CODEWITHSUNDEEP
c++
Sick654
Sick654

Reputation: 79

Is it possible to predict random number of srand(time(0))?

my rand number is rand()%6+1 aka dice rolling, when its based on "time", is it possible to make a console app that foresees the future numbers in the time I want to? for example predict a number on time 14:40:32 on a certain day in future?

Upvotes: 1

Views: 2712

Answers (3)

vitaut
vitaut

Reputation: 55594

Yes provided that you use the same implementation of rand i.e. link with the same version of the standard library. All you need is to get the time_t value for the time you are interested in pass it to srand and call rand to get the value.

For example, if time_t holds the number of seconds since the epoch (which is the case for most implementations), then you can do the following to get the value returned by rand with a 10-second-in-the-future seed:

std::srand(std::time(nullptr) + 10);
std::cout << std::rand();

(Leaving aside the questions of whether it's a good idea to use rand at all.)

Upvotes: 1

LSerni
LSerni

Reputation: 57418

Yes and no. If you have a value of time_t, then just run the same library version of srand() on that value, and rand() will definitely yield the same sequence.

But you need to be sure that

  • the random libraries in the two applications use the same implementation (I think it's Mersenne Twister, but I'd need to check)
  • the clock of the two applications is synchronised. If you think that the master application's clock is 14:30:17, but it's really 14:30:18, then entering 14:30:17 in the monitor application will (of course) get different values.
  • the sequence of calls to rand() in both applications is the same, i.e., the number of calls between the srand() and the rand() you are interested in is known by you.

The last point might be a showstopper.

Say that you know that the app was initialised with srand(T) and you know T. Now yes, you know all the future extractions of its rand(). But you still need to know at which point in the sequence you are.

The number extracted at 19:30:17 GMT will not depend on the '19:30:17 GMT', but on how many numbers have been extracted before since the call to srand().

TL;DR if you know the value that time(0) passed to srand(), you cannot predict the output of the rand() call at a given time. You can predict the output of the n-th call to rand() for any given n.

Upvotes: 0

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

... for example predict a number on time 14:40:32 on a certain day in future?

It's possible when knowing how exactly rand() generates the pseudo random number on a certain seed (which is available for most compilers open source code implementation).

You have a certain seed number given from your date and time, thus you can just inspect the sequence of random numbers generated consecutively.

Upvotes: 0

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