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How to return a new array that its values are greater than its 2nd value. If the array that was passed to the function has less than two elements, function should return false.
For example,
greaterThanSecond([1,3,5,7])
should return [5, 7].
greaterThanSecond([0, -3, 2, 5])
should return [0, 2, 5].
greaterThanSecond([2])
should return false.
This is what I tried.
function valGreaterThanSecond(arr) {
for (let newArr of arr) {
if (newArr > arr[1]) {
return [newArr]
}else {
return false
}
}
}
You can try a one-liner:
[0, -3, 2, 5].filter((element, index, array) => element > array[1])
The filter function has 3 parameters:
The inspected elment
Index of the inspected element
The original array
The filter iterate through the array and you can compare to the original array's second element
function valGreaterThanSecond(arr) {
let resultArray = []
console.log(arr.length)
let checkValue = arr[1]
if(arr.length < 2)
return false
else
{
for(let i = 0 ; i < arr.length ; i++){
if(arr[i]!=checkValue && arr[i]>checkValue)
resultArray.push(arr[i])
}
return resultArray
}
}
console.log(valGreaterThanSecond([2]))
try this approach
You can try this
function greaterThanSecond(arr) {
if (arr.length < 2)
return false;
return arr.filter((item) => item > arr[1])
}
console.log(greaterThanSecond([0,-3,2,5]))
You can try this:
const greaterThanSecond = arr => {
if (arr.length > 1){
return arr.filter(e => e > arr[1])
}
return false
}
console.log(greaterThanSecond([1,3,5,7]))
In this function, at first you should check if the length of the array is not less than 2. Then we filter the array by checking if each number in the array is bigger than the second number and keep those ones in the array.
First let's try fixing your code before I am suggesting another approach.
function valGreaterThanSecond(arr) {
let newArr = [];
if (arr.length < 2) return false;
for (let elem of arr) {
if (elem > arr[1]) {
newArr = [...newArr, elem]
}
}
return newArr;
};
console.log(valGreaterThanSecond([1,3,5,7]));
The problem with your function is that once you found a number that is greater than the second element you immediately return it, thus exiting the function and returning this one element in an array, which is not what you want. If you found an element in the array that is not greater than the second element you immediately return false, which is not the behavior you want as well. In your example of [1,3,5,7] you will return false because 1 < 3 and you are out of the function at this point.
Different approach using reduce
I would like to suggest a different approach since there are a few answers using filter which is the first thing I would think about myself.
Here is a solution using the reduce function for arrays.
const greaterThanSecond = (arr) => {
if (arr.length < 2) return false;
const result = arr.reduce((acc, curr) => {
if (curr > arr[1]) return [...acc, curr];
return acc;
}, []);
return result;
}
console.log(greaterThanSecond([1,3,5,7]));
I have an array of following strings:
['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0']
...etc.
I need a solution that will give me following ordered result
['4.5.0', '4.21.0', '4.22.0', '5.1.0', '5.5.1', '6.1.0'].
I tried to implement a sort so it first sorts by the numbers in the first position, than in case of equality, sort by the numbers in the second position (after the first dot), and so on...
I tried using sort() and localeCompare(), but if I have elements '4.5.0' and '4.11.0', I get them sorted as ['4.11.0','4.5.0'], but I need to get ['4.5.0','4.11.0'].
How can I achieve this?
You could prepend all parts to fixed size strings, then sort that, and finally remove the padding again.
var arr = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
arr = arr.map( a => a.split('.').map( n => +n+100000 ).join('.') ).sort()
.map( a => a.split('.').map( n => +n-100000 ).join('.') );
console.log(arr)
Obviously you have to choose the size of the number 100000 wisely: it should have at least one more digit than your largest number part will ever have.
With regular expression
The same manipulation can be achieved without having to split & join, when you use the callback argument to the replace method:
var arr = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
arr = arr.map( a => a.replace(/\d+/g, n => +n+100000 ) ).sort()
.map( a => a.replace(/\d+/g, n => +n-100000 ) );
console.log(arr)
Defining the padding function once only
As both the padding and its reverse functions are so similar, it seemed a nice exercise to use one function f for both, with an extra argument defining the "direction" (1=padding, -1=unpadding). This resulted in this quite obscure, and extreme code. Consider this just for fun, not for real use:
var arr = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
arr = (f=>f(f(arr,1).sort(),-1)) ((arr,v)=>arr.map(a=>a.replace(/\d+/g,n=>+n+v*100000)));
console.log(arr);
Use the sort compare callback function
You could use the compare function argument of sort to achieve the same:
arr.sort( (a, b) => a.replace(/\d+/g, n => +n+100000 )
.localeCompare(b.replace(/\d+/g, n => +n+100000 )) );
But for larger arrays this will lead to slower performance. This is because the sorting algorithm will often need to compare a certain value several times, each time with a different value from the array. This means that the padding will have to be executed multiple times for the same number. For this reason, it will be faster for larger arrays to first apply the padding in the whole array, then use the standard sort, and then remove the padding again.
