
let numToFind = target - nums[j]
if(numToFind in hash && hash[numToFind] !== j) {
return [hash[numToFind], j]
}
}
return false
};
console.log(twoSum([1,2,3,5,7], 5))
In Python:
def twoSum(self, nums: List[int], target: int) -> List[int]:
myMap = {}
for i in range(len(nums)):
myMap[nums[i]] = i
for j in range(len(nums)):
numToFind = target - nums[j]
if numToFind in myMap and myMap[numToFind] != j:
return [myMap[numToFind], j]
print(twoSum([1,2,3,5,7], 5))
In Java:
import java.util.*;
class Solution {
public int[] twoSum(int[] nums, int target) {
Map<Integer, Integer> map = new HashMap<>();
for(Integer i = 0; i < nums.length; i++) {
map.put(nums[i], i);
}
for(Integer j = 0; j < nums.length; j++) {
Integer numToFind = target - nums[j];
Integer myInt = map.get(numToFind);
if(map.containsKey(numToFind) && myInt != j) {
return new int[] {myInt , j};
}
}
return new int[] {0, 0};
}
}
System.out.println(twoSum([1,2,3,5,7], 5))

Processing an array by sub arrays

Suppose we have an array of variable length, and I want to process it by chunks that are of a maximum length of 100, and do it in the minimum number of chunks. So for an array of length 241, it would be 3 sub arrays of sizes 41, 100, 100 (or 100, 100, 41).
curr_len = arr.length;
offset = curr_len%100;
doSomethingWithSubArray(arr.slice(offset))
for(j = offset; j <= curr_len; j = j+100){
doSomethingWithSubArray(arr.slice(j,j+100))
}
I'm sure there are more elegant ways of doing this, possibly without the special case before the for loop. Any ideas?
I'd expect the last chunk to be of smaller size. The code then would be:
for (var i=0; i<arr.length; i+=100)
doSomethingWithSubArray(arr.slice(i, 100));
This is exactly what my splitBy function does:
Array.prototype.splitBy = function(n) {
/* get: number of items per array
return: array of n-sized arrays with the items (last array may contain less then n) */
for (var r=[], i=0; i<this.length; i+=n)
r.push(this.slice(i, i+n));
return r;
}
Then write only:
arr.splitBy(100).forEach(doSomethingWithSubArray);
use chunk function~
function chunk(a, s){
for(var x, i = 0, c = -1, l = a.length, n = []; i < l; i++)
(x = i % s) ? n[c][x] = a[i] : n[++c] = [a[i]];
return n;
}
console.log(chunk([1,2,3,4,5,6,7,8,9,10], 3));
it's functional style recursive solutions.
no var, no loop, no count, because it's more cleary
var chunk = function(arr, n){
if (arr.length == 0) return [];
var head = arr.slice(0, n), rest = arr.slice(n);
return [head].concat( chunk(rest, n) );
};
console.log(chunk([1,2,3,4,5,6,7,8,9,10], 3));​
Not really, using reduce looks like this:
var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
var splitArrays = array.reduce(function(arr, cur, i) {
if (i % 3 === 0) arr.push([]);
arr[i / 3 | 0].push(cur);
return arr;
}, []);
//splitArrays looks like:
//[[1,2,3],[4,5,6],[7,8,9],[10,11]]
More generic function
function splitArray(array, num) {
return array.reduce(function(arr, cur, i) {
if (i % num === 0) arr.push([]);
arr[i / num | 0].push(cur);
return arr;
}, []);
}
Make your doSomethingWithSubArray function accept a starting index and return a next unprocessed index or null if there's no more work. Put this "iterator" in a while loop. Do rest of work that you want to do between chunks (update UI?) right after calling this "iterator" in a while condition.

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