I've got an array. I push items multiple times into this array using a function. Below is an simplified version of the code.
var arr = [];
function pushItems(i){
//do something with i
var abc = "string"
arr.push(abc);
//do something with i
var xyz = "string"
arr.push(xyz);
}
Sometimes abc value is pushed before xyz. Sometimes xyz gets pushed before abc value. My question is how do I always have the abc value ahead of 'xyz' value?
So basically I need the array values to be [abc1, xyz1, abc2, xyz2, abc3, xyz3, ...] so on. How do I order the push accordingly?
This is wrong. According to the specification of this method:
The push() method adds one or more elements to the end of an array and
returns the new length of the array.
Please have a look here.
For a more formal approach please see the ECMAScript specification here.
The arguments are appended to the end of the array, in the order in
which they appear. The new length of the array is returned as the
result of the call.
Update
But even if the elements are added at the end of the array, I'm
looking a way of ordering my array.
You can use the sort function for this reason passing to it an appropriate function that will do the compare. For instance, let we have the following array
var array = [4,1,2,5,3];
and we want to order it in a descending order, we could do this like below:
var array = array.sort(function(a,b){ return b-a; });
Since you need your base64-strings to be in an arbitrary order in the array, sort them by an identifier you define.
var firstObj = {id: 0, base64: 'asdf'}
var secondObj = {id: 1, base64: 'qwer'}
var arr = []
// do stuff
// callback needs to have something along these lines:
function base64isLoaded(obj){
arr[obj.id] = obj.base64;
}
Now the 'front' image (as you gave this as example) can be given id: 0, so it ends up in the '0' spot of the array. I can't really help more without more information about how your code is structured.
EDIT: From your comment ("passing multiple items into pushItems"), I am going to assume that i (the argument) is an array and you iterate this array to transform each element into a base64 encoded string. You then want these encoded strings added to arr in the same order, correct?
easily done, simply make i an array of objects:
var i = [{source: 'abc'}, {source: 'xyz'}];
pushItems(i){
for(var c = 0; c < i.length; c++){
makeIntoBase64(i[c]);
}
}
makeIntoBase64(obj){
// this is whatever function that transforms it and takes a callback when it is done
transform(obj.source, function(result){ //pass the source to be encoded
//result should be base64 encoded string
obj.encoded = result;
});
}
after all this, the array i has objects with both .source and .encoded. If you need to know when ALL encoding is done, create a counter and add one to it in the transform callback, and check if counter === i.length every time. When it is, you know you have loaded all base64 strings and can run another function, adding these images to your catalogue or whatever else you need this for :)
Related
I've been trying to build a 2D array in javascript, but not having much success. I'm pulling some data from a DB and I then want to combine some fields into a 2D array in order to use it elsewhere in the code. Ideally what I want to end up with is:
mapLocs = [
['name a','location a',1],
['name b','location b',2],
['name c','location c',3]
]
here is the code I am using to build the mapLocs array:
for(i = 0;i < phtLen;i++){
var x = i + 1;
var myLocs = new Array(myPhotogs[i].phtName,myPhotogs[i].phtLoc,x);
console.log(myLocs);
mapLocs[i] = new Array(myLocs);
}
}
which is pretty much the method that I've gathered from reading similar problems here. The console.log() outputs an array consisting of the three elements I want, but if I try to access mapLocs it doesn't seem to consist of three arrays as I would have expected, but of three elements each of which is made up of the three elements in the myLoc array if that makes sense? So:
console.log(mapLocs[0][0]); // Joe Bloggs, SW1A 1AA, 1
where I was expecting just 'Joe Bloggs' and
console.logs(mapLocs[0][1]); // undefined
What am I doing wrong?
The explicit new Array() constructor does not take an array and make a new array identical to the argument array, but takes a list of arguments that you wish to be contained within the new array. So in the line
mapLocs[i] = new Array(myLocs)
mapLocs[i] is actually being set to
[[Joe Bloggs, SW1A 1AA, 1]]
Instead, you could say
mapLocs[i] = myLocs.slice()
which will clone myLocs and place it at index i in mapLocs, resulting in the output you want.
I have an array that is initialized like such var generationObject = [{string:"", score: 0}];
which I then fill dynamically:
for(var i = 0; i < amount_offspring; i++)
{
// "load" text into array and send the string to see if it evolves
generationObject[i].string = evolve(start_text, characters, mutation_rate);
// then score the string
generationObject[i].score = score(target_text, generationObject.string);
}
I then want to sort this array by score. I don't know what's best, to sort it in the for loop or sort the entire array afterwards.
I will then take the string of the highest scoring object and pass it through the function again, recursively.
So what would be a good way to go about this sort function? I've seen some here use this
generationObject.sort(function(a, b) {
return (a.score) - (b.score);
});
But I'm not sure if .sort is still supported? This didnt seem to work for me though.
generationObject is an array, not an object, so score(target_text, generationObject.string); could be the problem, as .string will be undefined. (Did you mean generationObject[i].string?)
Try building your array like this:
var generationObject = []
for(var i = 0; i < amount_offspring; i++)
{
evolved_string = evolve(start_text, characters, mutation_rate)
generationObject.push({
string: evolved_string,
score: score(target_text, evolved_string)
})
}
And then Array.prototype.sort should do the trick.
You should write your sorting logic outside the for loop, since if you put it inside, the object array will be sorted N times, where N being the iterations of your loop. The following are two ways to do it-
By using sort() function- To clarify your question, sort() is still supported across almost all the browsers. If you are still concerned about the browser compatibility, you can check the MDN documentation to see the list of supported browsers.
generationObject = generationObject.sort(function(a, b) {
return parseInt(a.score) - parseInt(b.score);
});
By using underscorejs-
In underscore, you can take advantage of the sortBy() function.