But for shorter arrays, this approach might still be the fastest. In that case, the so-called natural sort option -- that can be achieved with the extra arguments of localeCompare -- will be more efficient than the padding method:
var arr = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
arr = arr.sort( (a, b) => a.localeCompare(b, undefined, { numeric:true }) );
console.log(arr);
More about the padding and unary plus
To see how the padding works, look at the intermediate result it generates:
[ "100005.100005.100001", "100004.100021.100000", "100004.100022.100000",
"100006.100001.100000", "100005.100001.100000" ]
Concerning the expression +n+100000, note that the first + is the unary plus and is the most efficient way to convert a string-encoded decimal number to its numerical equivalent. The 100000 is added to make the number have a fixed number of digits. Of course, it could just as well be 200000 or 300000. Note that this addition does not change the order the numbers will have when they would be sorted numerically.
The above is just one way to pad a string. See this Q&A for some other alternatives.
If you are looking for a npm package to compare two semver version, https://www.npmjs.com/package/compare-versions is the one.
Then you can sort version like this:
// ES6/TypeScript
import compareVersions from 'compare-versions';
var versions = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
var sorted = versions.sort(compareVersions);
You could split the strings and compare the parts.
function customSort(data, order) {
function isNumber(v) {
return (+v).toString() === v;
}
var sort = {
asc: function (a, b) {
var i = 0,
l = Math.min(a.value.length, b.value.length);
while (i < l && a.value[i] === b.value[i]) {
i++;
}
if (i === l) {
return a.value.length - b.value.length;
}
if (isNumber(a.value[i]) && isNumber(b.value[i])) {
return a.value[i] - b.value[i];
}
return a.value[i].localeCompare(b.value[i]);
},
desc: function (a, b) {
return sort.asc(b, a);
}
}
var mapped = data.map(function (el, i) {
return {
index: i,
value: el.split('')
};
});
mapped.sort(sort[order] || sort.asc);
return mapped.map(function (el) {
return data[el.index];
});
}
var array = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0'];
console.log('sorted array asc', customSort(array));
console.log('sorted array desc ', customSort(array, 'desc'));
console.log('original array ', array);
.as-console-wrapper { max-height: 100% !important; top: 0; }
You can check in loop if values are different, return difference, else continue
var a=['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
a.sort(function(a,b){
var a1 = a.split('.');
var b1 = b.split('.');
var len = Math.max(a1.length, b1.length);
for(var i = 0; i< len; i++){
var _a = +a1[i] || 0;
var _b = +b1[i] || 0;
if(_a === _b) continue;
else return _a > _b ? 1 : -1
}
return 0;
})
console.log(a)
Though slightly late this would be my solution;
var arr = ["5.1.1","5.1.12","5.1.2","3.7.6","2.11.4","4.8.5","4.8.4","2.10.4"],
sorted = arr.sort((a,b) => {var aa = a.split("."),
ba = b.split(".");
return +aa[0] < +ba[0] ? -1
: aa[0] === ba[0] ? +aa[1] < +ba[1] ? -1
: aa[1] === ba[1] ? +aa[2] < +ba[2] ? -1
: 1
: 1
: 1;
});
console.log(sorted);
Here's a solution I developed based on #trincot's that will sort by semver even if the strings aren't exactly "1.2.3" - they could be i.e. "v1.2.3" or "2.4"
function sortSemVer(arr, reverse = false) {
let semVerArr = arr.map(i => i.replace(/(\d+)/g, m => +m + 100000)).sort(); // +m is just a short way of converting the match to int
if (reverse)
semVerArr = semVerArr.reverse();
return semVerArr.map(i => i.replace(/(\d+)/g, m => +m - 100000))
}
console.log(sortSemVer(["1.0.1", "1.0.9", "1.0.10"]))
console.log(sortSemVer(["v2.1", "v2.0.9", "v2.0.12", "v2.2"], true))
This seems to work provided there are only digits between the dots:
var a = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0']
a = a.map(function (x) {
return x.split('.').map(function (x) {
return parseInt(x)
})
}).sort(function (a, b) {
var i = 0, m = a.length, n = b.length, o, d
o = m < n ? n : m
for (; i < o; ++i) {
d = (a[i] || 0) - (b[i] || 0)
if (d) return d
}
return 0
}).map(function (x) {
return x.join('.')
