I need to test out a Java program 20 times and need to set the random seed so that the tests can be repeated. If I were to set the initial seed as 0 and then increment by 1 at each run (i.e. 1,2,3 etc), would this method still ensure complete randomness, even though the seeds are not far apart?
Thank you
Any seed will provide the same level of randomness as any other seed for a standard PRNG like the one included with Java. So it's fine to use an incrementing seed for tests.
You might want to consider using a better random number generator though. The one included with Java yields noticeable repeating patterns if you render the values as an image (I can't find a reference offhand, but I recall it being apparent). I'd recommend the Mersenne Twister (there are Java versions) as an alternative that's fast and has a very long period so you won't easily see patterns.
If you use a seed of 0 the sequence of random numbers will be repeatable from that point. You would only need to use a different random seed if you wanted a different sequence. i.e. you should be able to set the seed once per test.
Why not just use the current time as the seed? ie, System.currentTimeMillis() ?
Do you need a number or a string? I use UUID.randomUUID().toString() to get a UUID for my tests when I need strings, but if you need a number then you can use System.nanoTime().
Related
I have to generate random numbers in two different places in the project and I do it using the system clock, but I have to get different numbers in both. How can I do that?
You could use a loop surrounded by calls to System.nanoTime(), then use the least significant bits of their difference as random seed. Once you have, say, 128 bits you can use it as seed for a pseudo random number generator.
There is an article posted in the wiki at java-gaming.org about generating pseudo-random numbers that might be worth perusing. Pseudo-random number generators He shows what he calls an "Old School" method of creating them using a combination of polynomials and modular division. Plus links to other types of PRNGs.
I am using a simple random calculations for a small range with 4 elements.
indexOfNext = new Random().nextInt(4); //randomize 0 to 3
When I attach the debugger I see that for the first 2-3 times every time the result is 1.
Is this the wrong method for a small range or I should implement another logic (detecting previous random result)?
NOTE: If this is just a coincidence, then using this method is the wrong way to go, right? If yes, can you suggest alternative method? Shuffle maybe? Ideally, the alternative method would have a way to check that next result is not the same as the previous result.
Don't create a new Random() each time, create one object and call it many times.
This is because you need one random generator and many numbers from its
random sequence, as opposed to having many random generators and getting
just the 1st numbers of the random sequences they generate.
You're abusing the random number generator by creating a new instance repeatedly. (This is due to the implementation setting a starting random number value that's a very deterministic function of your system clock time). This ruins the statistical properties of the generator.
You should create one instance, then call nextInt as you need numbers.
One thing you can do is hold onto the Random instance. It has to seed itself each time you instantiate it and if that seed is the same then it will generate the same random sequence each time.
Other options are converting to SecureRandom, but you definitely want to hold onto that random instance to get the best random number performance. You really only need SecureRandom is you are randomly generating things that have security implications. Like implementing crypto algorithms or working around such things.
"Random" doesn't mean "non-repeating", and you cannot judge randomness by short sequences. For example, imagine that you have 2 sequences of 1 and 0:
101010101010101010101010101010101
and
110100100001110100100011111010001
Which looks more random? Second one, of course. But when you take any short sequence of 3-4-5 numbers from it, such sequence will look less random than taken from the first one. This is well-known and researched paradox.
A pseudo-random number generator requires a seed to work. The seed is a bit array which size depends on the implementation. The initial seed for a Random can either be specified manually or automatically assigned.
Now there are several common ways of assigning a random seed. One of them is ++currentSeed, the other is current timestamp.
It is possible that java uses System.currentTimeMillis() to get the timestamp and initialize the seed with it. Since the resolution of the timestamp is at most a millisecond (it differs on some machines, WinXP AFAIK had 3ms) all Random instances instantiated in the same millisecond-resolution window will have the same seed.
Another feature of pseudo-random number generators is that if they have the same seed, they return the same numbers in the same order.
So if you get the first pseudo-random number returned by several Randoms initialized with the same seed you are bound to get the same number for a few times. And I suspect that's what's happening in your case.
It seems that the number 1 has a 30% chance of showing its self which is more than other numbers. You can read this link for more information on the subject.
You can also read up on Benford's law.
I can't understand what was the meaning of Seed in java.util.Random ? I had read Why does this code print “hello world”? question and I am still confuse about seed . Can anyone describe me kindfully what was seed actually mean ? Thanks.
In documentation for setSeed() method ... what does mean seed - the initial seed ?
public void setSeed(long seed)
Sets the seed of this random number generator using a single long seed. The general contract of setSeed is that it alters the state of this random number generator object so as to be in exactly the same state as if it had just been created with the argument seed as a seed. The method setSeed is implemented by class Random by atomically updating the seed to
(seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)
and clearing the haveNextNextGaussian flag used by nextGaussian().
