I'm writing a custom sort function that I'm prototyping into Array. (PLEASE don't post answers explaining to me how I shouldn't bother prototyping into Array for whatever reason you feel prototyping into Array isn't a good idea).
so, my method looks like this:
//method
Array.prototype.mySort = function(memberName, ascOrDesc){
var labelRow = this.shift();
var ret = this.sort((function (a,b){
if(ascOrDesc > 0)
return (a[memberName] > b[memberName])?1:-1;
return (a[memberName] < b[memberName])?1:-1;
}));
ret.unshift(labelRow)
return ret;
}
Notice how this.shift() will affect the Array IN PLACE.
However, I'm not clear on how this is accomplished. If I wanted to write my own myShift method, at some point I'd need to say something to the effect of
this = this.myShift();
which is obviously illegal.
So, I'm trying to understand how shift() gets access to the array's members and is able to remove the first one in-place. And if I'm allowed to do something analogous, or if this is somehow baked in and not available to me to use.
You can access the array using this inside the method.
You can for example implement the shift method as:
Array.prototype.myShift = function() {
if (this.length == 0) return null;
var result = this[0];
for (var i = 1; i < this.length; i++) {
this[i-1] = this[i];
}
this.length--;
return result;
};
The problem is that you can't assign to this. This means you can't do things like this:
Array.prototype.myShift = function() {
this = this.slice(1);
};
This is because Array.prototype.slice returns a new array and does not modify the old array. Other methods, however, such as Array.prototype.splice, do modify the old array. So you can do something like this:
Array.prototype.myShift = function() {
return this.splice(0, 1)[0];
};
This will have exactly the same behaviour as the standard Array.prototype.shift method. It modifies the current array, so you can do this:
var labelRow = this.myShift();
Related
I have a function that tries to systematically add arrays to another multidimensional array. At each step of the way the arrays being added are calculated correctly, however, these calculations change the previously entered values. I've tried using slice but I'm clearly doing it wrong :(.
Please see code below - it is the return posMatrix that is being affected.
function allPossibilities(hand) {
var startingHandLength = hand.length;
var potHand = Array.prototype.slice.call(hand);
var scores = new Array();
var posMatrix = new Array();
var nextCard = 1;
var progressStage = true;
var finished = false;
var shallowArr = new Array();
do {
scores = calculateScores(potHand);
var maxScore = Math.max.apply(null, scores)
shallowArr = potHand.slice();
if (maxScore>16.5)
{posMatrix.push([shallowArr,maxScore])
console.log(posMatrix);
debugger;
if (potHand.length !== startingHandLength)
{
do{
if(potHand[potHand.length-1][1] < 10)
{
potHand[potHand.length-1][1]++;
progressStage = true;
}
else {potHand.pop();
potHand[potHand.length-1][1]++;}
}
while(progressStage === false)
}
}
else
{
potHand.push(["Imaginary",1,"Imaginary"]);
}
progressStage=false;
if(potHand.length === startingHandLength)
{finished = true;}
}
while(finished === false);
return posMatrix;
}
If the starting hand > 16.5, the function works as none of the other code gets to run. But otherwise it does not. The final return should be an array where each element is looks like this: [[array],number]. The number seems to come out fine, but since it is not an object it is not affected. I would expect the [array]s to be different from one another, currently they are all the same.
Slice returns a shallow copy of array, since you have multidimensional array so you need to deep clone of array
JSON.parse(JSON.stringify(array))
Or you can use loadash cloneDeep
You made a shallow copy of hand (which, BTW, you should've included). With statements like this
potHand[potHand.length-1][1]++;
you're accessing and modifying elements of hand, too.
Here, potHand[potHand.length-1] is an object, and it's en element of hand (not a copy - the same element).
I'm writing a subclass of arrays in Javascript to have better support for matrix operations (I know others exist, this is partially for me to re-teach myself linear algebra), and what I want is to have some properties that are reset whenever any values in the matrix are adjusted. Some calculations like the determinant are computationally intensive, and I'd like to be able to store them to avoid re-calculation, but then they need to be reset to null whenever any matrix elements are changed.