})
'use strict';
var arr = ['5.1.2', '5.1.1', '5.1.1', '5.1.0', '5.7.2.2'];
Array.prototype.versionSort = function () {
var arr = this;
function isNexVersionBigger (v1, v2) {
var a1 = v1.split('.');
var b2 = v2.split('.');
var len = a1.length > b2.length ? a1.length : b2.length;
for (var k = 0; k < len; k++) {
var a = a1[k] || 0;
var b = b2[k] || 0;
if (a === b) {
continue;
} else
return b < a;
}
}
for (var i = 0; i < arr.length; i++) {
var min_i = i;
for (var j = i + 1; j < arr.length; j++) {
if (isNexVersionBigger(arr[i], arr[j])) {
min_i = j;
}
}
var temp = arr[i];
arr[i] = arr[min_i];
arr[min_i] = temp;
}
return arr;
}
console.log(arr.versionSort());
This solution accounts for version numbers that might not be in the full, 3-part format (for example, if one of the version numbers is just 2 or 2.0 or 0.1, etc).
The custom sort function I wrote is probably mostly what you're looking for, it just needs an array of objects in the format {"major":X, "minor":X, "revision":X}:
var versionArr = ['5.5.1', '4.21.0', '4.22.0', '6.1.0', '5.1.0', '4.5.0'];
var versionObjectArr = [];
var finalVersionArr = [];
/*
split each version number string by the '.' and separate them in an
object by part (major, minor, & revision). If version number is not
already in full, 3-part format, -1 will represent that part of the
version number that didn't exist. Push the object into an array that
can be sorted.
*/
for(var i = 0; i < versionArr.length; i++){
var splitVersionNum = versionArr[i].split('.');
var versionObj = {};
switch(splitVersionNum.length){
case 1:
versionObj = {
"major":parseInt(splitVersionNum[0]),
"minor":-1,
"revision":-1
};
break;
case 2:
versionObj = {
"major":parseInt(splitVersionNum[0]),
"minor":parseInt(splitVersionNum[1]),
"revision":-1
};
break;
case 3:
versionObj = {
"major":parseInt(splitVersionNum[0]),
"minor":parseInt(splitVersionNum[1]),
"revision":parseInt(splitVersionNum[2])
};
}
versionObjectArr.push(versionObj);
}
//sort objects by parts, going from major to minor to revision number.
versionObjectArr.sort(function(a, b){
if(a.major < b.major) return -1;
else if(a.major > b.major) return 1;
else {
if(a.minor < b.minor) return -1;
else if(a.minor > b.minor) return 1;
else {
if(a.revision < b.revision) return -1;
else if(a.revision > b.revision) return 1;
}
}
});
/*
loops through sorted object array to recombine it's version keys to match the original string's value. If any trailing parts of the version
number are less than 0 (i.e. they didn't exist so we replaced them with
-1) then leave that part of the version number string blank.
*/
for(var i = 0; i < versionObjectArr.length; i++){
var versionStr = "";
for(var key in versionObjectArr[i]){
versionStr = versionObjectArr[i].major;
versionStr += (versionObjectArr[i].minor < 0 ? '' : "." + versionObjectArr[i].minor);
versionStr += (versionObjectArr[i].revision < 0 ? '' : "." + versionObjectArr[i].revision);
}
finalVersionArr.push(versionStr);
}
console.log('Original Array: ',versionArr);
console.log('Expected Output: ',['4.5.0', '4.21.0', '4.22.0', '5.1.0', '5.5.1', '6.1.0']);
console.log('Actual Output: ', finalVersionArr);
Inspired from the accepted answer, but ECMA5-compatible, and with regular string padding (see my comments on the answer):
function sortCallback(a, b) {
function padParts(version) {
return version
.split('.')
.map(function (part) {
return '00000000'.substr(0, 8 - part.length) + part;
})
.join('.');
}
a = padParts(a);
b = padParts(b);
return a.localeCompare(b);
}
Usage:
['1.1', '1.0'].sort(sortCallback);
const arr = ["5.1.1","5.1.12","5.1.2","3.7.6","2.11.4","4.8.5","4.8.4","2.10.4"];
const sorted = arr.sort((a,b) => {
const ba = b.split('.');
const d = a.split('.').map((a1,i)=>a1-ba[i]);
return d[0] ? d[0] : d[1] ? d[1] : d[2]
});
console.log(sorted);
This can be in an easier way using the sort method without hardcoding any numbers and in a more generic way.
enter code here
var arr = ['5.1.2', '5.1.1', '5.1.1', '5.1.0', '5.7.2.2'];
splitArray = arr.map(elements => elements.split('.'))
