[Resolved] Math Question (1,2,3,4) -> (1,2,4,8)

[davidjam]

Hello all,

OK, I have a multi-part multi-page form. The status of the form (i.e. incomplete, skipped, complete) is handled in an array: parts 1,2,3,4.

I keep track of what parts are completed in the database as a courtesy to return users. I keep track in a MySQL set() column. Each set element has a decimal value: 1,2,4,8,.. respectively.

If the user is on, say, part 3, completes it, and the set() column is updated ("update table set col = col | 4 where id = ..") I need some nifty math to automatically convert 3 to its decimal equivelent of 4 (and of course any other part number, 4 to 8, 5 to 16, etc). Maybe this is not possible...

(BTW The tip for updating a set() column was found here -- go to the user notes below. Credit to: Monte Ohrt. Also, for this to work the initial create table must have the set() column as NOT NULL.)

[mtmosier]

You want the pow function. 2 is your base and the exponential is (part - 1), so 2 in this example.

[davidjam]

Thanks a lot!

But if I take the pow() function it returns the wrong values:

1,2,3,4 become 2,4,8,16

which wouldn't work for the set column.

So I just went like this:

$set_val = (pow(2,$part)/2);

That seems to work. But thanks for sending me in the right direction mtmosier! I am not very smart, but given enough time I can figure things out. 🙂

[mtmosier]

Reread my original message please. You need to use part - 1. The divide will work too, but the minus one is better. (More logically correct, likely faster [negligible].)

[Weedpacket]

You might also want to look at the left-shift operator <<; a<<b is equivalent to a*pow(2,b), unless you're talking about numbers in the billions (which I take it you're not!)

[davidjam]

Sorry mtmosier for not reading your post better. I changed my code accordingly.

weedpacket: thanks also, although I don't understand how it would fit into my problem. damn! I wish I took the academic stream in high school. Now I regret it! 🙁

[davidjam]

OK, now that I know how to "shift" those values to their equivelents for the set_col, I want to integrate this into my code.

$array = array("username" => "..", "basic_info" => "..", "shipping_etc" => "..");
$current_part = "basic_info";
$current_part_num = ???? 
$set_value = pow(2,($current_part_num-1));
$update_sql = new sql("update table set set_col = set_col | {$set_value} where id = {$id}");

Really, this is an array question now. How do I get the numerical position of $current_part.

[mtmosier]

If it is always "part n" then you could just use substr. Then if you want to explicitly convert the result to a number you can use intval, or just use it like a number and php will automatically convert it for you. (Note that substr should be used whether you use intval or not.)

It doesn't really matter, but why are you using 'part n' as the array key anyway, instead of just numbers?

[davidjam]

Sorry, in an attempt to make my code as simple as possible, I made it more confusing. The key names are the actual names of the parts (see my previous post again -- edited). I need to know what the numerical position in the array is. I looked through all the array functions on php.net and couldn't find anything.

[mtmosier]

Well, that's different then.

If you were working with the array while looping through it you could simply keep a counter. Or if you didn't need the keys while working with it you could use array_values to reindex the array.

If given only the part name and the array to work with (without a loop), like in your example, you could use array_keys and array_search.

The more logical way might be to redo your array as a numerically indexed multi-dimensional array, like this

$array = array();
$array[0] = array('Part Name' => "username", 'Value' => "..");
$array[1] = array('Part Name' => "basic_info", 'Value' => "..");
$array[2] = array('Part Name' => "shipping_etc", 'Value' => "..");

[davidjam]

Originally posted by mtmosier
... The more logical way might be to redo your array as a numerically indexed multi-dimensional array ...

Thank you again. I appreciate the time you put into your replies. Yes, this does seem like the best solution, so I have gone ahead and rebuilt the array with a numeric index.

🙂

[davidjam]

OK, I'm bumping this thread because I now have the inverse question. How do I check to see if a part decimal equivelent (i.e. part 1 = 1, part 2 = 2, part 3 = 4) is in the mysql result for that row?

I know I can use a strpos() on the part name but I want to avoid using this if there is a mathematical (and presumably faster) way of doing it.

OK so I retreive from the database the set column as set_col + 0, so I get the decimal value of the column. Let me just make this into an example:

part 1 --> "username" --> dec. value 1
part 2 --> "basic_info" --> dec. value 2
part 3 --> "shipping_etc" --> dec. value 4

Let's say the set column for this row has a value of "username, basic_info" and therefore a dec. value of 3. OK, in my loop as I cycle through the part numbers, let's say I am on the iteration part 2. Is there a mathematical way of "seeing" if part 2 (with a dec. value of 2) is IN THE MYSQL RETURNED VALUE OF 3??

[EDIT]OK, I'm moving this post over to a new thread. I guess it's being skipped because it says [resolved]. Hey , that's so logical![/EDIT]

[mtmosier]

Convert your "part 2" to "dec value 2", then check whether the result of a bitwise & of the mysql set value with the dec value is a non-zero number.

Okay, that was confusing. This page gives a little tutorial about shifting bits, and using the bitwise operators. It's about java but the concepts are the same.

Here's some code to demonstrate.

$part_num = 2;
$dec_value = 1 << $part_num - 1;  // Same as pow(2, $part_num - 1)
$set_info = 3;

if ($set_info & $dec_value) {
    //  Part is in the set
}