| www.delorie.com/djgpp/v2faq/faq193.html | search |
| Previous | Next | Up | Top |
Q: I keep getting the same random numbers each time I run my program. How do I get a different series on every run?
rand and random. The former is part of the ANSI C Standard, and is therefore very portable to other
environments. The latter is available on almost every Unix platform, but is generally unsupported by DOS/Windows compilers. On the other hand, series produced by random have better
qualities than those produced by rand. In particular, the least-significant bits in the numbers produced by random are much more random than those you get from
rand, so if you need, say, a random number between 0 and 4, and portability is not an issue, you will get better results with random () % 5. However, the DJGPP
implementation of rand is quite good, so when portability is important, you should use rand.
Both rand and random return a pseudo-random integer in the range [0..RAND_MAX], where RAND_MAX is defined in the stdlib.h header.
By default, every time you restart a program, you get the same series of pseudo-random numbers. This is important in some applications, because it allows to reproduce exactly the results of running
a program which used random series, and thus makes debugging easier. But sometimes, e.g. in a game, you will want a different series every time. To achieve that, you need to initialize the random
series with a different seed. Two functions provided for this purpose, srand and srandom, will seed the series generated, respectively, by rand and
random. You seed the series with a single call to srand or srandom, and then proceed by calling rand or random as usual.
A popular way of getting a different seed every run is to use the current system clock as the seed, like this:
srand (time (NULL));
If the 1-second granularity of the values returned by time is not enough for you (e.g., if you need to generate more than one series every second), use gettimeofday or
uclock, or use the values returned by rand as an argument to srandom (or vice versa).
| webmaster donations bookstore | delorie software privacy |
| Copyright ⌐ 1998 by Eli Zaretskii | Updated Sep 1998 |
You can help support this site by visiting the advertisers that sponsor it! (only once each, though)