//now lets sort based on the elements on the corresponding index of each array
//mapped.sort(function(a, b) {
// if (a.value > b.value) {
// return 1;
// }
// if (a.value < b.value) {
// return -1;
// }
// return 0;
//});
//here we compare the first element with the first element of the next version number and that is [5.1.2,5.7.2] 5,5 and 1,7 and 2,2 are compared to identify the smaller version...In the end use the join() to get back the version numbers in the proper format.
sortedArray = splitArray.sort((a, b) => {
for (i in a) {
if (parseInt(a[i]) < parseInt(b[i])) {
return -1;
break
}
if (parseInt(a[i]) > parseInt(b[i])) {
return +1;
break
} else {
continue
}
}
}).map(p => p.join('.'))
sortedArray = ["5.1.0", "5.1.1", "5.1.1", "5.1.2", "5.7.2.2"]
sort 1.0a notation correct
use native localeCompare to sort 1.090 notation
function log(label,val){
document.body.append(label,String(val).replace(/,/g," - "),document.createElement("BR"));
}
const sortVersions = (
x,
v = s => s.match(/[a-z]|\d+/g).map(c => c==~~c ? String.fromCharCode(97 + c) : c)
) => x.sort((a, b) => (a + b).match(/[a-z]/)
? v(b) < v(a) ? 1 : -1
: a.localeCompare(b, 0, {numeric: true}))
let v=["1.90.1","1.090","1.0a","1.0.1","1.0.0a","1.0.0b","1.0.0.1","1.0a"];
log(' input : ',v);
log('sorted: ',sortVersions(v));
log('no dups:',[...new Set(sortVersions(v))]);
In ES6 you can go without regex.
const versions = ["0.4", "0.11", "0.4.1", "0.4", "0.4.2", "2.0.1","2", "0.0.1", "0.2.3"];
const splitted = versions.map(version =>
version
.split('.')
.map(i => +i))
.map(i => {
let items;
if (i.length === 1) {
items = [0, 0]
i.push(...items)
}
if (i.length === 2) {
items = [0]
i.push(...items)
}
return i
})
.sort((a, b) => {
for(i in a) {
if (a[i] < b[i]) {
return -1;
}
if (a[i] > b[i]) {
return +1;
}
}
})
.map(item => item.join('.'))
const sorted = [...new Set(splitted)]
If ES6 I do this:
versions.sort((v1, v2) => {
let [, major1, minor1, revision1 = 0] = v1.match(/([0-9]+)\.([0-9]+)(?:\.([0-9]+))?/);
let [, major2, minor2, revision2 = 0] = v2.match(/([0-9]+)\.([0-9]+)(?:\.([0-9]+))?/);
if (major1 != major2) return parseInt(major1) - parseInt(major2);
if (minor1 != minor2) return parseInt(minor1) - parseInt(major2);
return parseInt(revision1) - parseInt(revision2);
});
**Sorted Array Object by dotted version value**
var sampleData = [
{ name: 'Edward', value: '2.1.2' },
{ name: 'Sharpe', value: '2.1.3' },
{ name: 'And', value: '2.2.1' },
{ name: 'The', value: '2.1' },
{ name: 'Magnetic', value: '2.2' },
{ name: 'Zeros', value: '0' },
{ name: 'Zeros', value: '1' }
];
arr = sampleData.map( a => a.value).sort();
var requireData = [];
arr.forEach(function(record, index){
var findRecord = sampleData.find(arr => arr.value === record);
if(findRecord){
requireData.push(findRecord);
}
});
console.log(requireData);
[check on jsfiddle.net][1]
[1]: https://jsfiddle.net/jx3buswq/2/
It is corrected now!!!
I have this code that pairs same elements in an array, with the expectation that the array will have an odd length and it should return the only element that couldn't get a pair. So I wrote the code just well, and it works fine for smaller arrays, but with very large big integers of over 1 billion, the time complexity became O(N**2) and then the need to refactor my code to get a much better performance for large arrays and large array elements. Here is my code below;
function solution(A) {
if(!Array.isArray(A)) return 0;
var temp = new Array(A.length);
var position = 0;
for(let i=0; i<A.length; i++){
if(temp.includes(A[i])){
position = temp.indexOf(A[i]);
index = A.indexOf(A[i]);
delete temp[position];
delete A[index];
delete A[i];
}else{
temp[i] = A[i];
}
}
for(let j=0; j<A.length; j++){
if(A[j] !== undefined) return A[j];
else continue;
}
}
To test it, source data can look like [2,3,6,7,3,5,5,6,2] and it will give an output of 7. But when the array is so large up to [1,2,....] with length n = n=999,999, or n = 5000,000,000, the time complexity increases exponentially.
You might use Object to store non-paired elements only.
Please note that you don't need to store all the array elements and their counts in the Object and then filter by count (like #StepUp does).
Everything's been done in a single loop.
The function returns Array of all non-paired elements:
const solution = A => Array.isArray(A) ?
