So I've written a few each functions like jQuery has, and to be honest they are nice but I got to thinking, what if the user at hand does not want to start at 0, or wants to manipulate the iterator. I've tried many ways of doing this and it seems to either crash or cause an infinite loop. Here are some examples.
//a being an array of elements
_$(a).each(function(i){
console.log(i);
i++;
});
//backend js
each:function(fn){
var i = fn[0]; // equals the first argument in the callback
for(i=0;i<this.length;)
fn.call(this[i],i);
return this;
},
Next Example
//a being an array of elements
var inta = 6;
_$(a).each(function(i){
console.log(i);
},inta++);
//backend js
each:function(fn,iterator){
var len = this.length;
var ct = iterator;
while(ct < len)
fn.call(this[ct],ct);
return this;
},
There are a few more examples and they don't work either. So question is how do I manipulate the iterator inside the function like the first example above. If not possible that is fine just curious as to this curious possibility.
So what I've read above from map, filter, reduce, some, every, they just won't work with what I need since I am writing a small minimal library. So I need to know exactly what to iterate over, which in my case is this but only this[INT]. So I created an optional parameter after the function to add to the iterator by.
each:function(fn,add){
/*
First check if add is assigned
If it is not assign it to be 1 + 1 (i++)
If it is, make sure it is not 0 (causing an infinite loop)
And that will result in ( i + add)
*/
add = 0 !== add && add || 1;
for(var i=0;i<this.length;i = i + add)
fn.call(this[i],i);
return this;
}
So the use case would be.
var domElements = document.querySelectorAll('*'); //10 items per se
_$(domElements).each(function(i){
console.log(i);
},2);
//logs
//0 2 4 6 8
This is just a very basic iterator for now, and like I've said for a small convenient library not boogered by lots of "bull".
Related
I got a nice solution to get HTML Comments from the HTML Node Tree
var findComments = function(el) {
var arr = [];
for (var i = 0; i < el.childNodes.length; i++) {
var node = el.childNodes[i];
if (node.nodeType === 8) {
arr.push(node);
} else {
arr.push.apply(arr, findComments(node));
}
}
return arr;
};
var commentNodes = findComments(document);
// whatever you were going to do with the comment...
console.log(commentNodes[0].nodeValue);
from this thread.
Everything I did was adding this small loop to print out all the nodes.
var arr = [];
var findComments = function(el) {
for (var i = 0; i < el.childNodes.length; i++) {
var node = el.childNodes[i];
if (node.nodeType === 8) {
arr.push(node);
} else {
arr.push.apply(arr, findComments(node));
}
}
return arr;
};
var commentNodes = findComments(document);
//I added this
for (var counter = arr.length; counter > 0; counter--) {
console.log(commentNodes[counter].nodeValue);
}
I keep getting this Error Message:
RangeError: too many arguments provided for a function call debugger
eval code:9:13
EDIT: i had a typo while pasting changed the code from i-- to counter--
see this comment in MDN docs about the use of apply to merge arrays:
Do not use this method if the second array (moreVegs in the example) is very large, because the maximum number of parameters that one function can take is limited in practice. See apply() for more details.
the other note from apply page:
But beware: in using apply this way, you run the risk of exceeding the JavaScript engine's argument length limit. The consequences of applying a function with too many arguments (think more than tens of thousands of arguments) vary across engines (JavaScriptCore has hard-coded argument limit of 65536), because the limit (indeed even the nature of any excessively-large-stack behavior) is unspecified. Some engines will throw an exception. More perniciously, others will arbitrarily limit the number of arguments actually passed to the applied function. To illustrate this latter case: if such an engine had a limit of four arguments (actual limits are of course significantly higher), it would be as if the arguments 5, 6, 2, 3 had been passed to apply in the examples above, rather than the full array.
As the array start from index of 0, actually the last item in the array is arr.length - 1.
you can fix it by:
for (var counter = arr.length - 1; counter >= 0; counter--)
Notice I've added arr.length -1 and counter >= 0 as zero is the first index of the array.
Adding the for loop is not the only thing you changed (and see the other answer about fixing that loop too). You also moved the declaration of arr from inside the function to outside, making arr relatively global.
Because of that, each recursive call to findComments() works on the same array, and the .apply() call pushes the entire contents back onto the end of the array every time. After a while, its length exceeds the limit of the runtime.
The original function posted at the top of your question has arr declared inside the function. Each recursive call therefore has its own local array to work with. In a document with a lot of comment nodes, it could still get that Range Error however.
I am creating a simple program that should utilize the bubble sort algorithm to sort a list of numbers in ascending order.
Just for testing purposes I have added the line alert(unsortedNumbers);and as you can see if you run it, the numbers do not change order no matter how many passes the algorithm does.
The program seems to be stuck in an infinite loop, as 'Another pass' is printed to the console repeatedly. As instructed by this line console.log("Another pass");
As with the bubble sort algorithm, once it does not have to swap any terms on a certain pass, we know this is the sorted list, I have created the variable swapped, however it looks like this is always 1. I think this may be caused by the swapArrayElements() function not swapping the terms.
Why is the function not swapping the index of the terms within the array?
