Changing the Array Prototype in JavaScript

Question

I'm trying to change the Array Prototype and access columns as if they are rows therefor doing a custom matrix transposition. I want to make sure that the access of each column doesn't require re-allocating and creating an entirely new object just to access a particular column.

For example...

var mcol = new column([[0,1,2],[3,4,5],[6,7,8]]);
alert(mcol[1]);

What I'm looking for is to read the column as if it was a row... (doing a 90 degree transform on the matrix)

mcol[1] = [1,4,7];

Any suggestions?

Marshall · Accepted Answer · 2012-08-01 22:47:47Z

2

You can use this constructor:

function column() {
    var arr = Array.prototype.slice.call(arguments);

    return arr;
}

Allowing you to do what you want.

var mcol = new column([0,1,2], [3,4,5], [6,7,8]);
alert(mcol[1]); // alerts [3, 4, 5]

answered Aug 1, 2012 at 22:47

Marshall

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1 Comment

Asher Over a year ago

What I was looking to do is do a matrix transform without copying the entire array. ROW 0 -> COLUMN 0 and ROW 1 -> COLUMN 1 and so on...

juandopazo · Accepted Answer · 2012-08-01 22:54:33Z

1

I'd suggest you make a Matrix constructor with a column method:

function Matrix() {
  this._rows = Array.prototype.slice.call(arguments);
}
Matrix.prototype.column = function (i) {
  // See (1)
  return this._rows.map(function (row) {
    return row[i];
  });
};

var m = new Matrix([0,1,2],[3,4,5],[6,7,8]);
console.log(m.column(1)); // [1,4,7]

(1) https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/map

answered Aug 1, 2012 at 22:54

juandopazo

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3 Comments

Asher Over a year ago

why does javascript need to reserve the word prototype. shouldn't we be able to use Array.slice.call(arguments);

juandopazo Over a year ago

What that line is doing is using a function that all arrays have on the arguments object which is a special object that looks like an array but isn't one. Array.slice isn't defined, but [].slice is defined. So you borrow from the array and call() on arguments.

juandopazo Over a year ago

Array.prototype.slice.call(arguments) is pretty much standard practice in the JavaScript community. It's a way of getting a real array from an arguments object. Map won't work for that. concat() also works.

Asher · Accepted Answer · 2012-08-08 19:07:09Z

The solution I have is a work around and involves copying the array to a new array which I don't like. It does override the Array prototype.

    Array.prototype.rotate = function () {
      var rotated = [];
      var columns = 1; 
      for(var col = 0; col < columns; col++) {
        var myrow = [];
        for(var row = 0; row < this.length; row++){ // this.length is the longest column
          if(this[row].length > columns){
            columns = this[row].length;
          }
          if(this[row].length > col){
            myrow.push(this[row][col]);
          } else {
            myrow.push(null);
          }
        }
        rotated.push(myrow);
      }
      return rotated;
    }

var mcol = [[0,1,2], [3,4,5], [6,7,8]];
mcol = mcol.rotate();
alert(mcol[1]);

Alerts [1,4,7]

Does anyone know of a solution that doesn't require someone to copy the entire array to a new array?

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