Object.keys(
A.reduce((r, k) => {
r[k] = r[k] || 0;
if (++r[k] > 1) delete r[k];
return r;
}, {})
) : [];
console.log(solution([2, 3, 6, 7, 3, 5, 5, 6, 2]))
We can try to find odd occurrences for one iteration by using great features of object. Object is key - value pair. So access to object key is O(1). So when we meet the same element, then we just increment value:
const hashMap = arr.reduce((a, c)=> {
a[c] = a[c] || 0;
a[c] += 1;
return a;
},{})
const result = Object.keys(hashMap).filter(key => hashMap[key] === 1);
An example:
let arr = [2, 3, 6, 7, 3, 5, 5, 6, 2];
const hashMap = arr.reduce((a, c)=> {
a[c] = a[c] || 0;
a[c] += 1;
return a;
},{})
const result = Object.keys(hashMap).filter(key => hashMap[key] === 1);
console.log(result);
My two 100% JavaScript solutions with optimized time complexity. The first one is using Set:
function solution(A) {
const pairs = new Set();
for (const num of A) {
if (pairs.has(num)) {
pairs.delete(num);
} else {
pairs.add(num);
}
}
const [unpaired] = pairs;
return unpaired;
}
The second one is using bitwise XOR:
function solution(A) {
let unpaired;
for (const num of A) {
unpaired ^= num;
}
return unpaired;
}
I am trying to arrange any kind of array input into a sorted array that also combines the equal pairs into an array inside the same array.
I do the following
const arrangeTheArray=(arr)=>
{
//checking input here
if(arr.length<2)
{
return arr;
} else {
//sorting the array
arr= arr.sort();
}
//displaying the sorted array
console.log(arr);
for(let i=1; i<arr.length;i++)
{
for(let j=0;j<i;j++)
{
//here I am looping and comparing the values of array
if(arr[j]===arr[i]){
//putting the value
arr[j]= [arr[i],arr[j]];
}
}
}
//displaying the final output
console.log(arr);
}
arrangeTheArray([0,2,2,1,1,6,3,1,0])
e.g array input : [0,2,2,1,1,6,3,1,0]
final out put: [[0,0],[1,1,1],[2,2],3,6]
You can use reduce and map.
Here idea is
First create a object with each digit as key and group the values by key.
Now map on the grouped data, if the length of element greater than one pass element as it is, else pass the 0th index value
let combine = (arr) =>{
let groups = arr.reduce((op,inp)=>{
op[inp] = op[inp] || []
op[inp].push(inp)
return op
},{})
let final = Object.values(groups).map(e=> e.length > 1 ? e : e[0])
return final
}
console.log(combine([0,2,2,1,1,6,3,1,0]))
const arrangeTheArray=(arr)=>{
if(arr.length<2)
{
return arr;
} else {
arr = arr.sort((a,b)=>a-b);
}
let final = []
for(let i=0;i<arr.length;i++){
let current = arr[i]
let j = i;
let temp = []
while(arr[j] === current){
temp.push(arr[j])
j++
}
i = j-1
temp = temp.length > 1 ? temp : temp[0]
final.push(temp)
}
console.log(final)
}
arrangeTheArray([0,2,2,1,1,6,3,1,0])
You can use Array.reduce to accumulate the common elements in an object.
Then use Object.values and Array.from to process the nested arrays into arrays of common elements and distinct elements:
const arrangeTheArray = (arr) => {
if (!Array.isArray(arr) && arr.length < 2){
return arr;
}
const pairs = arr.reduce((acc, ele) => {
if(acc[ele]){
acc[ele].push(ele);
}else{
acc[ele] = [ele];
}
return acc;
}, {});
return Array.from(Object.values(pairs), ele => ele.length > 1 ? ele : +ele.join())
}
console.log(arrangeTheArray([0,2,2,1,1,6,3,1,0]));
You could sort the array with a callback for numbers and reduce the array by checking the predecessor p and the actual value v and push either the value or an array of the last group and the value.
function arrangeTheArray(array) {
return array
.sort((a, b) => a - b)
.reduce(
(r, v, i, { [i - 1]: p }) => r.concat([p !== v ? v : [].concat(r.pop(), v)]),
[]
);
}
console.log(arrangeTheArray([0, 2, 2, 1, 1, 6, 3, 1, 0]));
.as-console-wrapper { max-height: 100% !important; top: 0; }
With a classsic while loop from the end, because the array shrinks.
function arrangeTheArray(array) {
var i = array.length - 1;
array.sort((a, b) => a - b);
while (i--) {
if (array[i] === (Array.isArray(array[i + 1]) ? array[i + 1][0] : array[i + 1])) {
array[i] = [].concat(array[i], ...array.splice(i + 1, 1));
}
}
return array;
}
console.log(arrangeTheArray([0, 2, 2, 1, 1, 6, 3, 1, 0]));
.as-console-wrapper { max-height: 100% !important; top: 0; }
How can I break the iteration of reduce() method?
for:
for (var i = Things.length - 1; i >= 0; i--) {
if(Things[i] <= 0){
break;
}
};
reduce()
Things.reduce(function(memo, current){
if(current <= 0){
//break ???
//return; <-- this will return undefined to memo, which is not what I want
}
}, 0)
You CAN break on any iteration of a .reduce() invocation by mutating the 4th argument of the reduce function: "array". No need for a custom reduce function. See Docs for full list of .reduce() parameters.
