I am looking to improve a function I wrote for array reversal. I was interested in writing my own just for practice and I came up with this:
function rArray(array){
var temp = [];
var len = array.length - 1;
for(i = len, index = 0 ; i >= 0; i--, index++){
temp.push(array[i]); // temp[index] = array[i];
}
return temp;
}
I am looking to 1.) improve speed, and two, create a more efficient function by leaving less of a footprint, I want to make a destructive reversing function. Can this be done with a for loop or must I use a while()? Thanks for the input.
You could get rid of index, since you aren't using it.
Or you could pre-allocate temp
var temp = new Array(len);
You can't do both, though, since you would need index to add to the pre-allocated temp. You could run some experiments to see at what length pre-allocation becomes preferable (my guess: several million).
Related
I would like to know if it is worth to convert an array into a set in order to search using NodeJS.
My use case is that this search is done lot of times, but not necessary on big sets of data (can go up to ~2000 items in the array from time to time).
Looking for a specific id in a list.
Which approach is better :
const isPresent = (myArray, id) => {
return Boolean(myArray.some((arrayElement) => arrayElement.id === id);
}
or
const mySet = new Set(myArray)
const isPresent = (mySet, id) => {
return mySet.has(id);
}
I know that theoretically the second approach is better as it is O(1) and O(n) for the first approach. But can the instantiation of the set offset the gain on small arrays?
#jonrsharpe - particularly for your case, I found that converting an array of 2k to Set itself is taking ~1.15ms. No doubt searching Set is faster than an Array but in your case, this additional conversion can be little costly.
You can run below code in your browser console to check. new Set(arr) is taking almost ~1.2ms
var arr = [], set = new Set(), n = 2000;
for (let i = 0; i < n; i++) {
arr.push(i);
};
console.time('Set');
set = new Set(arr);
console.timeEnd('Set');
Adding element in the Set is always costly.
Below code shows the time required to insert an item in array/set. Which shows Array insertion is faster than Set.
var arr = [], set = new Set(), n = 2000;
console.time('Array');
for (let i = 0; i < n; i++) {
arr.push(i);
};
console.timeEnd('Array');
console.time('Set');
for (let i = 0; i < n; i++) {
set.add(i);
};
console.timeEnd('Set');
I run the following code to analyze the speed of locating an element in the array and set. Found that set is 8-10 time faster than the array.
You can copy-paste this code in your browser to analyze further
var arr = [], set = new Set(), n = 100000;
for (let i = 0; i < n; i++) {
arr.push(i);
set.add(i);
}
var result;
console.time('Array');
result = arr.indexOf(12313) !== -1;
console.timeEnd('Array');
console.time('Set');
result = set.has(12313);
console.timeEnd('Set');
So for your case array.some is better!
I will offer a different upside for using Set: your code is now more semantic, easier to know what it does.
Other than that this post has a nice comparison - Javascript Set vs. Array performance but make your own measurements if you really feel that this is your bottleneck. Don't optimise things that are not your bottleneck!
My own heuristic is a isPresent-like utility for nicer code but if the check is done in a loop I always construct a Set before.
I have found this code written online(not by me), and was hoping to get an answer on what the mathematical formula or concept is that makes this function work. I am curious as to how this person designed this. First, I will explain the requirements that the function must produce, then I will supply the code, and a link to a working code pen for further hacking. P.S. The problem uses the word "vector", but since this is Javascript, vector just means array.
Function Requirements
Given a vector of vectors of words, ex.
[['quick', 'lazy'], ['brown', 'black', 'grey'], ['fox', 'dog']].
Write a function that prints all combinations of one word from the first vector, one word from the second vector, etc.
The solution may not use recursion. The number of vectors and number of elements within each vector may vary.
Example output: 'quick, brown, dog', 'lazy black fox' etc.
My Current Level Of Understanding
I am already aware that by using the principle of multiplication, to find the number of possible combinations available in this scenario is to just multiply the lengths of each inner vector by each other. For this specific example, we get a total of 12(2x3x2) different possible combinations. Where I fall off however, is inside the 4 nested for loops section of the program.
