Length of Array within Array - javascript

I have an object that I am pushing some ints, strings and arrays into an array. And I want to get the length of the array that is within said array.
This is my code
var all_categories = [];
all_categories.push({
title: theTitle,
id: theId,
sub: subcategories
});
Now I know that all_categories.length is the general way of getting the length and I believe that I can't run all_categories[0].sub[0].length will not work because the function does not exist.
Suggestions for a solution or work around?

In your statement all_categories[0].sub[0].length refers to the length of the first element of array named sub.
In order to see length of the array you should call:
all_categories[0].sub.length

Assuming that subcategories is the array you want the length of, take out the second [0]. You aren't trying to get the length of the first subcategory, you're trying to get the number of subcategories.

Related

How to push data to specific sub-array in an empty 2-dimensional array

I'm trying to setup a 2-d array, which should receive values at specific sub-arrays. I created my array with:
myArray= Array(100).fill([])
Now, let's say I want to push a value to say sub-array number 40
I'm doing this like that:
myArray[40].push("myValue")
I would expect the value to be pushed only to the myArray[40] instead it is pushed as the first element of every of the hundred sub-arrays.
I searched for the solution for quite some time, but I still have no idea what I'm doing wrong. Please help.
fill will push the same value to each element, not a copy of it. That's fine when it's a number or string or something else immutable. But now each copy is a reference to the same object.
There are a number of ways to fix this. Here's one (switched to ten elements for demonstration):
const myArray = [...Array(10)].map((_, i) => [])
myArray[4].push('myValue')
console.log(myArray)

Array length is not correct in javascript

I have an array like below
arr=[];
arr[0]={"zero": "apple"};
arr[1]={"one": "orange"};
arr["fancy"]="what?";
but i am getting length as 2 when i do console.log(arr.length) even though i am able to console all the values .
and not able to get all values while doing console.log(JSON.stringify(arr))
What is the issue here.
here is the link to fiddle fiddle
.length is a special property in Javascript arrays, which is defined as "the biggest numeric index in the array plus one" (or 2^32-1, whatever comes first). It's not "the number of elements", as the name might suggest.
When you iterate an array, either directly with for..of or map, or indirectly with e.g. JSON.stringify, JS just loops over all numbers from 0 to length - 1, and, if there's a property under this number, outputs/returns it. It doesn't look into other properties.
The length property don't work as one will expect on arrays that are hashtables or associative arrays. This property only works as one will expect on numeric indexed arrays (and normalized, i.e, without holes). But there exists a way for get the length of an associative array, first you have to get the list of keys from the associative array using Object.keys(arr) and then you can use the length property over this list (that is a normalized indexed array). Like on the next example:
arr=[];
arr[0]={"zero": "apple"};
arr[1]={"one": "orange"};
arr["fancy"]="what?";
console.log(Object.keys(arr).length);
And about this next question:
not able to get all values while doing console.log(JSON.stringify(arr))
Your arr element don't have the correct format to be a JSON. If you want it to be a JSON check the syntax on the next example:
jsonObj = {};
jsonObj[0] = {"zero": "apple"};
jsonObj[1] = {"one": "orange"};
jsonObj["fancy"] = "what?";
console.log(Object.keys(jsonObj).length);
console.log(JSON.stringify(jsonObj));
From MDN description on arrays, here, "Arrays cannot use strings as element indexes (as in an associative array) but must use integers."
In other words, this is not Javascript array syntax
arr["fancy"]="what?";
Which leads to the error in .length.

