PHP array for loop - php

Question

I want to show the array value $result[] from the for loop calculation. However, it shows me nothing on the page. Is there is anything wrong in the below code?
$sql= "SELECT * FROM items where itemID =3 ";
$result1= mysql_query($sql) or die (mysql_error());
while ($row= mysql_fetch_array($result1)){
$quantity[] = $row ['quantity'];
$month[] = $row ['month'];
}
$alpha = 0.3;
for($i = 1; $i > 12; $i++){
$result[$i] = ($quantity[$i] - $result[$i-1]) * $alpha + $result[$i-1];
}
foreach ($result as $key => $value ){
echo "$value";
}

Answer 1

Your for loop has an error. You have
for($i = 1; $i > 12; $i++)
but it should be
for($i = 1; $i < 12; $i++)

Answer 2

This is not directly the answer to your question, but there are few things that hasn't been mentioned that concern the way you query and process your data:
Your SELECT statement doesn't have specific order specified. Since order of records is not preserved you can get records out of correct order and get invalid calculations. Use ORDER BY (e.g. ORDER BY month) or make use of month values and extract exactly previous month's value from array(s) (if it is what you're doing in your code).
Your current code relies on the fact that the resultset from DB will contain (at least) 12 records. If for some reason it will produce less records your for loop will brake.
It's uncertain from the information in the question but it looks like you might need a year in your query unless the table contains records only for one year.
Now, you can calculate the whole thing on DB side with a query like this
SELECT i.month,
COALESCE((i.quantity - p.quantity) * 0.3 + p.quantity, 0) value
FROM items i LEFT JOIN items p
ON i.itemID = p.itemID
AND i.`year` = p.`year`
AND i.month = p.month + 1
WHERE i.itemID = 3
AND i.`year` = 2013
ORDER BY month
SQLFiddle
That's assuming (and I'm not sure about that) you actually need to read previous month's quantity values for your calculations and month column is of integer type

Answer 3

There is an obvious flaw in the logic. You try to get the $i index form $quantity. However, you can't be sure $quantity will have this index.
Supposing that itemId is not the primary key, I would do something like this:
$sql = "SELECT * FROM `items` WHERE `itemID` = 3";
$result1= mysql_query($sql) or die (mysql_error());
while ($row= mysql_fetch_assoc($result1)){
$quantity[] = $row ['quantity'];
}
$alpha = 0.3;
$i = 1
foreach ($quantity as $elem) {
if ($i >= 12)
break;
$result[$i] = ($elem - $result[$i-1]) * $alpha + $result[$i-1];
$i++
}
foreach ($result as $value ){
echo $value;
}