I used default_random_engine to generate a list of different numbers, but they happen to be the same. I wonder to generate different numbers? I know srand() does. But can I use this default engine?
default_random_engine engine{ static_cast<unsigned int>(time(0)) };
uniform_real_distribution<double>randomNumber{ 50,100 };
for(int i=0; i<10;i++){
a[i]= randomNumber(engine);
}
That behavior is by design for random number generators. You need to set the 'seed' for the random generator when you start your program.
If you have the C++ extensions #WhozGraig recommends they're best. It's very sloppy but many people use the time() function to get a different number each time the program runs.
Related
Is there any way to achieve same random int numbers sequence in different operating system with same seed?
I have tried this code:
std::default_random_engine engine(seed);
std::uniform_int_distribution<int> dist(0, N-1);
If I ran this code on one machine multiple times with same seed, sequence of dist(engine) is the same, but on different operating system sequence is different.
Yes there is, but you need a different or to put it exactly, the same PRNG on each platform. std::default_random_engine engine is a implementation defined PRNG. That means you may not get the same PRNG on every platform. If you do not have the same one then your chances of getting the same sequence is pretty low.
What you need is something like std::mt19937 which is required to give the same output for the same seed. In fact all of the defined generators in <random> besides std::default_random_engine engine will produce the same output when using the same seed.
The other thing you need to know is that std::uniform_int_distribution is also implementation defined. The formula it has to use is defined but the way it achieves that is left up to the implementor. That means you may not get the exact same output. If you need portability you will need to roll you own distribution or get a third party one that will always be the same regardless of platform.
I'm trying to find a random number generator that will give me a single random number each time I run it. I have spent a week trying dozens of different ones, both from this site and others. Every time I run it, it gives me the same number! The only time it changes is if I change the range, and then it just gives me the new number over and over.
I am running Code::Blocks ver. 16.01 on Windows 7. Can anyone help?? I'm at my wits' end!
This code gives me a decently ramdom string of numbers, but still the same string each time!
#include <iostream>
#include <random>
int main()
{
std::random_device rd;
std::mt19937 eng(rd()); std::uniform_int_distribution<> distr(0, 10);
for(int n=0; n<100; ++n)
std::cout << distr(eng) << '\t';
}
I have tried the code on my compiler app on my phone as well.
Every pseudo random number generator will return the same sequence of numbers for the same initial seed value.
What you want to do is to use a different seed every time you run the program. Otherwise you'll just be using the same default seed every time and get the same values.
Picking good seeds is not as easy as you might think. Using the output from time(nullptr) for example still gives the same results if two copies of the program run within the same second. Using the value of getpid() is also bad since pid values wrap and thus sometimes you'll get the same value for different runs. Luckily you have other options. std::seed_seq lets you combine multiple bad sources and returns a good (or rather, pretty good) seed value you can use. There is also std::random_device which (on all sane implementations) returns raw entropy - perfect for seeding a pseudo random generator (or you can just use it directly if it is fast enough for your purpose) or you can combine it with std::seed_seq and the bad sources to seed a generator if you are worried it might be implemented as a prng on your implementation.
I would advice you to read this page: http://en.cppreference.com/w/cpp/numeric/random for an overview of how to deal with random number generation in modern C++.
The standard allows std::random_device to be implemented in terms of a pseudo-random number generator if there is no real random source on the system.
You may need to find a different entropy source, such as the time, or user touch co-ordinates.
I've been searching for a better solution than my own and I haven't really been able to find one that I understand or that works for me.
I have made the simple game where the computer randomly generates a number which you then guess a number and if it is higher the computer says higher and so on..
The problem is my randomly generated number, after looking up alot of information regarding the <random>, uniform_int_distribution and default_random_engine. I have found out that the computer generates a random number, but if you run the program again the same random number will be generated.
My solution:
uniform_int_distribution<unsigned> u(0,100); // code to randomly generate numbers between 0 and 100
default_random_engine e; // code to randomly generate numbers
size_t userInput; // User input to find out where to look in the vector
vector<int> randomNumbers; //vector to hold the random numbers
unsigned start = 0, ending = 101, cnt = 0; // used in the game not important right now
cout << "Please enter a number between 1 and 1000 for randomness" << endl;
cin >> userInput;
for(size_t i = 0; i < 1000; ++i){ //for loop to push numbers into the vector
randomNumbers.push_back(u(e));
}
unsigned guess = randomNumbers[userInput]; // finally the number that the user will have to guess in the game
My solution right now is to use a vector where I push alot of randomly generated numbers in then ask the user to type a number which then the computer uses for the game. But there should be a better way of doing this. And my question is therefore
Is there a better way to randomly generate numbers to use in the game?
Either use std::random_device in place of std::default_random_engine, or else think of a way to provide a different number to the engine each time it is run.
This number is called a "seed" and can be passed as an optional parameter to the constructor. Since std::default_random_engine is implementation-specific, and different engines do different things about seeding, you generally want to choose a specific engine if you're providing a seed. A deterministic pseudo-random number generator will produce the same sequence of outputs for any given seed, so you want to use a different seed each time.
For no-security uses like a guessing game, the most "obvious" thing to use as a seed is the current time. Generally speaking this is different each time the program is run, although obviously if you can run the program twice in less than the granularity of the clock then that's not the case. So using the time to seed your random engine is pretty limited but will do the job for a toy program.
