Posted by Peter Fox on 10/24/06 09:02

Following on from Daz's message. . .
>Hello all,
>
>my question is more regarding advice on a script design. I have about
>3600 entries in my database, the user submits a list, which is then
>checked against those in the database to confirm whether or not they
>already own a particular item. If they do, then it's not added to the
>user table, whereas if it is, then it _is_ added to the user table.
>However, if the item is not in the database, the user is advised of
>this. So basically, I need to figure out a quick way to compare the
>users submited items (probably 50 to 700 items), with those in an array
>that I have created using the items from the database.
As I read this you are simply trying to decide which items in list U are
not in list D (U=user's list D=database list).

Two methods spring to mind.
1 - (Possibly not suitable for PHP)
You set up two arrays of bits with the position in the array being the
'ID'.
So if the U list has items 3,4 and 6 the array looks like 00011010000...
and similarly with the D list and now you can AND (etc) to give set
operations.


2 - (Probably better for PHP)
Sort both lists
Set two pointers to start (lowest) of both lists (call them pU and pD)
repeat until end of both lists reached
Compare the pointed to items
if D[pD] == U[pU] then "U already has this D". Bump both pointers
if D[pD] < U[pU] then "U doesn't have this D". Bump pD.
if D[pD] >U[pU] then "This U isn't in D". Bump pU.

With any luck your D list should be pre-sorted as a result of the DB
query.

For speed you may want to bulk your updates by doing the logic and all
of the 'what goes in which category' first.

--
PETER FOX Not the same since the porcelain business went down the pan
peterfox@eminent.demon.co.uk.not.this.bit.no.html
2 Tees Close, Witham, Essex.
Gravity beer in Essex <http://www.eminent.demon.co.uk>

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