Array.prototype.reduce((acc, curr, i, array))
The 4th argument is the array being iterated over.
const array = ['apple', '-pen', '-pineapple', '-pen'];
const x = array
.reduce((acc, curr, i, arr) => {
if(i === 2) arr.splice(1); // eject early
return acc += curr;
}, '');
console.log('x: ', x); // x: apple-pen-pineapple
WHY?:
The one and only reason I can think of to use this instead of the many other solutions presented is if you want to maintain a functional programming methodology to your algorithm, and you want the most declarative approach possible to accomplish that. If your entire goal is to literally REDUCE an array to an alternate non-falsey primitive (string, number, boolean, Symbol) then I would argue this IS in fact, the best approach.
WHY NOT?
There's a whole list of arguments to make for NOT mutating function parameters as it's a bad practice.
UPDATE
Some of the commentators make a good point that the original array is being mutated in order to break early inside the .reduce() logic.
Therefore, I've modified the answer slightly by adding a .slice(0) before calling a follow-on .reduce() step, yielding a copy of the original array.
NOTE: Similar ops that accomplish the same task are slice() (less explicit), and spread operator [...array] (slightly less performant). Bear in mind, all of these add an additional constant factor of linear time to the overall runtime ... + O(n).
The copy, serves to preserve the original array from the eventual mutation that causes ejection from iteration.
const array = ['apple', '-pen', '-pineapple', '-pen'];
const x = array
.slice(0) // create copy of "array" for iterating
.reduce((acc, curr, i, arr) => {
if (i === 2) arr.splice(1); // eject early by mutating iterated copy
return (acc += curr);
}, '');
console.log("x: ", x, "\noriginal Arr: ", array);
// x: apple-pen-pineapple
// original Arr: ['apple', '-pen', '-pineapple', '-pen']
Don't use reduce. Just iterate on the array with normal iterators (for, etc) and break out when your condition is met.
You can use functions like some and every as long as you don't care about the return value. every breaks when the callback returns false, some when it returns true:
things.every(function(v, i, o) {
// do stuff
if (timeToBreak) {
return false;
} else {
return true;
}
}, thisArg);
Edit
A couple of comments that "this doesn't do what reduce does", which is true, but it can. Here's an example of using every in a similar manner to reduce that returns as soon as the break condition is reached.
// Soruce data
let data = [0,1,2,3,4,5,6,7,8];
// Multiple values up to 5 by 6,
// create a new array and stop processing once
// 5 is reached
let result = [];
data.every(a => a < 5? result.push(a*6) : false);
console.log(result);
This works because the return value from push is the length of the result array after the new element has been pushed, which will always be 1 or greater (hence true), otherwise it returns false and the loop stops.
There is no way, of course, to get the built-in version of reduce to exit prematurely.
But you can write your own version of reduce which uses a special token to identify when the loop should be broken.
var EXIT_REDUCE = {};
function reduce(a, f, result) {
for (let i = 0; i < a.length; i++) {
let val = f(result, a[i], i, a);
if (val === EXIT_REDUCE) break;
result = val;
}
return result;
}
Use it like this, to sum an array but exit when you hit 99:
reduce([1, 2, 99, 3], (a, b) => b === 99 ? EXIT_REDUCE : a + b, 0);
> 3
Array.every can provide a very natural mechanism for breaking out of high order iteration.
const product = function(array) {
let accumulator = 1;
array.every( factor => {
accumulator *= factor;
return !!factor;
});
return accumulator;
}
console.log(product([2,2,2,0,2,2]));
// 0
You can break every code - and thus every build in iterator - by throwing an exception:
function breakReduceException(value) {
this.value = value
}
try {
Things.reduce(function(memo, current) {
...
if (current <= 0) throw new breakReduceException(memo)
...
}, 0)
} catch (e) {
if (e instanceof breakReduceException) var memo = e.value
else throw e
}
You can use try...catch to exit the loop.
try {
Things.reduce(function(memo, current){
if(current <= 0){
throw 'exit loop'
//break ???
//return; <-- this will return undefined to memo, which is not what I want
}
}, 0)
} catch {
// handle logic
}
As the promises have resolve and reject callback arguments, I created the reduce workaround function with the break callback argument. It takes all the same arguments as native reduce method, except the first one is an array to work on (avoid monkey patching). The third [2] initialValue argument is optional. See the snippet below for the function reducer.