That's because your random number is actually what we call a pseudorandom number generator
It's just a machine that given a starting number generates a large list of seemingly random numbers. As you don't provide a starting number, the generated list of random numbers is thus always the same. One easy way to fix this is to use the current time as a starting value or 'seed', which is an argument of the constructor of std::default_random_engine.
You can also use your machines real random number generator std::random_device as a replacement for std::default_random_engine
Why not simply:
#include <ctime> // for time()
#include <cstdlib> // for srand()
srand(time(NULL)); // Initializes the rand() function
int randomNumber = rand()%100; // Random number between 0 and 99.
What this does is the rand() seed is set at the current time, meaning that every execution of the program will have a different seed for rand().
Still just pseudo-random solution, though suitable for your purposes.
I have a for loop, and inside it I'm generating random numbers and seeding it with time(msecs),but it's not generatin them randomly, what can I do to solve that?
note:
I know why rand() is not generating them randomly, I'm asking for the solution, and I'm using Qt creator if that will help.
for(int j=0;j<5;j++){
qsrand(QDateTime::currentDateTime().time().msec());//for every j itteratio i must
for(int i=1;i<s;i++){ //have new sequence and ret is
ret.push_back(rand()%s); //vector i'm using Qt
} //s=4 for now,but s=[1;50]
qDebug()<<"new sequence ...";
}
it generates
[2,1,1],[2,1,1],[2,1,1],[2,1,1],[2,1,1];
Depends of what is you application.
If you need to generate secure random numbers that can't be guessed, then you should use CSPRNG https://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator.
For this purpouse there is /dev/urandom availabe for unix clones.
In case you need random numbers for simulation or something similar where you need random numbers you should use high quality super fast, random and large PRNG called mersenne twister.
It is available in standard library of c++11 or in boost. (headers only)
http://en.cppreference.com/w/cpp/numeric/random/mersenne_twister_engine
http://www.boost.org/doc/libs/1_55_0/doc/html/boost_random.html
As many other said, make sure you seed only once.
If you want a long sequence of random numbers, like here, then you probably want to use a pseudo-random number generator (unless this is for cryptography!). C++11 brought support for random and pseudo-random numbers into the standard library, which makes it very easy to use. Just seed the PRNG with a true random number before the loop, then just use the output from that to get (pseudo-)random numbers. You will also need to include the random header.
std::random_device rd;
std::default_random_engine e1{rd()};
std::cout << e1(); // random number
std::cout << e1(); // another random number
std::uniform_int_distribution<> dist{1, 6};
std::cout << dist(e1); // random integer between 1 and 6
If this is for cryptographical uses, see what Luka Rahne said.
My random numbers that output, output in the same sequence every time I run my game. Why is this happening?
I have
#include <cstdlib>
and am using this to generate the random numbers
randomDiceRollComputer = 1 + rand() % 6;
You need to seed your random number generator:
Try putting this at the beginning of the program:
srand ( time(NULL) );
Note that you will need to #include <ctime>.
The idea here is to seed the RNG with a different number each time you launch the program. By using time as the seed, you get a different number each time you launch the program.
You need to give the randum number generator a seed. This can be done by taking the current time, as this is hopefully some kind of random.
#include <cstdlib>
#include <ctime>
using namespace std;
int main()
{
int r;
srand(time(0));
r = rand();
return 0;
}
The rand() function is specifically required to produce the same sequence of numbers when seeded with a given seed (by calling srand()); each possible seed value specifies a sequence. And if you never call srand(), you get the same sequence you would have gotten by calling srand(1) before any call to rand().
(This doesn't apply across different C or C++ implementations.)
This can be useful for testing purposes. If there's a bug in your program, for example, you can reproduce it by re-running it with the same seed, guaranteeing that (barring other unpredictable behaviors) you'll get the same sequence of pseudo-random numbers.
Calling srand(time(NULL)) is the usual recommended way to get more or less unpredictable pseudo-random numbers. But it's not perfect. If your program runs twice within the same second, you'll probably get the same sequence, because time() (typically) has a resolution of 1 second. And typical `rand() implementations are not good enough for cryptographic use; it's too easy for an attacker to guess what numbers you're going to get.
There are a number of other random number implementations. Linux systems have two pseudo-devices, /dev/random and /dev/urandom, from which you can read reasonably high-quality pseudo-random byte values. Some systems might have functions like random(), drand48(), and so forth. And there are numerous algorithms; I've heard good things about the Mersenne Twister.
For something like a game, where you don't expect or care about players trying to cheat, srand(time(NULL)) and rand() is probably good enough. For more serious purposes, you should get advice from someone who knows more about this stuff than I do.
Section 13 of the comp.lang.c FAQ has some very good information about pseudo-random number generation.
Pseudorandom number generators take a starting number, or seed, and then generate the next number in the sequence from this. That's why they're called pseudorandom, because if they always use the same starting value, they will generate the same sequence of numbers like the C standard lib generator does. This can be fixed by giving the generator a starting value that will change the next time the program is run like the current time.
Anyway, the code you're looking for like others have said is:
srand(time(0)); //Seed the generator, give it a starting value