Resizing tilemaps the wrong and slow way

public function resize(width:int, height:int):void
{
    var wallstring:String = walls.saveToString();

    walls = new Tilemap(GFX.WALLS, width << 4, height <<4, 16, 16);

    walls.loadFromString(limit(wallstring, width, height));
}

protected function limit(string:String, cols:int, rows:int):String
{
    var limited:String = '';
    var array:Array = csvToArray(string);

    for(var y:int = 0; y < rows; y++)
    {
        for(var x:int = 0; x < cols; x++)
        {
            var id:int = array[y] ? array[y][x] : 0;
            limited += id ? id : '0';
            if(x < cols-1) limited += ',';
        }

        if(y < rows-1) limited += '\n';
    }

    return limited;
}

protected function csvToArray(string:String):Array
{
    var arr:Array = [];

    var row:Array = string.split('\n');
    var rows:int = row.length;

    var col:Array;
    var cols:int;

    for(var y:int = 0; y < rows; y++)
    {
        if(row[y] == '') continue;

        col = row[y].split(',');
        cols = col.length;
        arr[y] = [];

        for(var x:int = 0; x < cols; x ++)
        {
            if(col[x] == '') continue;
            arr[y][x] = uint(col[x]);
        }
    }

    return arr;
}