Javascript slice isn't giving me correct array length values

Why does it say length 1 instead of 4?
The following is what I'm trying to push and slice. I try and append items.image_urls and slice them into 5 each.
items.image_urls is my dictionary array.
var final_push = []
final_push.push(items.image_urls.splice(0,5))
console.log(final_push.length)## gives me 1...?
var index = 0
final_push.forEach(function(results){
index++ ##this gives me one. I would need 1,2,3,4,5,1,2,3,4,5. Somehting along that.
}
items.image_urls looks like this:
It's an iteration of arrays with image urls.
In your example items.image_urls.splice(0,5) returns an array of items removed from items.image_urls. When you call final_push.push(items.image_urls.splice(0,5));, this whole array is pushed as one item to the final_push array, so it now looks like [["url1", "url2", "url3", "url4", "url5"]] (2-dimensional array). You can access this whole array by calling final_push[some_index].
But what you want instead is to add every element of items.image_urls.splice(0,5) to the final_push. You can use a spread operator to achieve this:
final_push.push(...items.image_urls.splice(0,5));
Spread syntax allows an iterable such as an array expression or string
to be expanded in places where zero or more arguments (for function
calls) or elements (for array literals) are expected
This is exactly our case, because push() expects one or more arguments:
arr.push(element1[, ...[, elementN]])
And here is an example:
let items = {
image_urls: ["url1", "url2", "url3", "url4", "url5", "url6", "url7", "url8", "url9", "url10"]
};
let final_push = [];
final_push.push(...items.image_urls.splice(0,5));
console.log(final_push.length);
console.log(JSON.stringify(final_push));
console.log(JSON.stringify(items.image_urls));
Note: do not confuse Array.prototype.slice() with Array.prototype.splice() - the first one returns a shallow copy of a portion of an array into a new array object while the second changes the contents of an array by removing existing elements and/or adding new elements and returns an array containing the deleted elements.
That seems to be a nested array. So if you would access index 0, and then work on that array like below it will probably work:
console.log(final_push[0].length); //should print 4
The author is mixing up splice and slice. Probably a typo :)
You start at the beginning (0) and then delete 5 items.

javascript converting array of ints to array of objects

I have an array of ints called SelectedItems. I have another array that contains objects that's called AvailableItems and that was parsed from json; these Item objects have the properties {ContainerID, ContainerName, ItemID, ItemName}.
I want to convert SelectedItems from an array of ints to an array of Items where each ItemID is replaced with the Item object that corresponds to the ItemID in AvailableItems. Each ItemID in SelectedItems is unique.
I started with 2 loops: one that loops through each element of SelectedItems but then I find myself looping through AvailableItems each time to find the corresponding ItemID with the object keys I need to copy into SelectedItems.
So basically I built a seemingly very inefficient loop. I was wondering if there was a better way to do it by avoiding a repeated loop inside a loop?
Sample data:
For AvailableItems, you have
{
ContainerID: i,
ContainerName: 'SomeName',
ItemID: j,
ItemName: 'SomeOtherName'
}
with may be 1,000 objects and then SelectedItems is array of ints
[23,43,64,34...]
Thanks.
You could put each object in the numeric array AvailableItems at the index that matches its ItemID.
So you know that the item with an ItemID of 5 is at AvailableItems[5] instead of having to loop through and find it.
Not sure what effect this will have if you have big gaps between different ItemID values, but you can try it and see if it works well.
UPDATE:
After a quick search, and reading this answer, it looks like having gaps between your indexes will not waste a bunch of memory. However, it will affect the result of checking AvailableItems.length. If you have an array with one entry, but the index of that entry is 500, then AvailableItems.length will return 501, even though there is only one entry in the array.
As long as you don't need to use the length function, this solution should work for you.
If you are able to get any arbitrary AvailableItem without looping through the whole array by just addressing it by its ID(if you have an index-based array where IDs are indexes) then you can go through the SelectedItems and check if it exists in AvailableItems, and if it does then you convert the SelecteItem into an object and add it to some temporary array for later use.

best way to append an item to a list within a JSON object?

I'm working on JavaScript and I have this JSON object:
var obj ={"name" : "objName",
"dynamicList" :[]};
and then I add some items to my list like this:
obj.dynamicList["someStringID"] = {"watever"};
It's important that I use a string as indexer for each item on my list (i didn't know this could be done until recently).
My only problem now is that whenever I ask for obj.dynamicList.lenght I get 0, unles I manually set the proper number... I was wondering if there's a better way to add items to my list?
Thanks!!
In Javascript, string index is not really an index. It's actually an attribute of the array object. You could set and get the value with the string index, but it's actually an empty array with some attributes. Not only .length, but also .sort(), .splice(), and other array function would not work. If there is a need to use array functions, I would use number as an index to make it a real item in the array.
If you have to use the string as an index, you couldn't rely on .length function. If there is no need to support IE prior to version 9, the Object.keys as suggested by #strimp099 should work. or you may have to create function to count the number of attributes for example:
function len(obj) {
var attrCount = 0;
for(var k in obj) {
attrCount++;
}
return attrCount;
}
and then call
len(obj.dynamicList);
Use the following the find the length of dynamicList object:
Object.keys(obj.dynamicList).length
To do this "the right way," you will have to make obj.dynamicList an object instead of an array; use {} instead of [] to set the initial value.
How to efficiently count the number of keys/properties of an object in JavaScript?
dynamiclist is an object, the length is not the length property you expect from an array.

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