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Usage of rand() is usually frowned upon despite using a seed via srand(). Why would that be the case? What better alternatives are available?
There are two parts to this story.
First, rand is a pseudorandom number generator. This means it depends on a seed. For a given seed it will always give the same sequence (assuming the same implementation). This makes it not suitable for certain applications where security is of a great concern. But this is not specific to rand. It's an issue with any pseudo-random generator. And there are most certainly a lot of classes of problems where a pseudo-random generator is acceptable. A true random generator has its own issues (efficiency, implementation, entropy) so for problems that are not security related most often a pseudo-random generator is used.
So you analyzed your problem and you conclude a pseudo-random generator is the solution. And here we arrive to the real troubles with the C random library (which includes rand and srand) that are specific to it and make it obsolete (a.k.a.: the reasons you should never use rand and the C random library).
One issue is that it has a global state (set by srand). This makes it impossible to use multiple random engines at the same time. It also greatly complicates multithreaded tasks.
The most visible problem of it is that it lacks a distribution engine: rand gives you a number in interval [0 RAND_MAX]. It is uniform in this interval, which means that each number in this interval has the same probability to appear. But most often you need a random number in a specific interval. Let's say [0, 1017]. A commonly (and naive) used formula is rand() % 1018. But the issue with this is that unless RAND_MAX is an exact multiple of 1018 you won't get an uniform distribution.
Another issue is the Quality of Implementation of rand. There are other answers here detailing this better than I could, so please read them.
In modern C++ you should definitely use the C++ library from <random> which comes with multiple random well-defined engines and various distributions for integer and floating point types.
None of the answers here explains the real reason of being rand() bad.
rand() is a pseudo-random number generator (PRNG), but this doesn't mean it must be bad. Actually, there are very good PRNGs, which are statistically hard or impossible to distinguish from true random numbers.
rand() is completely implementation defined, but historically it is implemented as a Linear Congruential Generator (LCG), which is usually a fast, but notoriously bad class of PRNGs. The lower bits of these generators have much lower statistical randomness than the higher bits and the generated numbers can produce visible lattice and/or planar structures (the best example of that is the famous RANDU PRNG). Some implementations try to reduce the lower bits problem by shifting the bits right by a pre-defined amount, however this kind of solution also reduces the range of the output.
Still, there are notable examples of excellent LCGs, like L'Ecuyer's 64 and 128 bits multiplicative linear congruential generators presented in Tables of Linear Congruential Generators of Different Sizes and Good Lattice Structure, Pierre L'Ecuyer, 1999.
The general rule of thumb is that don't trust rand(), use your own pseudo-random number generator which fits your needs and usage requirements.
What is bad about rand/srand is that rand—
Uses an unspecified algorithm for the sequence of numbers it generates, yet
allows that algorithm to be initialized with srand for repeatable "randomness".
These two points, taken together, hamper the ability of implementations to improve on rand's implementation (e.g., to use a cryptographic random number generator [RNG] or an otherwise "better" algorithm for producing pseudorandom numbers). For example, JavaScript's Math.random and FreeBSD's arc4random don't have this problem, since they don't allow applications to seed them for repeatable "randomness" — it's for exactly this reason that the V8 JavaScript engine was able to change its Math.random implementation to a variant of xorshift128+ while preserving backward compatibility. (On the other hand, letting applications supply additional data to supplement "randomness", as in BCryptGenRandom, is less problematic; even so, however, this is generally seen only in cryptographic RNGs.)
Also:
The fact that the algorithm and the seeding procedure for rand and srand are unspecified means that even reproducible "randomness" is not guaranteed between rand/srand implementations, between versions of the same standard library, between operating systems, etc.
If srand is not called before rand is, rand behaves similarly as though srand(1) were first called. In practice, this means that rand can only be implemented as a pseudorandom number generator (PRNG) rather than as a nondeterministic RNG, and that rand's PRNG algorithm can't differ in a given implementation whether the application calls srand or not.
