Question

In Javascript, I have a matrix with a variable number of rows and columns, which I wish to store in a multi-dimensional array.

The problem is that I need extra 3 columns and 3 extra rows with negative indexes in the matrix too. So the result for a 10×10 matrix should be a 13×13 array with indexes from -3 to 9.

I define the array with:

  var numberofcolumns = 10;
  var numberofrows = 10;
  var matrix = [];
  for (var x = -3; x < numberofcolumns; x++) {
    matrix[x] = [];
  }

Is this the right way to do this? Or is there a better way?

Answer 1

You could define the matrix as an object instead. You would lose some array functionality but you could still access matrix[-3] for example.

  var numberofcolumns = 10;
  var numberofrows = 10;
  var matrix = {};
  for (var x = -3; x < numberofcolumns; x++) {
    matrix[x] = [];
  }
  for (x in matrix) {
    console.log(matrix[x]);
  }

Or you could define your own class starting as an object or array and work from there. Here’s something to get you started:

function Matrix() { };

Matrix.prototype.LBound = function()
{
    var n;
    for (i in this) {
        if (!isNaN(i) && (isNaN(n) || n > i))
            n = parseInt(i);
    }
    return n;
};

Matrix.prototype.UBound = function()
{
    var n;
    for (i in this) {
        if (!isNaN(i) && (isNaN(n) || n < i))
            n = parseInt(i);
    }
    return n;
};

Matrix.prototype.length = function()
{
    var length = this.UBound() - this.LBound();
    return isNaN(length) ? 0 : length+1;
};

Matrix.prototype.forEach = function(callback, indexes)
{
    if (!indexes) var indexes = [];
    for (var i = this.LBound(); i <= this.UBound() ; i++)
    {
        indexes[Math.max(indexes.length-1, 0)] = i;
        callback(this[i], indexes);
        if (this[i] instanceof Matrix)
        {
            var subIndexes = indexes.slice();
            subIndexes.push("");
            this[i].forEach(callback, subIndexes);
        }
    }
};

Matrix.prototype.val = function(newVal)
{
    if (newVal) 
    {
        this.value = newVal;
        return this;
    }
    else
    {
        return this.value;
    }
};

Then you’d create your matrix as such

var numberofcolumns = 10;
var numberofrows = 10;

var matrix = new Matrix();

for (var i = -3; i < numberofcolumns; i++) {
    matrix[i] = new Matrix();
        for (var j = -4; j < numberofrows; j++) {
            matrix[i][j] = new Matrix();
            matrix[i][j].val("test " + i + " " + j);
        }
}

And you can run some cool functions on it

console.log("Upper bound: " + matrix.LBound());
console.log("Lower bound: " + matrix.UBound());
console.log("Length: " + matrix.length());

matrix.forEach(function(item, index) 
    {
        if (item.val()) 
            console.log("Item " + index + " has the value \"" + item.val() + "\""); 
        else
            console.log("Item " + index + " contains " + item.length() + " items"); 
    });

DEMO: http://jsfiddle.net/uTVUP/