Returns a (stably) sorted copy of list, ranked in ascending order by the results of running each value through iteratee. iteratee may also be the string name of the property to sort by (eg. length).
You can simply do this in underscorejs-
generationObject = _.sortBy(generationObj, 'score');
I send a php array like:
$var = array (
0=> 4,
1=> 6,
2=> 8,
...
as json_encode($var); into the uri and then I receive it into javascript file is still ok here but when I push it into new array like this :
this.patg.push(attd);
Is inserted like this below .
var attds = ["4,6,7,8,9,5558,5560,5573,5574,5586,5589,5606"]
I know I have to find the problem. but
Questions:
could you please tell me why this could happends or help me to deal with it.
but in any case just for knowledge . how you would add the extra " " surrounding the , that i miss to be an array , or is that crazy idea to fix this?
If you are receiving a string value and you want to use it as an array of integers you should split it into an array first:
var receivedData = "4,6,7,8,9,5558,5560,5573,5574,5586,5589,5606";
var dataArray = receivedData.split(",");
Afterwards you can use it with another array, however be aware that if you already have a defined array into which you want to push the dataArray you shouldn't push but concat instead.
In other words if you have:
var previousArray = [1,2,3];
previousArray.push(dataArray);
You will get
[1,2,3,[4,6,7,8,9,5558,5560,5573,5574,5586,5589,5606]]
meaning that the whole array is pushed onto the 4th position of previousArray.
If, on the other hand, you concat the arrays will merge:
var previousArray = [1,2,3];
previousArray.concat(dataArray);
[1,2,3,4,6,7,8,9,5558,5560,5573,5574,5586,5589,5606]
Source: http://www.w3schools.com/jsref/jsref_concat_array.asp
I have this code that fetches data and puts it into an array:
this.$httpGetTest(this.test.testId)
.success(function (data: ITestQuestion[]) {
self.test.qs = data;
});
It works and populates the array starting with self.test.qs[0].
However many times my code references this array (which contains a list of questions 1...x)
I must always remember to subract 1 from the question number and so my code does not look clear. Is there a way that I could place an entry ahead of all the others in the array so that:
self.test.qs[0] is null
self.test.qs[1] references the first real data for question number 1.
Ideally I would like to do this by putting something after the self.test.qs = and before data.
Push values at start of array via unshift
self.test.qs.unshift(null);
You need to use Splice(), It works like:
The splice() method changes the content of an array, adding new elements while removing old elements.
so try:
self.test.qs.splice(0, 0, null);
Here mid argument 0 is to set no elements to remove from array so it will insert null at zero and move all other elements.
Here is demo:
var arr = [];
arr[0] = "Hello";
arr[1] = "Friend";
alert(arr.join());
arr.splice(1,0,"my");
alert(arr.join());
You can start off with an array with a null value in it, then concat the questions array to it.
var arr = [null];
arr = arr.concat(data);
You could do something like:
x = [null].concat([1, 2, 3]);
Though there isn't anything wrong with doing something like:
x[index-1]
I'd prefer it to be honest, otherwise someone might assume that the index value returned is 0 based.
I need to map specific numbers to string values. These numbers are not necessarily consecutive, and so for example I may have something like this:
var obj = {};
obj[10] = "string1";
obj[126] = "string2";
obj[500] = "string3";
If I'm doing a search like this obj[126] would it be faster for me to use an object {} or an array []?
There will be no difference. ECMAScript arrays, if sparse (that is don't have consecutive indices set) are implemented as hash tables. In any case, you are guaranteed the O(n) access time, so this shouldn't concern you at all.
I created a microbenchmark for you - check out more comprehensive test by #Bergi. On my browser object literal is a little bit slower, but not significantly. Try it yourself.
A JS-array is a object, so it should not matter what you choose.
Created a jsperf test (http://jsperf.com/array-is-object) to demonstrate this.
Definetely an object should be the best choice.
If you have such code:
var arr = [];
arr[10] = 'my value';
, your array becomes an array of 11 values
alert(arr.length); // will show you 11
, where first 10 are undefined.
Obviously you don't need an array of length 1000 to store just
var arr = [];
arr[999] = 'the string';
Also I have to notice that in programming you have to chose an appropriate classes for particular cases.
Your task is to make a map of key: value pairs and object is the better choice here.
If your task was to make an ordered collection, then sure you need an array.
UPDATE:
Answering to your question in comments.
Imagine that you have two "collections" - an array and an object. Each of them has only one key/index equal to 999.
If you need to find a value, you need to iterate through your collection.
For array you'll have 999 iterations.
For object - only one iteration.
http://jsfiddle.net/f0t0n/PPnKL/
var arrayCollection = [],
objectCollection = {};
arrayCollection[999] = 1;
objectCollection[999] = 1;
var i = 0,
l = arrayCollection.length;
for(; i < l; i++) {
if(arrayCollection[i] == 1) {
alert('Count of iterations for array: ' + i); // displays 999
}
}
i = 0;
for(var prop in objectCollection) {
i++;
if(objectCollection[prop] == 1) {
alert('Count of iterations for object: ' + i); // displays 1
}
}
Benchmark
In total:
You have to design an application properly and take into account possible future tasks which will require some different manipulations with your collection.
If you'll need your collection to be ordered, you have to chose an array.
Otherwise an object could be a better choice since the speed of access to its property is roughly same as a speed of access to array's item but the search of value in object will be faster than in sparse array.