EDIT (Jul. 8, 2020):
There is one more important thing that's bad about rand and srand. Nothing in the C standard for these functions specifies a particular distribution that the "pseudo-random numbers" delivered by rand have to follow, including the uniform distribution or even a distribution that approximates the uniform distribution. Contrast this with C++'s uniform_int_distribution and uniform_real_distribution classes, as well as the specific pseudorandom generator algorithms specified by C++, such as linear_congruential_engine and mt19937.
EDIT (begun Dec. 12, 2020):
Yet another bad thing about rand and srand: srand takes a seed that can only be as big as an unsigned. unsigned must be at least 16 bits and in most mainstream C implementations, unsigned is either 16 or 32 bits depending on the implementation's data model (notably not 64 bits even if the C implementation adopts a 64-bit data model). Thus, no more than 2^N different sequences of numbers can be selected this way (where N is the number of bits in an unsigned), even if the underlying algorithm implemented by rand can produce many more different sequences than that (say, 2^128 or even 2^19937 as in C++'s mt19937).
Firstly, srand() doesn't get a seed, it sets a seed. Seeding is part of the use of any pseudo random number generator (PRNG). When seeded the sequence of numbers that the PRNG produces from that seed is strictly deterministic because (most?) computers have no means to generate true random numbers. Changing your PRNG won't stop the sequence from being repeatable from the seed and, indeed, this is a good thing because the ability to produce the same sequence of pseudo-random numbers is often useful.
So if all PRNGs share this feature with rand() why is rand() considered bad? Well, it comes down to the "psuedo" part of pseudo-random. We know that a PRNG can't be truly random but we want it to behave as close to a true random number generator as possible, and there are various tests that can be applied to check how similar a PRNG sequence is to a true random sequence. Although its implementation is unspecified by the standard, rand() in every commonly used compiler uses a very old method of generation suited for very weak hardware, and the results it produces fair poorly on these tests. Since this time many better random number generators have been created and it is best to choose one suited to your needs rather than relying on the poor quality one likely to provided by rand().
Which is suitable for your purposes depends on what you are doing, for example you may need cryptographic quality, or multi-dimensional generation, but for many uses where you simply want things to be fairly uniformly random, fast generation, and money is not on the line based on the quality of the results you likely want the xoroshiro128+ generator. Alternatively you could use one of the methods in C++'s <random> header but the generators offered are not state of the art, and much better is now available, however, they're still good enough for most purposes and quite convenient.
If money is on the line (e.g. for card shuffling in an online casino, etc.), or you need cryptogaphic quality, you need to carefully investigation appropriate generators and ensure they exactly much your specific needs.
rand is usually -but not always-, for historical reasons, a very bad pseudo-random number generator (PRNG). How bad is it is implementation specific.
C++11 has nice, much better, PRNGs. Use its <random> standard header. See notably std::uniform_int_distribution here which has a nice example above std::mersenne_twister_engine.
PRNGs are a very tricky subject. I know nothing about them, but I trust the experts.
Let me add another reason that makes rand() totally not usable: The standard does not define any characteristic of random numbers it generates, neither distribution nor range.
Without definition of distribution we can't even wrap it to have what distribution we want.
Even further, theorically I can implement rand() by simply return 0, and anounce that RAND_MAX of my rand() is 0.
Or even worse, I can let least significant bit always be 0, which doesn't violate the standard. Image someone write code like if (rand()%2) ....
Pratically, rand() is implementation defined and the standards says:
There are no guarantees as to the quality of the random sequence produced and some implementations
are known to produce sequences with distressingly non-random low-order bits. Applications with
particular requirements should use a generator that is known to be sufficient for their needs
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf p36
If you use rand(), you will basically have the same result after generating your random number.
So even after using srand(), it will be easy to predict the number generated if someone can guess the seed you use. This is because the function rand() uses a specific algorithm to produce such numbers
With some time to waste, you can figure out how to predict numbers generated by the function, given the seed. All you need now is to guess the seed. Some people refer to the seed as the current time. So if can guess the time at which you run the application, I ll be able to predict the number
IT IS BAD TO USE RAND()!!!!
