I have multiple keys in my localstorage: task-1,task-2, task-3 ... I'm searching a way to get the total of the keys starting with the string "tasks-" in order to place it in my loop instead of localStorage.length.
In fact if I have an other app that uses the localstorage, my loop returns me a length corresponding to ALL the keys in the localStorage, including the ones that are out of the app in question.
i=0;
if(localStorage.getItem("task-"+i) != null){
for( i = 0; i < localStorage.length; i++){
console.log(i)
}
}
Thanks a lot for your help
If you control the creation of these items, it would make much more sense to store them in an object and use JSON to convert between object and string:
var tasksObject = {
task1: ...,
task2: ...,
...
};
window.localStorage.setItem('tasks', JSON.stringify(tasksObject));
... later ...
var tasks = JSON.parse(window.localStorage.getItem('tasks'));
for (var task in tasks) {
// do stuff
}
The statements should be switched. Loop over everything and see if each item matches your condition.
var count = 0;
for(i = 0; i < localStorage.length; i++){
if(localStorage.getItem("task-"+i) != null){
console.log(i)
count++;
}
}
// count contains number of items beginning with task
var i, e, tasks=[];
for (i=0; (e=localStorage["task-"+i])!==undefined; i++) {
console.log(e);
tasks.push(e);
}
And how about
localStorage['num-tasks'] = tasks.length;
Related
I have both an object and an array:
var elementsArr = [];
var elements = {
polygon: 734,
infoContent: "huhu",
id: 0
}
I am going to have multiple "elements" and push each one into "elementsArr".
for(var i=0; i<20; i++){
elements.id = id +=1;
elementsArr.push(elements);
}
My problem now is how to access a specific "id" within the object elements, within the array elementsArr and pushing this into another array.
I tried this but it doesn't seem to be working:
var ids = [];
for(var m=0; m<elementsArr.length; m++){
if(elements.id == elementsArr[m]){
ids.push(elements.id);
}
How do I do this?
Your code is pushing the same object onto the array over and over again.
One way to fix that would be to write a function to get a new element:
function Element(id) {
return {
polygon: 734,
infoContent: "huhu",
id: id
};
}
for(var i=0; i<20; i++){
elementsArr.push(Element(i));
}
If there are going to be a lot of elements, and the "id" values are all relatively small numbers, you'd be better off storing the elements such that the "id" is also the index:
for (var i = 0; i < 20; i++)
elementsArr[i] = Element(i);
To get the element with "id" 17, then:
var element17 = elementsArr[17];
If you really want to search, however, you can use .find():
var element17 = elementsArr.find(function(elem) { return elem.id === 17; });
or a simple loop:
for (var i = 0; i < elementsArr.length; ++i) {
if (elementsArr[i].id === 17) {
// found it!
}
}
You can extract the "id" values from the array with a simple call to .map():
var ids = elementsArr.map(function(elem) { return elem.id; });
or another for loop:
var ids = [];
for (var i = 0; i < elementsArr.length; ++i)
ids.push(elementsArr[i].id);
There are a couple of ways to do this. If you are able to work in ES6 then a WeakMap would be great.
However, I'm going to give you an ES5 instead.
Option 1 Loop through the data.
If you aren't accessing the data often, then looping through as you've done so may be the best choice.
Option 2 Set up your own index
On the other hand, if you need faster lookup, you can set up your own separate index to look things up.
var elementsLookup = {}; // Create an empty object.
for(var i=0; i<20; i++){
elements.id = id +=1;
elementsLookup[id] = elements; // Stash off a copy
elementsArr.push(elements);
}
Then you can look things up with a
var elem = elementsLookup[2]; // Get the element with an id of 2.
It is easy to do it with Array prototyping. Here is a function findById
Array.prototype.findById = function(id){
for(var x = 0 ; x < this.length; x++){
if(this[x].id == id){
return this[x];
}
}
return false;
}
You can use this function to recieve any id from the array or false if it does not exists
elementsArr.findById(17);
I'm not sure if this has been asked already, if it has, let me know in the comments and I'll delete the question. What's faster, looping through an object or looping through an array? For instance, if I have two arrays that are the exact same length(required for the script to run properly, not gonna get into details), each position in one array is associated with a position in the other array. So it looks like this:
var array_variable = ["get_ped_bridges", "get_rail_bridges"];
var array_url = ["http://eg.com/rest/services/Nope/MapServer/1", "http://eg.com/rest/services/Nope/MapServer/2"];
for(var j = 0; j < array_variable.length; j++){
console.log(array_variable[j], array_url[j]);
}
So as you can see, each url matches up to a position in the array_variable array. In an object, it would look something like this:
var object = {
one: {
variable: "get_ped_bridges",
url: "http://eg.com/rest/services/Nope/MapServer/1"
},
two: {
variable: "get_rail_bridges",
url: "http://eg.com/rest/services/Nope/MapServer/2"
}
}
So, if the lengths of both were substantially longer(the actual length would probably be around 20 positions), which loop would process faster?
