Element-wise Operations In Javascript

问题:

I'm doing some physics simulations which of course involve vectors. This has become very difficult for me because to the best of my knowledge javascript doesn't support anything like this...

#with the aid of numpy
>>> a = np.array([1,2,3])
>>> b = np.array([9,2,7])
>>> a+b
array([10,  4, 10])

I've been able to work around this limitation by defining functions that will achieve the same thing, but my formulas end up looking like this:

add(x, add( mult(v,dt), mult(mult( a(x), .5), Math.pow(dt,2))))

So my question is whether there are better ways to achieve this functionality, whether they be features of the language I am unaware of, libraries that address this issue, or more effective ways to deal with it.

Thanks for the help all.

回答1:

Check out Sylvester. I think it might be what you are looking for.

But if you wanted to implement the objects yourself, then it might be better to do a more OOP approach. JavaScript is a prototype-based language, so it different a little bit from other OOP languages, but its still pretty easy to implement your own prototypes.

Something like:

Vector = function(items) {
    this.items = items
}

Vector.prototype.add = function(other) {
    var result = []
    for(var i = 0; i < this.items; i++) {
        result.push( this.items[i] + other.items[i])
    }

    return new Vector(result);
}

Vector.prototype.subtract = function(other) { /* code to subtract */ }
Vector.prototype.multiply = function(other) { /* code to multiply */ }

And then use them like this:

var a = new Vector([1,2,3]);
var b = new Vector([5,0,1]);

var result = a.add(b)
result.items // [6,2,4]

Or if you wanted to, you could also extend the Array class with some functions with

Array.prototype.vectorAdd = function(other) { /* code to add another array as a vector */ };

And call that using

[1,2,3].vectorAdd([5,0,1])

Hopefully, that might give you a starting point to make your code a little more readable.

Just another note: Unfortunately in this case, JavaScript doesn't support operation overloading so you can't do neat stuff like a+b. You'll have to do something like a.add(b). but as long you return an appropriate object you can chain methods together. Like:

a.add(b).multiply(c).subtract(d);

ps. the presented code might be a little "off", I just typed it up off the top of my head, so treat it more like pseduocode :)

回答2:

Don't know if this will help, but you can add methods to Array or Number by extending the constructor's .protoype object.

Example: http://jsfiddle.net/9JwLd/

Array.prototype.add = function( b ) {
    var a = this,
        c = [];
    if( Object.prototype.toString.call( b ) === '[object Array]' ) {
        if( a.length !== b.length ) {
            throw "Array lengths do not match.";
        } else {
            for( var i = 0; i < a.length; i++ ) {
                c[ i ] = a[ i ] + b[ i ];
            }
        }
    } else if( typeof b === 'number' ) {
        for( var i = 0; i < a.length; i++ ) {
            c[ i ] = a[ i ] + b;
        }
    }
    return c;
};

var a = [1,2,3];
var b = [9,2,7];

   // pass an Array
var c = a.add( b );  // [10,4,10]

   // pass a number
var d = a.add( 5 );  // [6,7,8]

---

The next version of JavaScript (ECMAScript) will likely include Array comprehensions, which may help as well. (Currently supported in SpiderMonkey.)

EXAMPLE: http://jsfiddle.net/dj6Eq/ (Test in newer versions of Firefox.)

var a = [1, 2, 3];
var b = [9, 2, 7];

var c = [a[n]+b[n] for (n in a) ];
var d = [a[n]+5 for (n in a) ];

---

EDIT: According to the proposal the syntax will be a little different than the current Mozilla implementation of Array comprehensions.

回答3:

Using zipWith from lodash (https://lodash.com/):

_.zipWith([1, 2, 3], [9, 2, 7], _.add);
// -> [10, 4, 10]

回答4:

we can use map function to add array elements

 function addvector(a,b){
 var d = a.map(function(e,i){
 return e + b[i]})
 document.write(d);
  }
  addvector([2,3,4],[4,7,90])

回答5:

For just adding arrays in js, you can use this function

function addArrays(ar1, ar2){
    var ar3 = [];
    for(var i in ar1)
        ar3.push(ar1[i] + ar2[i]);
    return ar3;
}

and then call it like so

var array1 = [1,4,3];
var array2 = [5,3,2];
var array3 = addArrays(array1,array2);
// value of array3 is [6,7,5]