java - What is random seed about? -


for example code below. has random class. produce same output everywhere . in case item seed?

source: link

import java.util.random; public class randomtest {     public static void main(string[] s) {         random rnd1 = new random(42);         random rnd2 = new random(42);          system.out.println(rnd1.nextint(100)+" - "+rnd2.nextint(100));         system.out.println(rnd1.nextint()+" - "+rnd2.nextint());         system.out.println(rnd1.nextdouble()+" - "+rnd2.nextdouble());         system.out.println(rnd1.nextlong()+" - "+rnd2.nextlong());     } }

42 seed, same javadoc says. so, seed? random number seldom truly random - it's pseudo-random instead. means it's generated function, said prng (pseudo random number genrator). being generated function, in turn, means output not random anymore, since it's predictable!

however, depending on needs, pseudo-randomness may enough - said enough because generating random bit expensive, , i'm not talking time or memory, money (see this link on wikipedia). so, example, if need random value place enemies in game, pseudo-random number ok - if building security-related software, want use true random number, or @ least cryptographically secure prng.

how can describe prng, 1 used in math.random()? it's function, initialized seed s returns array of values a. note that, each integer s, defined 1 , 1 array a. example (values not actual):

                first call     second call     third call seed: 14329            .18             .82             .5 seed:  3989             .7             .02            .93

so seed prng known value when want result predictable - example testing purposes or ensure that, each time run level 1 in game, enemies placed in same (pseudo) random places - otherwise don't need explicitely pass seed.


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