Fluctuating(decrease-increase) random numbers,how??

This is a discussion on Fluctuating(decrease-increase) random numbers,how?? within the C++ Programming forums, part of the General Programming Boards category; How can you get a random number that fluctuates. See, when I start my program successively,like 25 runs per minute ...

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    Fluctuating(decrease-increase) random numbers,how??

    How can you get a random number that fluctuates. See, when I start my program successively,like 25 runs per minute the number just increases is there a way that the random number can fluctuate?

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    Huh?

    My guess would be that you are seeding the random number generator with "time()", and thus your initial number will be dependent on current time. Since time has a tendency to increase, it will [up to a point] increase. After all, a randum number generator takes a seed, modifies it mathematically (usually a multiply, add and divide/modulo), so changing the seed will affect the actual random number output.

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    So what should i use instead of time()?

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    Quote Originally Posted by freddyvorhees View Post
    So what should i use instead of time()?
    Well, that's a good question. You need a number that changes more rapidly/less predictably than time() provides. Unfortunately, it's hard to find one that is portable, available on most systems, and rapidly changing/less predictable.

    Some suggestions:
    In Linux/Unix "gettimeofday" provides microseconds, so that would change 1 million times more often.
    In Windows, GetSystemTime provided millisecond time.
    Both of these return a struct, so you don't just get a large count, you have to combine different parts of the struct to get a "large number".

    Edit: In windows, QueryPerformanceCounter() will give you a 64-bit number that is changing at least on a microsecond basis - often much faster than that.

    Another option would be to use the TSC (TimeStampCounter). There are inline assembler versons here: http://en.wikipedia.org/wiki/RDTSC

    You can also (as "Prelude" on this board suggests) calculate a hash of the time_t value that you get back from time() - this will "shuffle" the changes to the time, which means that it changes more substantially for the same amount of time-change. Have a look at Prelue's site:
    http://eternallyconfuzzled.com/arts/jsw_art_rand.aspx

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    Mats
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    Please don't PM me for help - and no, I don't do help over instant messengers.

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    Nice, Thanks!

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    It sounds like the problem is just that srand() is being called multiple times. You can still use time, but you should call srand a total of once in your entire application.

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