How to change a value in an array by key name

This has been driving me nuts
I have this array where in certain cases I need to change a value in the array from NULL to some other value. I can change the value by using the numeric key, but not by the key name. Here is the example:

while($row = @mysql_fetch_array($r)){
		echo "<tr>\n";
		for($i=0; $i < sizeof($_SESSION['parts']['columns_selected']); ++$i){
			$row['price'] = 1.11;
			echo "<td>".$row[$i]."</td>\n";
		}
		echo "</tr>\n";
	}

PHP:

I would think that every row would have a price of 1.11, but this does not work.
If I change line 4 to:

$row[12] = 1.11;

PHP:

it works as expected and every row displays 1.11 for price. The problem is that the number of items in the array changes depending on what the user selects, so it will not always be $row[12].
I hope someone can help me figure this out.

Here is some additional information:
One row of the output of print_r(array($row)) with sensitive info removed:

Array ( [0] => Array ( [0] => 4 [request_id] => 4 [1] => 12345001 [part_number] => 12345001 [2] => 4-Port Hub [part_description] => 4-Port Hub [3] => XXX [vendor] => XXX [4] => 4 [quantity] => 4 [5] => XXX [first_name] => XXX [6] => XXX [last_name] => XXX [7] => XXX [address] => XXX [8] => XXX [city] => XXX [9] => XX [state] => XX [10] => XXXXX [zip] => XXXXX [11] => AZAA-123456 [po_number] => AZAA-123456 [12] => 25.99 [price] => 25.99 [13] => 103.96 [(price * quantity)] => 103.96 [14] => Received-closed [status] => Received-closed ) )

PHP:

 

I think your codes should be changed to:

[COLOR="red"]$columns = $_SESSION['parts']['columns_selected'];[/COLOR]
while($row = @mysql_fetch_array($r)){
        [COLOR="red"]$row['price'] = 1.11;[/COLOR]
        echo "<tr>\n";
        for($i=0; $i < sizeof([COLOR="red"]$columns[/COLOR]); ++$i){
            echo "<td>".[COLOR="red"]$row[$columns[$i]][/COLOR]."</td>\n";
        }
        echo "</tr>\n";
}

Code (markup):

 

Thanks, but $_SESSION['parts']['columns_selected'] does not contain the data I need to fill in the table. It is only used to obtain the number of loops to run.
I sill am having the problem with replacing the data in the array when using the key name.