I am working on a MCQ module and I need to fetch random questions from my database. The problem is that I seem to get duplicates.
A:
What methods have you tried? I don't know if any RNG can guarantee non-repeating numbers in theory, but you can always store a table of results and throw away any repetitions based on that.
Peter
2009-03-23 16:35:14
Be careful doing something like this. If you are generating truly random numbers, there will *eventually* be repetitions. Discarding the repetitions makes the process unrandom.
theycallmemorty
2009-03-23 16:38:00
I agree--it would make things less random; in his case though, with his aim that he fetch random questions, it makes sense to view the problem as something like a raffle. He's drawing random numbers from a hat: once one is drawn, it can't be drawn again.
Peter
2009-03-23 16:42:29
i hv first produces the random no. by mt_rand() and then store them in a string for further reference and check each time for the no. compiaring with previous values.......but the repitiotion test fails dome time......
PROFESSOR
2009-03-23 18:17:16
This kind of stuff should be a comment on his question, not an answer.
TravisO
2009-03-23 19:05:46
A:
Without any more info i can suggest a rudimentary solution. (but please update your question with more info)
I'm guessing you have users, because then you could save into a table (be it temporary or not), what questions said user has already gotten.
If you don't have users, you can use the SESSION_ID as a user identifier for that user.
So when you fetch a question for the first time, and the user answers it, it saves the info you need to save, and then the user's id and the question's id into a table.
When fetching the next question, you do a check to see if the user has that question id in this new table.
Ólafur Waage
2009-03-23 16:35:49
+6
A:
Sounds like you want to shuffle the questions, not randomize access to them. So your algorithm would be something like this.
- Get the all question (or question keys) you want to display.
- Shuffle them
- Retrieve/ display in them in the shuffled order
for shuffling check out: Fisher-Yates shuffle algorithm
Greg Dean
2009-03-23 16:36:53
+1
A:
If you're using MySql and you have reasonable small amount of data, you can use ORDER BY RAND()
soulmerge
2009-03-23 16:44:47
Other RDBMSes have similar syntax. For example, I believe you can use "ORDER BY NEWID()" in MSSQL and "ORDER BY DBMS_CRYPTO.randominteger" in Oracle.
Ben Blank
2009-03-23 17:19:52
A:
See http://stackoverflow.com/questions/203382/do-stateless-random-number-generators-exist
Any sequence of pseudo-random numbers will eventually repeat. How you obtain your pseudo-random numbers?
cdonner
2009-03-23 16:45:49
+5
A:
If you're fetching them from database, use SQL to do your job. e.g. fetching 20 random questions (without repeating):
SELECT * FROM questions ORDER BY RAND() LIMIT 20
vartec
2009-03-23 16:53:19
A:
If you have a very large number of rows you can add a column to the table which stores a number between 0 and 1 and then fetch with a query:
SELECT * FROM `mytable` WHERE `randcolumn` > RAND() LIMIT 20
This means that your database doesn't have to randomly order the entire table to provide just 20 rows.
David Caunt
2009-03-23 19:27:00