Essentially, it seems like what i want is the deprecated Array.observe(). And the replacement, proxies, seem like a lot of overhead for this one thing. As alluded to in some of the comments on Detecting Changes in a Javascript Array using the proxy object that were not directly addressed, I don't want to have to access my matrices only ever through proxies. I use a lot of handy [i][j] indexing and [mat[i], mat[j]] = [mat[j], mat[i]] in the code I've written so far.
class Matrix extends Array {
constructor() {
var args = [];
for (var i = 0; i < arguments.length; i++) {
if (Array.isArray(arguments[i])) {
args.push(new Matrix(...arguments[i]));
} else {
args.push(arguments[i]);
}
}
super(...args);
this._determinant = null;
}
determ(forceRecalculate = false) {
if (this._determinant === null || forceRecalculate) {
this.upperEchelon();
}
return this._determinant;
}
upperEchelon(reduced = false) {
//There's a lot of code here but in the process of doing this other thing
//you get 99% of the way to calculating the determinant so it does this
this._determinant = factor;
}
}
Basically, I want anything like mat[0][0] = 10 or mat.push([2,4,5]) that updates the values in the matrix to set mat._determinant = null. Or any equivalent method of flagging that it needs to be re-calculated next time it's asked for. I'm not opposed to using proxies necessarily if someone can help me figure out the implementation, I would just rather have this set-to-null-on-update property be inherent to my class functionality.
What I really want is a way to overload base methods like [] a la C# so the functions that do the updating would trigger this without changing syntax, but I've resigned myself to not having that in JS.
While a Proxy would work, it would also be pretty slow. A different approach would be for every method that needs to use the value of _determinant go through a different function first to check to see if the _determinant needs to be updated (and if so, updates it). This way, the expensive recalculation is not done every time the array changes, but only just in time for the result to be used. For example:
class Matrix extends Array {
constructor() {
var args = [];
for (var i = 0; i < arguments.length; i++) {
if (Array.isArray(arguments[i])) {
args.push(new Matrix(...arguments[i]));
} else {
args.push(arguments[i]);
}
}
super(...args);
this._determinant = null;
}
// next method is effectively a recursive deep join
// could also use toString if it doesn't interfere with anything else
getString() {
const itemsStr = this.map((item) => (
item instanceof Matrix
? item.getString()
: item
))
.join(',');
const result = '[' + itemsStr + ']';
return result;
}
getDeterm() {
const newString = this.getString();
if (newString !== this._lastString) {
this._lastString = newString;
this.upperEchelon();
}
return this._determinant;
}
upperEchelon() {
console.log('running upperEchelon');
this._determinant = Math.random();
}
}
const m = new Matrix([2, 3, 4], 5);
console.log(m.getDeterm());
// Not calculated again:
console.log(m.getDeterm());
// Mutation, next call of getDeterm will run upperEchelon:
m[0][0] = 1;
console.log(m.getDeterm());
I pass 2 arrays to a function and want to move a specific entry from one array to another. The moveDatum function itself uses underscorejs' methods reject and filter. My Problem is, the original arrays are not changed, as if I was passing the arrays as value and not as reference. The specific entry is correctly moved, but as I said, the effect is only local. What do I have to change, to have the original arrays change as well?
Call the function:
this.moveDatum(sourceArr, targetArr, id)
Function itself:
function moveDatum(srcDS, trgDS, id) {
var ds = _(srcDS).filter(function(el) {
return el.uid === uid;
});
srcDS = _(srcDS).reject(function(el) {
return el.uid === uid;
});
trgDS.push(ds[0]);
return this;
}
Thanks for the help
As mentioned in the comments, you're assigning srcDS to reference a new array returned by .reject(), and thus losing the reference to the array originally passed in from outside the function.
You need to perform your array operations directly on the original array, perhaps something like this:
function moveDatum(srcDS, trgDS, id) {
var ds;
for (var i = srcDS.length - 1; i >= 0; i--) {
if (srcDS[i].uid === id) {
ds = srcDS[i];
srcDS.splice(i,1);
}
}
trgDS.push(ds);
return this;
}
I've set up the loop to go backwards so that you don't have to worry about the loop index i getting out of sync when .splice() removes items from the array. The backwards loop also means ds ends up referencing the first element in srcDS that matches, which is what I assume you intend since your original code had trgDS.push(ds[0]).
If you happen to know that the array will only ever contain exactly one match then of course it doesn't matter if you go forwards or backwards, and you can add a break inside the if since there's no point continuing the loop once you have a match.
(Also I think you had a typo, you were testing === uid instead of === id.)
