Re: Way for computing random primes in standard C.

fieldfallow wrote:
Is there a function in the standard C library which returns a prime
number which is also pseudo-random?

No. :) The ANSI C standard doesn't have that sort of content in it.

Assuming there isn't, as it appears from the docs that I have, is there
a better way than to fill an array of range 0... RAND_MAX with
pre-computed primes and using the output of rand() to index into it to
extract a random prime.

You can use the primeAt() function here:

http://www.pobox.com/~qed/primeat.zip

indexed by a pseudo random number. This will give exactly even
probabilities for all the primes that are at most 32 bits.

Now, of course you may need a range larger than 0 ... RAND_MAX. You
can build that from code found here:

http://www.pobox.com/~qed/random.html

Also what is meant by reinitialising the rand() function with srand(1)?
Does it mean that further calls to rand() will return numbers with a new
starting point? If so, how is it different to calling srand() with a
seed value such as that returned by the time() function?

rand() outputs numbers in a deterministic sequence indexed by the seed
passed to srand(). Calling srand(time(NULL)) makes the sequence change
over time (usually different for every second of time.)

Thank you all for the help. I find this group very useful.

(That is so ironic ...)

--
Paul Hsieh
http://www.pobox.com/~qed/
http://bstring.sf.net/

.

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