The short answer is that generally an array is faster.
This is because of continuos indexing in an array, where it is always exactly known where the next element is (because it's stored contiguously)
you can refer to this previous answer for more info: Array vs. Object efficiency in JavaScript
<script src="http://broofa.com/Tools/JSLitmus/JSLitmus.js"></script>
<script>
JSLitmus.test('Array', function() {
var array_variable = ["get_ped_bridges", "get_rail_bridges"];
var array_url = ["http://eg.com/rest/services/Nope/MapServer/1", "http://eg.com/rest/services/Nope/MapServer/2"];
for (var j = 0, len = array_variable.length; j < len; j++) {
//
}
for (var j = 0, len = array_url.length; j < len; j++) {
//
}
});
JSLitmus.test('Object', function() {
var object = {
one: {
variable: "get_ped_bridges",
url: "http://eg.com/rest/services/Nope/MapServer/1"
},
two: {
variable: "get_rail_bridges",
url: "http://eg.com/rest/services/Nope/MapServer/2"
}
};
for (var i in object) {
//
}
});
</script>
I am confused about how to iterate on multiple values.
for example : values.categories[0].num[0].entry[0].label;
Do I need to write three for loops in order to iterate through categories, num and entry.
Because categories[0] will always identify the 1st position, but am looking for generic categories[i].
Can you please help me out whether to write three for loops or better option is there to achieve.?
This is what I have tried:
var result = [];
for (var i = 0; i < categories.length; i++) {
var abc = categories[i].num;
for (var j = 0; j < abc.length; j++){
var def = num[i].entry;
}
for(var k = 0; k < def.length; k++){
var ghi = entry[i].label;
result.push(ghi)
console.log(result);
}
}
you can use the each function of jquery.
$.each(categories, function(ci, num) {
// This set the index of the array in ci and the value in num = categories[ci]
$.each(num, function(ni, entry) {
// etc ...
});
});
if you want it to stop the iteration you can return false inside the callback function.
OK.. I really must be crazy. Why would these not print out 3-5 for "a"? I've tried two different methodologies to have the array store 3-5 below, and both of them seem.. obvious. There must be something in the underlying translation that I'm just not seeing.
<script>
var articlesKey = [];
for(var i = 3; i < 6; i++) {
articlesKey.push(i);
document.write('<br>i:'+i);
}
for (a in articlesKey)
document.write("<br>a:"+a);
articlesKey = [];
var count = 0;
for(var i = 3; i < 6; i++) {
articlesKey[count] = i;
document.write('<br>i:'+i);
count++;
}
for (a in articlesKey)
document.write("<br>a:"+a);
</script>
It prints out:
i:3
i:4
i:5
a:0
a:1
a:2
i:3
i:4
i:5
a:0
a:1
a:2
for( a in articlesKey) iterates a through the KEYS of articlesKey (letting you then get the values as articlesKey[a]). There is nothing wrong here.
That's very simply because a represents the array index in the loop, not the value at the index. Fix:
document.write("<br>a:"+articlesKey[a]);
I am having issues with a couple arrays below and the match method. On my page I call the Checkout() function and it sets a temporary array equal to the array I've been building with different options. It then loops through the temporary array and removes html from one of the elements. The issue is that when I alert the array Remote.Cart.products before the loop it is exactly as it was built, but when I call the function again the exact same alert shows the new updated values even though I am not modifying the Remote.Cart.products array anywhere in the function.
function Checkout() {
tmp = null;
tmp = Remote.Cart.products;
alert( Remote.Cart.products );
for ( i = 0, li = tmp.length; i < li; i++ ) {
for ( j = 0, lj = tmp[ i ][1].length; j < lj; j++ ) {
tmp[ i ][1][j][1] = tmp[ i ][1][j][1].match(/<a\s+[^>]*href="([^\"]*)"[^>]*>(.*)<\/a>/i)[2];
}
}
}
Your help / insight is much appreciated!
You are using the same array. Just a different variable that points to the same array. In memory it's the same object.
You need to rebuild the array in your loop so that you can get an identical but new array.
More info on copying arrays and other objects can be found here: http://my.opera.com/GreyWyvern/blog/show.dml/1725165
Clint, you have to understand that tmp and Remote.Cart.products are different names for the same array. If you want to clone the array, do:
var tmp = [];
for(var i = 0; i < Remote.Cart.products.length; i++)
{
tmp[i] = []
for(var j = 0; j < Remote.Cart.products[i].length; j++)
{
tmp[i][j] = [];
for(var k = 0; k < Remote.Cart.products[i][j].length; k++)
{
tmp[i][j][k] = Remote.Cart.products[i][j][k].slice();
}
}
}
EDIT: Nesting corrected thanks to Squeegy
"even though I am not modifying the Remote.Cart.products array anywhere in the function"
tmp = Remote.Cart.products;
...
tmp[ i ][1][j][1] = ...
It sure looks like you are.