Copy over every match before deleting it using methods which modify Arrays, e.g. splice.
function moveDatum(srcDS, trgDS, id) { // you pass an `id`, not `uid`?
var i;
for (i = 0; i < srcDS.length; ++i) {
if (srcDS[i].uid === uid) {
trgDS.push(srcDS[i]);
srcDS.splice(i, 1);
// optionally break here for just the first
i--; // remember; decrement `i` because we need to re-check the same
// index now that the length has changed
}
}
return this;
}
I have a JSON response like this:
var errorLog = "[[\"comp\",\"Please add company name!\"],
[\"zip\",\"Please add zip code!\"],
...
Which I'm deserializing like this:
var log = jQuery.parseJSON(errorLog);
Now I can access elements like this:
log[1][1] > "Please add company name"
Question:
If I have the first value comp, is there a way to directly get the 2nd value by doing:
log[comp][1]
without looping through the whole array.
Thanks for help!
No. Unless the 'value' of the first array (maybe I should say, the first dimension, or the first row), is also it's key. That is, unless it is something like this:
log = {
'comp': 'Please add a company name'
.
.
.
}
Now, log['comp'] or log.comp is legal.
There are two was to do this, but neither avoids a loop. The first is to loop through the array each time you access the items:
var val = '';
for (var i = 0; i < errorLog.length; i++) {
if (errorLog[i][0] === "comp") {
val = errorLog[i][1];
break;
}
}
The other would be to work your array into an object and access it with object notation.
var errors = {};
for (var i = 0; i < errorLog.length; i++) {
errors[errorLog[i][0]] = errorLog[i][1];
}
You could then access the relevant value with errors.comp.
If you're only looking once, the first option is probably better. If you may look more than once, it's probably best to use the second system since (a) you only need to do the loop once, which is more efficient, (b) you don't repeat yourself with the looping code, (c) it's immediately obvious what you're trying to do.
No matter what you are going to loop through the array somehow even it is obscured for you a bit by tools like jQuery.
You could create an object from the array as has been suggested like this:
var objLookup = function(arr, search) {
var o = {}, i, l, first, second;
for (i=0, l=arr.length; i<l; i++) {
first = arr[i][0]; // These variables are for convenience and readability.
second = arr[i][1]; // The function could be rewritten without them.
o[first] = second;
}
return o[search];
}
But the faster solution would be to just loop through the array and return the value as soon as it is found:
var indexLookup = function(arr, search){
var index = -1, i, l;
for (i = 0, l = arr.length; i<l; i++) {
if (arr[i][0] === search) return arr[i][1];
}
return undefined;
}
You could then just use these functions like this in your code so that you don't have to have the looping in the middle of all your code:
var log = [
["comp","Please add company name!"],
["zip","Please add zip code!"]
];
objLookup(log, "zip"); // Please add zip code!
indexLookup(log, "comp"); // Please add company name!
Here is a jsfiddle that shows these in use.
Have you looked at jQuery's grep or inArray method?
See this discussion
Are there any jquery features to query multi-dimensional arrays in a similar fashion to the DOM?
I need to create an array from tree elements in Javascript and being a newbie I don't know how to achieve this.
pseudo-code :
function make_array_of_tree_node(tree_node)
{
for (var i = 0; i < tree_node.childCount; i ++) {
var node = tree_node_node.getChild(i);
if (node.type ==0) {
// Here I'd like to put a link (node.title) in an array as an element
} else if (node.type ==6) {
// Here the element is a folder so a I need to browse it
make_array_of_tree_node(node)
}
}
}
// Some code
make_array_of_tree_node(rootNode);
// Here I'd like to have access to the array containing all the elements node.title
You can declare an array like this:
var nodes = [];
Then you can add things to it with:
nodes.push(something);
That adds to the end of the array; in that sense it's kind-of like a list. You can access elements by numeric indexes, starting with zero. The length of the array is maintained for you:
var len = nodes.length;
What you'll probably want to do is make the array another parameter of your function.
edit — To illustrate the pattern, if you've got a recursive function:
function recursive(data, array) {
if ( timeToStop ) {
array.push( data.whatever );
}
else {
recursive(data.subData, array);
}
}
Then you can use a second function to be the real API that other code will use:
function actual(data) {
var array = [];
recursive(data, array); // fills up the array
return array;
}
In JavaScript, furthermore, it's common to place the "recursive" function inside the "actual" function, which makes the recursive part private and keeps the global namespace cleaner.