The implementation of setSeed by class Random happens to use only 48 bits of the given seed. In general, however, an overriding method may use all 64 bits of the long argument as a seed value.
Parameters:
seed - the initial seed
I would expect if I can understand exactly meaning of seed , I am sure I will understand clearly to this answer.
A pseudo-random number generator produces a sequence of numbers. It
isn't truly random, but generally a mathematical calculation which
produces an output that matches some desirable distribution, and
without obvious patterns. In order to produce such a sequence, there
must be state stored for the generator to be able to generate the next
number in that sequence. The state is updated each time using some
part of the output from the previous step.
Seeding explicitly initialises this state. A 'seed' is a starting
point, from which something grows. In this case, a sequence of
numbers.
This can be used either to always generate the same sequence (by using
a known constant seed), which is useful for having deterministic
behaviour. This is good for debugging, for some network applications,
cryptography, etc.
Or, in situations where you want the behaviour to be unpredictable
(always different each time you run a program, a card game perhaps),
you can seed with a number likely to be continually changing, such as
time.
The 'randomness' of the sequence does not depend on the seed chosen,
though it does depend on not reseeding the sequence.
Taken from What is a seed in relation to a random number generation algorithm and why is computer time used to create this seed more often than not?
This should answer your question.
The pseudorandom number generator is implemented in terms of an integer which is, each time you ask for a number, transformed into another integer by the pseudorandom sequence generator function.
The initial value of that internal integer is termed the seed. The idea is to set it differently each time you instantiate Random because the pseudorandom sequence is fully deterministic once the seed is assigned.
If you use the nullary constructor, new Random(), then System.currentTimeMillis() will be used for the seed, which is good enough for almost all cases.
java.util.Random.setSeed(long seed): Sets the seed of this random number generator using a single long seed
Syntax:
public void setSeed(long seed)
Parameters:
seed - the initial seed
Every Random constructed with the same seed will generate the same
pattern of numbers every time.
So basically we set seed with a long value when we want to get the
same random number sequence every time (like in video
games,debugging,etc)
I strongly recommend to go through this answer:https://stackoverflow.com/a/23127798/9080948
and this video:https://youtu.be/86_cnhqSyh0
Whenever we create an object of Random class in java. We either of the constructor
Random()
Random(long seed)
What is the purpose of seed here in the 2nd constructor and how can I use it to my benefit i.e. manipulate its use?
The answer above sums it up clearly. As per java api docs from oracle, the first constructor
Random()
"Creates a new random number generator. This constructor sets the seed of the random number generator to a value very likely to be distinct from any other invocation of this constructor. "
The seed is probably a derivative of the current time, or the current time itself. That should be enough to be "very likely to be distinct from any other invocation". Which, in essence, is most likely what you need, most of the time.
So why have another constructor that takes a seed?
Simply put, if you want to generate the same set of random numbers over and over, you use the same seed on your Random constructor. This is useful when doing experiments on different control sets, and you don't want to bother creating your own table of random inputs, but still want the same set of random input on a different experiment/control set.
There's no such thing as truly random numbers in computing. The available methods for getting a random number across all programming languages is nothing but an algorithm to simulate random numbers.
In some languages (C++, I know for sure), an unseeded random number generator will return the same series of numbers on every fresh execution of the program.
What is common is to seed the random number generator with the current time (which will be random enough for most purposes) so that the algorithm starts with a random number each time.
Pseudo-random number generators maintain some set of state information, which is advanced through some recurrence relation to determine the next value of the state. The output of a PRNG is some function of the state. Java's Random class uses a Linear Congruential Generator. LCG's work using the recurrence relationship Ui+1 = (A Ui + C) % M for some constant integer values A, C, and M. Java's current implementation uses a 48-bit state but uses 32 bits or less of it on each iteration of the recurrence.
Based on this, you can see that if you start with the same state you will get the exact same sequence of values out of your PRNG. This can be useful if you want to be able to reproduce exactly the same sequence of "randomness", for instance for debugging or for comparing two experiments head-to-head.
If you invoke the constructor without an argument, it picks a starting value for the state with a promise that different invocations are very likely to be distinct from each other. If you supply a seed to the constructor, that seed's value is used to set the initial state.
Will two java.security.SecureRandom instances which are seeded with the same value initially give the same sequence of random numbers?
I am asking this because I want the same sequence of random numbers in both the client and the server. What if both of them are using the same seed value. Will the sequence be the same, or is there any way that the sequence can be made the same?
From the API docs:
If two instances of Random are created with the same seed, and the same sequence of method calls is made for each, they will generate and return identical sequences of numbers.
What if both of them are using the same seed value. Will the sequence be the same?
No, they definitely won't. At least not in Oracle's Java 7 SDK implementation. See my sample code in this SO post. It appears that the implementation may elect to use additional sources of randomness, in addition to the provided seed.