Whoever wrote the code, clearly understands some concept or formula that I do not. Just two examples, are the "previous" variable used inside the loops, and the strategic placement of where they decide to increment the j variable. It seems to me that they might be aware of some mathematical formula.
Code
Below is the code without comments. If you however go to this codepen, I have included the same code with plenty of comments that explain how the program works, so you don't have to trace everything out from scratch. You can also test the output in the built-in console.
function comboMaker(vector) {
var length = vector.length;
var solutions = 1;
for (var i = 0; i < length; i++) {
solutions *= vector[i].length;
}
var combinations = [];
for (var i = 0; i < solutions; i++) {
combinations[i] = [];
}
var previous = 1;
for (var i = 0; i < length; i++) {
for (var j = 0; j < solutions;) {
var wordCount = vector[i].length;
previous *= vector[i].length;
for (var l = 0; l < wordCount; l++) {
for (var k = 0; k < (solutions/previous); k++) {
combinations[j][i] = vector[i][l];
j++
}
}
}
}
for (var i = 0; i < solutions; i++) {
console.log(combinations[i].join(" "));
}
}
comboMaker([['quick', 'lazy'], ['brown', 'black', 'grey'], ['fox', 'dog']]);
You can consider combination of items as number in mixed radix numeric system.
Radix for every position is equal to number of items in corresponding array (here {2,3,2}). Overall number of combination M is product of all radixes.
You can generate combination either
by making for-loop with counter in range 0..M-1 and separating every digit from this counter and getting corresponding item. Pseudocode
M = ProductOfLengthsOfArrays
for c = 0..M-1
t = c
combination = {}
for i = 0..NumOfArrays-1
d = t %% Array[i].Length //modulo operation
t = t / Array[i].Length //integer division
combination.add(Array[i][d])
output combination
by counting in mixed radix from 0 to M-1
if item is last in the array, get first item and increment the next array
else get the next item in the same array
A cooperation of .reduce() and .map() allows us to come up with a very efficient single liner answer for this question.
var data = [['quick', 'lazy'], ['brown', 'black', 'grey'], ['fox', 'dog'],['jumps','runs']],
result = data.reduce((p,c) => p.reduce((r,fw) => r.concat(c.map(sw => fw + " " + sw)),[]));
console.log(result);
I have a doubt about how can be affected to speed the use of object data arrays, that is, use it directly or preasign them to simple vars.
I have an array of elements, for example 1000 elements.
Every array item is an object with 10 properties (for example).
And finally I use some of this properties to do 10 calculations.
So I have APPROACH1
var nn = myarray.lenght;
var a1,a2,a3,a4 ... a10;
var cal1,cal2,.. cal10
for (var x=0;x<nn;x++)
{ // assignment
a1=my_array[x].data1;
..
a10 =my_array[x].data10;
// calculations
cal1 = a1*a10 +a2*Math.abs(a3);
...
cal10 = (a8-a7)*4 +Math.sqrt(a9);
}
And APPROACH2
var nn = myarray.lenght;
for (var x=0;x<nn;x++)
{
// calculations
cal1 = my_array[x].data1*my_array[x].data10 +my_array[x].data2*Math.abs(my_array[x].data3);
...
cal10 = (my_array[x].data8-my_array[x].data7)*4 +Math.sqrt(my_array[x].data9);
}
Assign a1 ... a10 values from my_array and then make calculations is faster than make the calculations using my_array[x].properties; or the right is the opposite ?????
I dont know how works the 'js compiler' ....
The kind of short answer is: it depends on your javascript engine, there is no right and wrong here, only "this has worked in the past" and "this don't seem to speed thing up no more".
<tl;dr> If i would not run a jsperf test, i would go with "Cached example" 1 example down: </tl;dr>
A general rule of thumb is(read: was) that if you are going to use an element in an array more then once, it could be faster to cache it in a local variable, and if you were gonna use a property on an object more then once it should also be cached.