I'm working on a test suite for my package, and as part of the tests I would like to run my algorithm on a block of data. However, it occured to me that instead of hardcoding a particular block of data, I could use an algorithm to generate it. I'm wondering if the C++11 <random> facilities would be appropriate for this purpose.
From what I understand, the C++11 random number engines are required to implement specific algorithms. Therefore, given the same seed they should produce the same sequence of random integers in the range defined by the algorithm parameters.
However, as far as distributions are concerned, the standard specifies that:
The algorithms for producing each of the specified distributions are implementation-defined.
(26.5.8.1 Random number distribution class templates / In general)
Which — unless I'm mistaken — means that the output of a distribution is pretty much undefined. And from what I've tested, the distributions in GNU libstdc++ and LLVM project's libc++ produce different results given the same random engines.
The question would therefore be: what would be the most correct way of producing pseudo-random data that would be completely repeatable across different platforms?
what would be the most correct way of producing pseudo-random data that would be completely repeatable across different platforms?
That would be obvious: write your own distribution. As you yourself pointed out, the engines are cross-platform since they implement a specific algorithm. It's the distributions that are implementation-defined.
So write the distributions yourself.
Please see this answer: https://stackoverflow.com/a/34962942/1151329
I had exactly this problem and writing my own distributions worked perfectly. I got the same sequences across linux, OSx, windows, x86 and ARM.
Quoting from cppreference:
std::random_device is a non-deterministic random number engine, although implementations are allowed to implement std::random_device using a pseudo-random number engine if there is no support for non-deterministic random number generation.
Is there a way to check whether current implementation uses PRNG instead of RNG (and then say exit with an error) and if not, why not?
Note that little bit of googling shows that at least MinGW implements std::random_device in this way, and thus this is real danger if std::random_device is to be used.
---edit---
Also, if the answer is no and someone could give some insight as to why there is no such function/trait/something I would be quite interested.
Is there a way to check whether current implementation uses PRNG instead of RNG (and then say exit with an error) and if not, why not?
There is a way: std::random_device::entropy will return 0.0 if it is implemented in terms of a random number engine (that is, it's deterministic).
From the standard:
double entropy() const noexcept;
Returns: If the implementation employs a random number engine, returns 0.0. Otherwise, returns an entropy estimate for the random numbers returned by operator(), in the range min() to log_2(max() + 1).
There is no 100% safe way to determine real randomness for sure. With a black box approach the best you could do do is to show evidence if it's not fully random:
first you could verify that the distribution seems random, by generating a lot of random munmbers and making statistics about their distribution (e.g. generate 1 million random numbers between 0 and 1000). If it appears that some numbers come out significantly more often than other, then obviously it's not really random.
THe next you can is to run several time a programme generating random numbers after the same initial seed. If you obtain the same sequence of random numbers then it's definitively PRNG and not real randmness. However, if you don't obtain the same sequence it does not proove tanything: the library could use some kind of auto-seed (using clock ticks or something else) to hide/improve the pseudo-randmness.
If your application highly depends on randomness quality (e.g. cryptographic quality) you should consider some more tests, such as those recommended by NIST SP 800-22
Xarn stated above:
However, said pessimism also precludes this method from differentiating between RNG and PRNG based implementation, making it rather unhelpful. Also VC++ could be realistic, but to check that would probably require a lot of insider knowledge about Windows.
If you debug into the Windows implementation, then you will find that you end up in RtlGenRandom, which is one of the better sources of cryptographically random bytes. If you debug into the Linux implementation, then you should end up reading from dev/urandom, which is also OK. The fact that they don't tell us that we're not using something awful, like rand, is annoying.
PS - you don't have to have internal Windows knowledge, you just need to attach the symbols to the debugger.
Background: I use rand(), std::rand(), std::random_shuffle() and other functions in my code for scientific calculations. To be able to reproduce my results, I always explicitly specify the random seed, and set it via srand(). That worked fine until recently, when I figured out that libxml2 would also call srand() lazily on its first usage - which was after my early srand() call.
I filled in a bug report to libxml2 about its srand() call, but I got the answer:
Initialize libxml2 first then.
