var myArray = new Array();
myArray['112'] = 0;
myArray.length
Why is length 113 in above sample? Shouldn't '112' add a object property for the array and create something similar to myArray = {"112":0}?
Besides this, why is the length 113 and not 1? Since myArray actually only contains 1 value
The array length is one more than the highest index, so you get 113.
No. The '112' string and a pure numeric 112 evaluate to the same thing when JS is doing the array lookup, so you get a slot in the array rather than a property.
Simplest to think of a JS Array indexes as properties that happen to be numbers, even in string form. It's more chameleonic than you'd think at first.
But if you add a property with some nonnumeric name, like myArray['foo'], it will do as you expect and the length won't change.
Consider this simple example:
var aa = [];
aa[3] = 'three';
alert( aa.length // 4
+ '\n' + aa[2] // undefined
+ '\n' + aa.hasOwnProperty('2') // false
);
The number 3 is used to assign the property name, but it is converted to a string and used as a standard property name (i.e. the string "3").
Adding a property named "3" has created one property and set the length to 4 since the length is always set to one more than the largest non-negative integer property name.
No other property is created, the array is "sparse", i.e. it doesn't have sequentially named (numbered) members. A for..in loop can also be used to see that there is only one property.
You got array of 0..112 elements - in total length of 113 elements.
Related
Why does this array have a length property of 0? And as a follow up to that question, why can't you access an item in this array with an index value?
Consider the following:
var box = [];
box['material'] = 'cardboard';
box['size'] = 'small';
box.length; //0
console.log(box);
//[ material: 'cardboard', size: 'small' ]
box[0]; //undefined
However, when I do the following:
var box = [];
box['0'] = true;
box['1'] = 'cardboard';
box['2'] = 'some value';
box.length; //3
box[0];// true
Why does it output the correct length in this case, since '0' is a string and not a number and access the correct item with the 0 index?
In JavaScript, the length of an array is always one more than the largest numeric (integer) property name. Arrays can have properties whose names are not numeric, but they don't count towards the length of the array (and they are ignored in some other important situations).
Object property names are always strings, but strings that are non-negative integer values when interpreted as numbers are special in arrays. Accessing properties with numeric values works because the numbers are first converted to strings; thus box[0] and box['0'] do exactly the same thing.
when setting box['material'] it creates a property called 'material' with value 'cardboard'. this doesn't add a element to the list!
you need to use .push() or asign the index you want.
also, the '0' string is cased to a number, so obj['0'] and obj[0] is the same (not just for 0, this happens for all numbers)
I think you’re confusing objects with arrays. While arrays are indeed objects in javascript, that doesn’t help us here.
What you are trying to instantiate is an associative array with key-value pairs “material”->“cardboard”, “size”->“small”. However, such a datastructure does not exist in javascript. Array must only have integers as indexes, as in
box = [];
box[0] = 'material';
box[1] = 'someValue';
The reason why your code (which only seems to be an associative array)
box = [];
box['0'] = true;
box['1'] = 'cardboard';
works is because of implicit type conversion. Because you’ve declared box to be an array, a string is not valid in this position, so javascript quietly converts it into an integer, so you again get a plain, boring, integer-indexed array as above.
What you can do, though, is create an object with attributes. So you might, for example, state
box = {}; // box is now an object
box.size = 'small'; // box now has an attribute called “size”
// which has the (string) value “'small'”
box.material = 'cardboard';
or at one go using the notation for object literals:
box = {
size: 'small',
material: 'cardboard'
};
This is the closest you can get to associative arrays in javascript. Note that objects do not have a length. If you want to access the number of attributes they have, you have to use Object.keys(box).length. To iterate over the keys, use
for(var key in box){
console.log('value of ' + key + ' is ' + box[key])
}
Note the array-like notation here, but remember: You’re dealing with objects, not arrays, though dealing with them looks similar. Since box is of type object, you will not only get the attributes you have defined yourself, but also any attributes which are defined for every object. To get only the keys you put in there yourself, use
for (var key in box) {
if (box.hasOwnProperty(key)) {
// do stuff
}
}
If you need to add keys, i.e. attributes at runtime, use (for example)
//supposing that box was instantiated as above
var listOfBoxAttributes = ['shape', 'color', 'weight'];
for(var i in listOfBoxAttributes)
box[listOfBoxAttributes[i]] = undefined;
What would the length property of the array x be?
var x=new Array();
x[0]="Monday";
x[1]="Tuesday";
x[3]="Thursday";
It would be 4.
The .length property is defined to be one more than the numeric value of the largest integer-like property name. The largest (when interpreted as a number) such property name in the example code is 3, so the .length value is therefor 4.
If you set a property of an array, such that the property name is an integer (or a string that looks like an integer), then length is updated to be one more than that integer value. Symmetrically, if you set length to some value, then all properties whose names are integers greater than or equal to the new value are implicitly deleted.
if you want to know the length of your array 'x' then you can just do something like:
var length = x.length;
keep in mind that arrays are 0 based. So the length here will be 4, however, if you want to access the indexes of your array you will need to use 0, 1, 2, 3.
further, if you want to know the length, after getting your var length you could do either of the following:
console.log(length);
alert(length);
I am using the following pattern to index an injection from pairs of numbers to numbers:
var myHash = {};
...
for (... billion of iterations ...)
var x = someNum;
var y = otherNum;
myHash[x + "," + y] = z;
The problem with this code is that I'm using a string as the key of myHash, which has been tested to be much slower than integer keys. My question is: what is a more intelligent way to combine 2 numbers before using them as keys of an object? I.E., how to combine 2 doubles into an unique Integer?
There is the definition of an array in JavaScript:
Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232 - 1. A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 232. The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.