Example:
You have this code:
// Data generation (not discussed here)
function GetLotsOfItems() {
var ret = [];
for (var i = 0; i < 1000; i++) {
ret[i] = { calc1: i * 4, calc2: i * 10, calc3: i / 5 };
}
return ret;
}
// Your calculation loop
var myArray = GetLotsOfItems();
for (var i = 0; i < myArray.length; i++) {
var someResult = myArray[i].calc1 + myArray[i].calc2 + myArray[i].calc3;
}
Depending on your browser (read:this REALLY depends on your browser/its javascript engine) you could make this faster in a number of different ways.
You could for example cache the element being used in the calculation loop
Cached example:
// Your cached calculation loop
var myArray = GetLotsOfItems();
var element;
var arrayLen = myArray.length;
for (var i = 0; i < arrayLen ; i++) {
element = myArray[i];
var someResult = element.calc1 + element.calc2 + element.calc3;
}
You could also take this a step further and run it like this:
var myArray = GetLotsOfItems();
var element;
for (var i = myArray.length; i--;) { // Start at last element, travel backwards to the start
element = myArray[i];
var someResult = element.calc1 + element.calc2 + element.calc3;
}
What you do here is you start at the last element, then you use the condition block to see if i > 0, then AFTER that you lower it by one (allowing the loop to run with i==0 (while --i would run from 1000 -> 1), however in modern code this is usually slower because you will read an array backwards, and reading an array in the correct order usually allow for either run-time or compile-time optimization (which is automatic, mind you, so you don't need to do anything for this work), but depending on your javascript engine this might not be applicable, and the backwards going loop could be faster..
However this will, by my experience, run slower in chrome then the second "kinda-optimized" version (i have not tested this in jsperf, but in an CSP solver i wrote 2 years ago i ended caching array elements, but not properties, and i ran my loops from 0 to length.
You should (in most cases) write your code in a way that makes it easy to read and maintain, caching array elements is in my opinion as easy to read (if not easier) then non-cached elements, and they might be faster (they are, at least, not slower), and they are quicker to write if you use an IDE with autocomplete for javascript :P
Okay so I have a 2D array that I am trying to alter using javascript. This is what I have so far:
for (var i = 0; i <= inputData.length; i++ {
inputData[0,0] = inputData[0,0];
inputData[i,0] = inputData[((i - 1) + 1/12), 0];
I want this to take array [i-1] value and then add 1/12 to it
for (j = 13; inputData.length; j += 13) {
delete inputData[j,0];
delete inputData[j,1];
}
Also, I want to delete the entire 2D array at every 13th increment value.
}
This is what I have so far. I am sure there are probably errors within it. Can you guys help me out here? Any help would be greatly appreciated.
Couple of things - you need to be careful when iterating over an array that you're removing from, your indexes will end up offset with respect to your data as soon as you do a delete. Secondly your syntax for deletion is off.
Normally in these situations I favour creating a new array containing the data I want to keep.
var inputData = [[1,1],[2,2],[3,3],[4,4]];
var b = [];
for (i=0; i < inputData.length; i++) {
if ((i + 1) % 13 != 0) {
var year_with_month = inputData[i][0] + i * 1/12;
var e = [year_with_month, inputData[i][1]]
b.push(e);
}
}
inputData = b;
Also, given a choice I'd use a library like underscore to make it easy to do the looping. I never manually write for loops anymore, took me a couple of attempts to get that one right :)
I have trouble dealing with my for loops now, I'm trying to compare two datum, basically it will compare 2 items, then it will write the matches and the mismatches on the webpage.
I managed to write the matches on the webpage, it was working good. But there's a bug in my mismatch compare.
It wrote all the data on the webpage X times, here's my JS code:
function testItems(i1, i2) {
var newArray = [];
var newArray2 = [];
var count = 0;
var count2 = 0;
for(var i = 0; i < i1.length; i++) {
for(var j = 0; j < i2.length; j++) {
if(i1[i] == i2[j]) {
newArray.push(i1[i]);
count++;
} if (i1[i] !== i2[j]) {
newArray2.push(i1[i]);
count2++;
}
}
}
count-=2;
count2-=2
writeHTML(count,count2, newArray, newArray2);
}
The result was horrible for the mismatches:
alt text http://www.picamatic.com/show/2009/03/01/07/44/2523028_672x48.jpg
I was expecting it to show the mistakes, not all the strings.