That's a perfectly legal call to be made from a library. You should
not expect that nobody else calls srand(), and the man page nowhere
states that using srand() multiple time should be avoided
This is actually my question now. If the general policy is that every lib can/should/will call srand(), and I can/might also call it here and there, I don't really see how that can be useful at all. Or how is rand() useful then?
That is why I thought, the general (unwritten) policy is that no lib should ever call srand() and the application should call it only once in the beginning. (Not taking multi-threading into account. I guess in that case, you anyway should use something different.)
I also tried to research a bit which other libraries actually call srand(), but I didn't find any. Are there any?
My current workaround is this ugly code:
{
// On the first call to xmlDictCreate,
// libxml2 will initialize some internal randomize system,
// which calls srand(time(NULL)).
// So, do that first call here now, so that we can use our
// own random seed.
xmlDictPtr p = xmlDictCreate();
xmlDictFree(p);
}
srand(my_own_seed);
Probably the only clean solution would be to not use that at all and only to use my own random generator (maybe via C++11 <random>). But that is not really the question. The question is, who should call srand(), and if everyone does it, how is rand() useful then?
Use the new <random> header instead. It allows for multiple engine instances, using different algorithms and more importantly for you, independent seeds.
[edit]
To answer the "useful" part, rand generates random numbers. That's what it's good for. If you need fine-grained control, including reproducibility, you should not only have a known seed but a known algorithm. srand at best gives you a fixed seed, so that's not a complete solution anyway.
Well, the obvious thing has been stated a few times by others, use the new C++11 generators. I'm restating it for a different reason, though.
You use the output for scientific calculations, and rand usually implements a rather poor generator (in the mean time, many mainstream implementations use MT19937 which apart from bad state recovery isn't so bad, but you have no guarantee for a particular algorithm, and at least one mainstream compiler still uses a really poor LCG).
Don't do scientific calculations with a poor generator. It doesn't really matter if you have things like hyperplanes in your random numbers if you do some silly game shooting little birds on your mobile phone, but it matters big time for scientific simulations. Don't ever use a bad generator. Don't.
Important note: std::random_shuffle (the version with two parameters) may actually call rand, which is a pitfall to be aware of if you're using that one, even if you otherwise use the new C++11 generators found in <random>.
About the actual issue, calling srand twice (or even more often) is no problem. You can in principle call it as often as you want, all it does is change the seed, and consequentially the pseudorandom sequence that follows. I'm wondering why an XML library would want to call it at all, but they're right in their response, it is not illegitimate for them to do it. But it also doesn't matter.
The only important thing to make sure is that either you don't care about getting any particular pseudorandom sequence (that is, any sequence will do, you're not interested in reproducing an exact sequence), or you are the last one to call srand, which will override any prior calls.
That said, implementing your own generator with good statistical properties and a sufficiently long period in 3-5 lines of code isn't all that hard either, with a little care. The main advantage (apart from speed) is that you control exactly where your state is and who modifies it.
It is unlikely that you will ever need periods much longer than 2128 because of the sheer forbidding time to actually consume that many numbers. A 3GHz computer consuming one number every cycle will run for 1021 years on a 2128 period, so there's not much of an issue for humans with average lifespans. Even assuming that the supercomputer you run your simulation on is a trillion times faster, your grand-grand-grand children won't live to see the end of the period.
Insofar, periods like 219937 which current "state of the art" generators deliver are really ridiculous, that's trying to improve the generator at the wrong end if you ask me (it's better to make sure they're statistically firm and that they recover quickly from a worst-case state, etc.). But of course, opinions may differ here.
This site lists a couple of fast generators with implementations. They're xorshift generators combined with an addition or multiplication step and a small (from 2 to 64 machine words) lag, which results in both fast and high quality generators (there's a test suite as well, and the site's author wrote a couple of papers on the subject, too). I'm using a modification of one of these (the 2-word 128-bit version ported to 64-bits, with shift triples modified accordingly) myself.
This problem is being tackled in C++11's random number generation, i.e. you can create an instance of a class:
std::default_random_engine e1
which allows you to fully control only random numbers generated from object e1 (as opposed to whatever would be used in libxml). The general rule of thumb would then be to use new construct, as you can generate your random numbers independently.