var list = ["w","o","r","l","d"," ","p","i","e","r","o","g","i"];
var result = reducer(list,(total,current,index,arr,stop)=>{
if(current === " ") stop(); //when called, the loop breaks
return total + current;
},'hello ');
console.log(result); //hello world
function reducer(arr, callback, initial) {
var hasInitial = arguments.length >= 3;
var total = hasInitial ? initial : arr[0];
var breakNow = false;
for (var i = hasInitial ? 0 : 1; i < arr.length; i++) {
var currentValue = arr[i];
var currentIndex = i;
var newTotal = callback(total, currentValue, currentIndex, arr, () => breakNow = true);
if (breakNow) break;
total = newTotal;
}
return total;
}
And here is the reducer as an Array method modified script:
Array.prototype.reducer = function(callback,initial){
var hasInitial = arguments.length >= 2;
var total = hasInitial ? initial : this[0];
var breakNow = false;
for (var i = hasInitial ? 0 : 1; i < this.length; i++) {
var currentValue = this[i];
var currentIndex = i;
var newTotal = callback(total, currentValue, currentIndex, this, () => breakNow = true);
if (breakNow) break;
total = newTotal;
}
return total;
};
var list = ["w","o","r","l","d"," ","p","i","e","r","o","g","i"];
var result = list.reducer((total,current,index,arr,stop)=>{
if(current === " ") stop(); //when called, the loop breaks
return total + current;
},'hello ');
console.log(result);
Reduce functional version with break can be implemented as 'transform', ex. in underscore.
I tried to implement it with a config flag to stop it so that the implementation reduce doesn't have to change the data structure that you are currently using.
const transform = (arr, reduce, init, config = {}) => {
const result = arr.reduce((acc, item, i, arr) => {
if (acc.found) return acc
acc.value = reduce(config, acc.value, item, i, arr)
if (config.stop) {
acc.found = true
}
return acc
}, { value: init, found: false })
return result.value
}
module.exports = transform
Usage1, simple one
const a = [0, 1, 1, 3, 1]
console.log(transform(a, (config, acc, v) => {
if (v === 3) { config.stop = true }
if (v === 1) return ++acc
return acc
}, 0))
Usage2, use config as internal variable
const pixes = Array(size).fill(0)
const pixProcessed = pixes.map((_, pixId) => {
return transform(pics, (config, _, pic) => {
if (pic[pixId] !== '2') config.stop = true
return pic[pixId]
}, '0')
})
Usage3, capture config as external variable
const thrusts2 = permute([9, 8, 7, 6, 5]).map(signals => {
const datas = new Array(5).fill(_data())
const ps = new Array(5).fill(0)
let thrust = 0, config
do {
config = {}
thrust = transform(signals, (_config, acc, signal, i) => {
const res = intcode(
datas[i], signal,
{ once: true, i: ps[i], prev: acc }
)
if (res) {
[ps[i], acc] = res
} else {
_config.stop = true
}
return acc
}, thrust, config)
} while (!config.stop)
return thrust
}, 0)
You cannot break from inside of a reduce method. Depending on what you are trying to accomplish you could alter the final result (which is one reason you may want to do this)
const result = [1, 1, 1].reduce((a, b) => a + b, 0); // returns 3
console.log(result);
const result = [1, 1, 1].reduce((a, b, c, d) => {
if (c === 1 && b < 3) {
return a + b + 1;
}
return a + b;
}, 0); // now returns 4
console.log(result);
Keep in mind: you cannot reassign the array parameter directly
const result = [1, 1, 1].reduce( (a, b, c, d) => {
if (c === 0) {
d = [1, 1, 2];
}
return a + b;
}, 0); // still returns 3
console.log(result);
However (as pointed out below), you CAN affect the outcome by changing the array's contents:
const result = [1, 1, 1].reduce( (a, b, c, d) => {
if (c === 0) {
d[2] = 100;
}
return a + b;
}, 0); // now returns 102
console.log(result);
Providing you do not need to return an array, perhaps you could use some()?
Use some instead which auto-breaks when you want. Send it a this accumulator. Your test and accumulate function cannot be an arrow function as their this is set when the arrow function is created.
const array = ['a', 'b', 'c', 'd', 'e'];
var accum = {accum: ''};
function testerAndAccumulator(curr, i, arr){
this.tot += arr[i];
return curr==='c';
};
accum.tot = "";
array.some(testerAndAccumulator, accum);
var result = accum.tot;
In my opinion this is the better solution to the accepted answer provided you do not need to return an array (eg in a chain of array operators), as you do not alter the original array and you do not need to make a copy of it which could be bad for large arrays.
So, to terminate even earlier the idiom to use would be arr.splice(0).
Which prompts the question, why can't one just use arr = [] in this case?
I tried it and the reduce ignored the assignment, continuing on unchanged.
The reduce idiom appears to respond to forms such as splice but not forms such as the assignment operator??? - completely unintuitive - and has to be rote-learnt as precepts within the functional programming credo ...
const array = ['9', '91', '95', '96', '99'];
const x = array
.reduce((acc, curr, i, arr) => {
if(i === 2) arr.splice(1); // eject early
return acc += curr;
}, '');
console.log('x: ', x); // x: 99195
The problem is, that inside of the accumulator it is not possible to just stop the whole process. So by design something in the outer scope must be manipulated, which always leads to a necessary mutation.