The issue you're seeing is because of the nested for loop. You are essentially doing a cross-compare: for every item in i1, you are comparing it to every item in i2 (remember that j starts again at 0 every time i advances... the two loops don't run in parallel).
Since I understand from the comments below that you want to be able to compare one array to the other, even if the items in each are in a different order, I've edited my original suggestion. Note that the snippet below does not normalize differences in case between the two arrays... don't know if that's a concern. Also note that it only compares i1 against i2... not both i1 to i2 and i2 to i1, which would make the task a little more challenging.
function testItems(i1, i2) {
var newArray = [];
var newArray2 = [];
for (var i = 0; i < i1.length; i++) {
var found = false;
for (var j = 0; j < i2.length; j++) {
if (i1[i] == i2[j]) found = true;
}
if (found) {
newArray.push(i1[i])
} else {
newArray2.push(i1[i])
}
}
}
As an alternative, you could consider using a hash table to index i1/i2, but since the example of strings in your comment include spaces and I don't know if you're using any javascript helper libraries, it's probably best to stick with the nested for loops. The snippet also makes no attempt to weed out duplicates.
Another optimization you might consider is that your newArray and newArray2 arrays contain their own length property, so you don't need to pass the count to your HTML writer. When the writer receives the arrays, it can ask each one for the .length property to know how large each one is.
Not directly related to the question but you should see this:
Google techtalks about javascript
Maybe it will enlighten you :)
Couple of things about your question. First you should use '!=' instead of '!==' to check inequality. Second I am not sure why you are doing decreasing counts by 2, suggests to me that there may be duplicates in the array?! In any case your logic was wrong which was corrected by Jarrett later, but that was not a totally correct/complete answer either. Read ahead.
Your task sounds like "Given two set of arrays i1 & i2 to find i1 {intersection} i2 and i1{dash} {UNION} i2{dash}) (Group theory notation). i.e. You want to list common elements in newArray and uncommon elements in newArray2.
You need to do this.
1) Remove duplicates in both the arrays. (For improving the program efficiency later on) (This is not a MUST to get the desired result - you can skip it)
i1 = removeDuplicate(i1);
i2 = removeDuplicate(i2);
(Implementation for removeDuplicate not given).
2) Pass through i1 and find i1{dash} and i1 {intersection} i2.
var newArray = [];
var newArray2 = [];
for (var i = 0; i < i1.length; i++)
{
var found = false;
for (var j = 0; j < i2.length; j++)
{
if (i1[i] == i2[j])
{
found = true;
newArray.push(i1[i]); //add to i1 {intersection} i2.
count++;
break; //once found don't check the remaining items
}
}
if (!found)
{
newArray2.push(i1[i]); //add i1{dash} to i1{dash} {UNION} i2{dash}
count2++;[
}
}
3) Pass through i2 and append i2{dash} to i1{dash}
for(var x=0; x<i2.length; x++)
{
var found = false;
//check in intersection array as it'd be faster than checking through i1
for(var y=0; y<newArray.length; y++) {
if( i2[x] == newArray[y])
{
found = true;
break;
}
}
if(!found)
{
newArray2.push(i2[x]); //append(Union) a2{dash} to a1{dash}
count2++;
}
}
writeHTML(count,count2, newArray, newArray2);
I have a feeling that this has to do with your second comparison using "!==" instead of "!="
"!==" is the inverse of "===", not "==". !== is a more strict comparison which does not do any type casting.
For instance (5 != '5') is false, where as (5 !== '5') is true. This means it's possible that you could be pushing to both arrays in the nested loop, since if(i1[i] == i2[j]) and if(i1[i] !== i2[j]) could both be true at the same time.
The fundamental problem here is that a pair of nested loops is NOT the right approach.
You need to walk a pointer through each dataset. ONE loop that advances both as needed.
Note that figuring out which to advance in case of a mismatch is a much bigger problem than simply walking them through. Finding the first mismatch isn't a problem, getting back on track after finding it is quite difficult.