Very good documentation
To address your concerns - I also think that it would be a bad practice to call srand() in a library like libxml. However, it's more that srand() and rand() are not designed to be used in the context you are trying to use them - they are enough when you just need some random numbers, as libxml does. However, when you need reproducibility and be sure that you are independent on others, the new <random> header is the way to go for you. So, to sum up, I don't think it's a good practice on library's side, but it's hard to blame them for doing that. Also, I could not imagine them changing that, as billion other pieces of software probably depend on it.
The real answer here is that if you want to be sure that YOUR random number sequence isn't being altered by someone else's code, you need a random number context that is private to YOUR work. Note that calling srand is only one small part of this. For example, if you call some function in some other library that calls rand, it will also disrupt the sequence of YOUR random numbers.
In other words, if you want predictable behaviour from your code, based on random number generation, it needs to be completely separate from any other code that uses random numbers.
Others have suggested using the C++ 11 random number generation, which is one solution.
On Linux and other compatible libraries, you could also use rand_r, which takes a pointer to an unsigned int to a seed that is used for that sequence. So if you initialize that a seed variable, then use that with all calls to rand_r, it will be producing a unique sequence for YOUR code. This is of course still the same old rand generator, just a separate seed. The main reason I meantion this is that you could fairly easily do something like this:
int myrand()
{
static unsigned int myseed = ... some initialization of your choice ...;
return rand_r(&myseed);
}
and simply call myrand instead of std::rand (and should be doable to work into the std::random_shuffle that takes a random generator parameter)
I want to supply a number, and then receive a set of random numbers. However, I want those numbers to be the same regardless of which computer I run it on (assuming I supply the same seed).
Basically my question is: in C++, if I make use of rand(), but supply srand() with a user-defined seed rather than the current time, will I be able to generate the same random number stream on any computer?
There are dozens of PRNGs available as libraries. Pick one. I tend to use Mersenne Twister.
By using an externally supplied library, you bypass the risk of a weird or buggy implementation of your language's library rand(). As long as your platforms all conform to the same mathematical semantics, you'll get consistent results.
MT is a favorite of mine because I'm a physicist, and I use these things for Monte Carlo, where the guarantee of equal-distribution to high dimensions is important. But don't use MT as a cryptographic PRNG!
srand() & rand() are not part of the STL. They're actually part of the C runtime.
Yes, they will produce the same results as long as it's the same implementation of srand()/rand().
Depending on your needs, you might want to consider using Boost.Random. It provides several high-quality random number generators.
Assuming the implementations of rand() are the same, yes.
The easiest way to ensure this is to include a known rand() implementation with your program - either included in your project's source code or in the form of a library you can manage.
No, the ANSI C standard only specifies that rand() must produce a stream of random integers between 0 and RAND_MAX, which must be at least 32767 (source). This stream must be deterministic only in that, for a given implementation on a given machine, it must produce the same integer stream given the same seed.
You want a portable PRNG. Mersenne Twister (many implementations linked at the bottom) is pretty portable, as is Ben Pfaff's homegrown C99-compliant PRNG. Boost.Random should be fine too; as you're writing your code in C++, using Boost doesn't limit your choice of platforms much (although some "lesser" (i.e. non-compliant) compilers may have trouble with its heavy use of template metaprogramming). This is only really a problem for low-volume embedded platforms and perhaps novel research architectures, so if by "any computer" you mean "any x86/PPC/ARM/SPARC/Alpha/etc. platform that GCC targets", any of the above should do just fine.
Write your own pseudorandom number routine. There are a lot of algorithms documented on the internet, and they have a number of applications where rand isn't good enough (e.g. Perlin Noise).
Try these links for starters:
http://en.wikipedia.org/wiki/Linear_congruential_generator
http://en.wikipedia.org/wiki/Pseudorandom_number_generator
I believe if you supply with srand with the same seed, you will get the same results. That's pretty much the definition of a seed in terms of pseudo random number generators.
Yes. For a given seed (starting value), the sequence of numbers that rand() returns will always be the same.