(Code does't seem to run properly on SO's code snippet tool, may have to copy into notepad document)
function main(){
var unsortedNumbers =[7,8,13,1,6,9,43,80]; //Declares unsorted numbers array
alert(unsortedNumbers);
var swapped = 0;
var len = unsortedNumbers.length;
function swapArrayElements(index_a, index_b) { //swaps swapArrayElements[i] with swapArrayElements[ii]
var temp = unsortedNumbers[index_a];
unsortedNumbers[index_a] = unsortedNumbers[index_b];
unsortedNumbers[index_b] = temp;
}
function finish(){
alert(unsortedNumbers);
}
function mainBody(){
for(var i =0;i<len;i++){
var ii =(i+1);
if (unsortedNumbers[i]>unsortedNumbers[ii]){
console.log("Swap elements");
swapArrayElements(i,ii);
swapped=1; // Variable 'swapped' used to check whether or not a swap has been made in each pass
}
if (ii = len){
if (swapped = 1){ // if a swap has been made, runs the main body again
console.log("Another pass");
alert(unsortedNumbers); //Added for debugging
swapped=0;
mainBody();
}else{
console.log("Finish");
finish();
}
}
}
}
mainBody();
}
<head>
</head>
<body onload="main()">
</body>
You have an error in your code:
if (ii = len) {
and also
if (swapped = 1){
it should be double equal
Invalid condition check causing infinite loop:
if (ii = len) & if (swapped = 1) should have == or === operator. This is causing infinity loop.
NOTE: Your code is not appropriate as per the best practices to avoid global variables. You should not use global variables and try
passing variables and returning them back after processing.
Refer this for avoiding globals.
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
Consider the Code below:
function splicer()
{
var arrayElements = ["elem1","elem2","elem3","elem4"];
for(var index in arrayElements)
{
arrayElements.splice(index,1);
}
alert("Elements: "+arrayElements);
}
The above function is supposed to remove all the elements from the array "arrayElements". But it won't.
Javascript engine maintains the "index" as it is and doesn't mind the array being modified.
People might expect something like "for each" loop that doesn't have this kind of issue
even the following code doesn't seem to work:
function splicer()
{
...
for(var index in arrayElements)
{
arrayElements.splice(index--,1);
}
...
}
even when changing the value of the variable "index" doesn't seem to work.
the changed value is available inside the "for(...){...}" block but, as the loop reaches the next iteration, the value gets reset and continues from the next index as clockwork.
so it seems code like this might be the only solution:
function splicer()
{
var arrayElements = ["elem1","elem2","elem3","elem4"];
for(var index=0;index<arrayElements.length;index++)
{
arrayElements.splice(index--,1);
}
alert("Elements: "+arrayElements);
}
Tested in: Firefox 16 Beta.
But placing a unary Operator inside a "splice()" method seems to be misleading at first sight.
This might be worth considering to the "W3C" or whomever it may concern so that they come up with a nice solution.
You may want to refer to John Resig's array.remove() link.
// Array Remove - By John Resig (MIT Licensed)
Array.prototype.remove = function(from, to) {
var rest = this.slice((to || from) + 1 || this.length);
this.length = from < 0 ? this.length + from : from;
return this.push.apply(this, rest);
};
Try this:
*Splice modifies the original array, hence tge loop skips the alternate values. *
var arrayElements = ["elem1","elem2","elem3","elem4"];
arrayElements.splice(0,arrayElements.length);
alert("Elements: "+arrayElements)
Hello I'm working on a problem that requires me to change an set array of numbers into an array that returns the original numbers as a function. So we get a return of a2 instead of a[2].
I dont want the answer I just need a hint. I know i can loop through the array and use .pop() to get the last value of the array, but then I dont know how to convert it to a function from there. any hints?
var numToFun = [1, 2, 3];
var numToFunLength = numToFun.length;
for (var i = 0; i < numToFunLength; i++) {
(function(num){
numToFun.unshift(function() {
return num;
});
}(numToFun.pop()))
}
DEMO
basically it pops out a number from the last, builds a function with that number returned, and put back into the first of the array. after one full cycle, all of them are functions.
here's the catch: how this works, it's up to you to research
why the loop does not look like the straightforward pop-unshift:
for (var i = 0; i < numToFunLength; i++) {
numToFun.unshift(function() { //put into first a function
return numToFun.pop() //that returns a number
});
}
and why i did this: (HINT: performance)
var numToFunLength = numToFun.length;
There's three important steps here:
Extract the number value from the array. Within a loop with an iterator of i, it might look like this:
var num = numArray[i];
This is important, because i will not retain its value that it had when you created the new function - it'll end up with the last value it had, once the for loop is finished. The function itself might look like this:
function() { return num; }
There's no reference to i any more, which is important - to understand better, read about closures. The final step would be to add the new function to the array of functions that you want.
...and you're done!
EDIT: See other's answers for good explanations of how to do this right, I will fix mine also though
As others have pointed out, one of the tricky things in javascript that many struggle with (myself included, obviously) is that scoping variables in javascript is dissimilar to many other languages; scopes are almost purely defined by functions, not the {} blocks of, for example, a for loop, as java/C would be.
So, below you can see (and in other answers here) a scoping function can aid with such a problem.
var numArray = [12, 33, 55];
var funcArray = [];
var numArrLength = numArray.length; // Don't do this in for loop to avoid the check multiple times
for(var j=0; j < numArrLength; j++) {
var scopeMe = function() {
var numToReturn = numArray[j];
console.log('now loading... ' + numToReturn);
var newFunc = function() {
return numToReturn;
};
return newFunc;
}();
funcArray.push(scopeMe);
};
console.log('now me');
console.log(funcArray);
console.log(funcArray[0]());
console.log(funcArray[1]());
console.log(funcArray[2]());
console.log(funcArray[1]()); // To ensure it's repeatable
EDIT my old bad answer below
What you'll want to do is something like
var funcArray = [];
for(...) {
var newFunc = function() {
return numArray.pop();
}
funcArray.push(newFunc);
}
The key here is that functions in javascript can be named variables, and passed around as such :)