As many others already mentioned throw with try...catch is not really an approach which can be called "solution". It is more a hack with many unwanted side effects.
The only way to do this WITHOUT ANY MUTATIONS is by using a second compare function, which decides whether to continue or stop. To still avoid a for-loop, it has to be solved with a recursion.
The code:
function reduceCompare(arr, cb, cmp, init) {
return (function _(acc, i) {
return i < arr.length && cmp(acc, arr[i], i, arr) === true ? _(cb(acc, arr[i], i, arr), i + 1) : acc;
})(typeof init !== 'undefined' ? init : arr[0], 0);
}
This can be used like:
var arr = ['a', 'b', 'c', 'd'];
function join(acc, curr) {
return acc + curr;
}
console.log(
reduceCompare(
arr,
join,
function(acc) { return acc.length < 1; },
''
)
); // logs 'a'
console.log(
reduceCompare(
arr,
join,
function(acc, curr) { return curr !== 'c'; },
''
)
); // logs 'ab'
console.log(
reduceCompare(
arr,
join,
function(acc, curr, i) { return i < 3; },
''
)
); // logs 'abc'
I made an npm library out of this, also containing a TypeScript and ES6 version. Feel free to use it:
https://www.npmjs.com/package/array-reduce-compare
or on GitHub:
https://github.com/StefanJelner/array-reduce-compare
You could to write your own reduce method. Invoking it like this, so it follows same logic and you control your own escape / break solution. It retains functional style and allows breaking.
const reduce = (arr, fn, accum) => {
const len = arr.length;
let result = null;
for(let i = 0; i < len; i=i+1) {
result = fn(accum, arr[i], i)
if (accum.break === true) {
break;
}
}
return result
}
const arr = ['a', 'b', 'c', 'shouldnotgethere']
const myResult = reduce(arr, (accum, cur, ind) => {
accum.result = accum.result + cur;
if(ind === 2) {
accum.break = true
}
return accum
}, {result:'', break: false}).result
console.log({myResult})
Or create your own reduce recursion method:
const rcReduce = (arr, accum = '', ind = 0) => {
const cur = arr.shift();
accum += cur;
const isBreak = ind > 1
return arr.length && !isBreak ? rcReduce(arr, accum, ind + 1) : accum
}
const myResult = rcReduce(['a', 'b', 'c', 'shouldngethere'])
console.log({myResult})
Another simple implementation that I came with solving the same issue:
function reduce(array, reducer, first) {
let result = first || array.shift()
while (array.length > 0) {
result = reducer(result, array.shift())
if (result && result.reduced) {
return result.reduced
}
}
return result
}
If you want to chain promises sequentially with reduce using the pattern below:
return [1,2,3,4].reduce(function(promise,n,i,arr){
return promise.then(function(){
// this code is executed when the reduce loop is terminated,
// so truncating arr here or in the call below does not works
return somethingReturningAPromise(n);
});
}, Promise.resolve());
But need to break according to something happening inside or outside a promise
things become a little bit more complicated because the reduce loop is terminated before the first promise is executed, making truncating the array in the promise callbacks useless, I ended up with this implementation:
function reduce(array, promise, fn, i) {
i=i||0;
return promise
.then(function(){
return fn(promise,array[i]);
})
.then(function(result){
if (!promise.break && ++i<array.length) {
return reduce(array,promise,fn,i);
} else {
return result;
}
})
}
Then you can do something like this:
var promise=Promise.resolve();
reduce([1,2,3,4],promise,function(promise,val){
return iter(promise, val);
}).catch(console.error);
function iter(promise, val) {
return new Promise(function(resolve, reject){
setTimeout(function(){
if (promise.break) return reject('break');
console.log(val);
if (val==3) {promise.break=true;}
resolve(val);
}, 4000-1000*val);
});
}
I solved it like follows, for example in the some method where short circuiting can save a lot:
const someShort = (list, fn) => {
let t;
try {
return list.reduce((acc, el) => {
t = fn(el);
console.log('found ?', el, t)
if (t) {
throw ''
}
return t
}, false)
} catch (e) {
return t
}
}
const someEven = someShort([1, 2, 3, 1, 5], el => el % 2 === 0)
console.log(someEven)
UPDATE
Away more generic answer could be something like the following
const escReduce = (arr, fn, init, exitFn) => {
try {
return arr.reduce((...args) => {
if (exitFn && exitFn(...args)) {
throw args[0]
}
return fn(...args)
}, init)
} catch(e){ return e }
}
escReduce(
Array.from({length: 100}, (_, i) => i+1),
(acc, e, i) => acc * e,
1,
acc => acc > 1E9
); // 6227020800
give we pass an optional exitFn which decides to break or not