In other words, if the index you specify is a number representing an integer between 0 and 0xFFFFFFFE, then it is used as an array index. Any other value is taken as a string and it is used to create an object member instead of an array item.
So if you have constraints on your indices which would fit the valid range (0 to 0xFFFFFFFE) then you're good. Otherwise, what you have is probably the fastest.
So the following represents string indices which are members of object myHash:
myHash[x + "," + y] = z;
Someone mentioned using an array of arrays. That would not help you. You'd get many arrays instead of many strings. It would probably be about the same if not slower. The idea is something like this:
myHash[x] = []; // initialize the sub-array (must be done only once per value of 'x'
myHash[x][y] = z; // save z in that array
I do not recommend the double array because it will initialize one array for each value of 'x' on top of myHash and that probably not any faster than having the string concatenation (especially because you'll have to test whether the myHash[x] array was already defined or not...).
So... it is possible to write:
myHash[3.3] = "that worked?";
But if after that you check out the length, you'll notice it is zero:
console.log("Hash length = " + myHash.length);
This is because 3.3 is not an integer.
I'm writing a JavaScript interpreter for extremely resource-constrained embedded devices (http://www.espruino.com), and every time I think I have implemented some bit of JavaScript correctly I realise I am wrong.
My question now is about []. How would you implement one of the most basic bits of JavaScript correctly?
I've looked through the JavaScript spec and maybe I haven't found the right bit, but I can't find a useful answer.
I had previously assumed that you effectively had two 'maps' - one for integers, and one for strings. And the array length was the value of the highest integer plus one. However this seems wrong, according to jsconsole on chrome:
var a = [];
a[5] = 42;
a["5"]; // 42
a.length; // 6
but also:
var a = [];
a["5"] = 42;
a[5]; // 42
a.length; // 6
So... great - everything is converted into a string, and the highest valued string that represents an integer is used (plus one) to get the length? Wrong.
var a = [];
a["05"] = 42;
a.length; // 0
"05" is a valid integer - even in Octal. So why does it not affect the length?
Do you have to convert the string to an integer, and then check that when converted back to a string, it matches?
Does anyone have a reference to the exact algorithm used to store and get items in an array or object? It seems like it should be very simple, but it looks like it actually isn't!
As the specs said, and was noted by others:
"A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 2^32-1."
That's explain why in your scenario "5" is considered an array index and "05" is not:
console.log("5" === String("5" >>> 0));
// true, "5" is equal to "5", so it's an index
console.log("05" === String("05" >>> 0));
// false, "05" is not equal to "5", so it's not an index
Note: the Zero-fill right shift is the shortest way in JS to have a substitute of ToUint32, shifting a number by zero.
See MDN
It's possible to quote the JavaScript array indexes as well (e.g.,
years["2"] instead of years[2]), although it's not necessary. The 2 in
years[2] eventually gets coerced into a string by the JavaScript
engine, anyway, through an implicit toString conversion. It is for
this reason that "2" and "02" would refer to two different slots on
the years object and the following example logs true:
console.log(years["2"] != years["02"]);
So with a["5"] you are accessing the array while a["05"] sets a property on the array object.
Arrays are just objects. That means they can have additional properties which are not considered elements of the array.
If the square bracket argument is an integer, it uses it to perform an assignment to the array. Otherwise, it treats it as a string and stores it as a property on the array object.
Edit based on delnan's comment and DCoder's comment, this is how JavaScript determines if it is an appropriate index for an array (versus just a property):
http://www.ecma-international.org/ecma-262/5.1/#sec-15.4
Arrays are also objects.
By doing this
a["05"] = 5;
You are doing the same thing as:
a.05 = 5;
However, the above will result in a syntax error, as a property specified after a dot cannot start with a number.
So if you do this:
a = [];
a["05"] = 5;
you still have an empty array, but the property of a named 05 has the value 5.
The number x is an array index if and only if ToString(ToUint32(x)) is equal to x (so in case of "05" that requirement is not met).
Can anyone explain why the second alert says 0 ?
var pollData = new Array();
pollData['pollType'] = 2;
alert(pollData['pollType']); // This prints 2
alert(pollData.length); // This prints 0 ??
The length of the array is only changed when you add numeric indexes. For example,
pollData["randomString"] = 23;
has no effect on length, but
var pollData = [];
pollData["45"] = "Hello";
pollData.length; // 46
changes the length to 46. Note that it doesn't matter if the key was a number or a string, as long as it is a numeric integer.
Besides, you are not supposed to use arrays in this manner. Consider it more of a side effect, since arrays are objects too, and in JavaScript any object can hold arbitrary keys as strings.
Because you haven't put anything into the array yet. You've only been assigning to a dynamically-created pollType attribute on the array object.
If you use numeric indices, then the array automagically takes care of length. For example:
var arr = [ ]; // same as new Array()
arr[2] = 'Banana!';
alert(arr.length); // prints 3 (indexes 0 through 2 were created)
The length property takes into consideration only those members of the array which names are indexes (like '1', '2', '3', ... ).
Arrays in JavaScript have numeric indexes only.
Use an object, which is essentially what you are doing above, setting properties on that array object.
array.length returns how many values are stored in the array. The first alert is returning the value of the position 'pollType'.
The reference guide I always use when needing help with javascript arrays is this page http://www.hunlock.com/blogs/Mastering_Javascript_Arrays
I'd also read what it says under the heading Javascript Does Not Support Associative Arrays, as you may run into problems with this also.
var pollData = Array();
function test() {
pollData[0] = 2
alert(pollData[0]);
alert(pollData.length);
}
//[x] is the array position; hence ['polltype'] is causing issues