Asked  7 Months ago    Answers:  5   Viewed   131 times

I need a 'good' way to initialize the pseudo-random number generator in C++. I've found an article that states:

In order to generate random-like numbers, srand is usually initialized to some distinctive value, like those related with the execution time. For example, the value returned by the function time (declared in header ctime) is different each second, which is distinctive enough for most randoming needs.

Unixtime isn't distinctive enough for my application. What's a better way to initialize this? Bonus points if it's portable, but the code will primarily be running on Linux hosts.

I was thinking of doing some pid/unixtime math to get an int, or possibly reading data from /dev/urandom.

Thanks!

EDIT

Yes, I am actually starting my application multiple times a second and I've run into collisions.

 Answers

49

The best answer is to <random>. If you are using a pre C++11 version you can alternatively look at Boost random number stuff.

But if we are talking about rand() and srand()
The best simplist way is just to use time():

int main()
{
    srand(time(nullptr));

    ...
}

Be sure to do this at the beginning of your program, and not every time you call rand()!


Side Note:

NOTE: There is a discussion in the comments below about this being insecure (which is true, but ultimately not relevant (read on)). So an alternative is to seed from the random device /dev/random (or some other secure real(er) random number generator). BUT: Don't let this lull you into a false sense of security. This is rand() we are using. Even if you seed it with a brilliantly generated seed it is still predictable (if you have any value you can predict the full sequence of next values). This is only useful for generating "pseudo" random values.

If you want "secure" you should probably be using <random> (Though I would do some more reading on a security informed site). See the answer below as a starting point: https://stackoverflow.com/a/29190957/14065 for a better answer.

Secondary note: Using random device actually solves the issues with starting multiple copies per second better than my original suggestion below (just not the security issue).


Back to original story:

Every time you start up, time() will return a unique value (unless you start the application multiple times a second). In 32 bit systems, it will only repeat every 60 years or so.

I know you don't think time is unique enough but I find that hard to believe. But I have been known to be wrong.

If you are starting a lot of copies of your application simultaneously you could use a timer with a finer resolution. But then you run the risk of a shorter time period before the value repeats.

OK, so if you really think you are starting multiple applications a second.
Then use a finer grain on the timer.

 int main()
 {
     struct timeval time; 
     gettimeofday(&time,NULL);

     // microsecond has 1 000 000
     // Assuming you did not need quite that accuracy
     // Also do not assume the system clock has that accuracy.
     srand((time.tv_sec * 1000) + (time.tv_usec / 1000));

     // The trouble here is that the seed will repeat every
     // 24 days or so.

     // If you use 100 (rather than 1000) the seed repeats every 248 days.

     // Do not make the MISTAKE of using just the tv_usec
     // This will mean your seed repeats every second.
 }
Tuesday, June 1, 2021
 
Guesser
answered 7 Months ago
19

That depends on what you are trying to achieve.

Randomization is performed as a function that has a starting value, namely the seed.

So, for the same seed, you will always get the same sequence of values.

If you try to set the seed every time you need a random value, and the seed is the same number, you will always get the same "random" value.

Seed is usually taken from the current time, which are the seconds, as in time(NULL), so if you always set the seed before taking the random number, you will get the same number as long as you call the srand/rand combo multiple times in the same second.

To avoid this problem, srand is set only once per application, because it is doubtful that two of the application instances will be initialized in the same second, so each instance will then have a different sequence of random numbers.

However, there is a slight possibility that you will run your app (especially if it's a short one, or a command line tool or something like that) many times in a second, then you will have to resort to some other way of choosing a seed (unless the same sequence in different application instances is ok by you). But like I said, that depends on your application context of usage.

Also, you may want to try to increase the precision to microseconds (minimizing the chance of the same seed), requires (sys/time.h):

struct timeval t1;
gettimeofday(&t1, NULL);
srand(t1.tv_usec * t1.tv_sec);
Tuesday, June 1, 2021
 
Student
answered 7 Months ago
48

The answer to my question has been given by @cderv :
https://github.com/rstudio/rmarkdown/issues/1877#issuecomment-679864674

The results = 'asis' is now more documented in https://bookdown.org/yihui/rmarkdown-cookbook/results-asis.html#results-asis . It is aimed at generating raw mardown content from a R chunk. Anything must result in a text output, and implicitly that means no knitr magic really happens for any R object in those chunk, because knitr does no adjustment when knit_printing the content (as it is juts text)

I think I would not use result = 'asis' to cat() a complex R object like an htmlwidget. You found a workaround but you may encounter other issues.

As this answer has been liked by @yihui, it gives a hint that cat + asis on htmlwidget should be used at one's own risk.

However, I'll personnaly continue to use the workarounds mentioned in the question, because as long as it works I find it very practical.

Thanks @atusi & @cderv for their valuable input.

Wednesday, July 7, 2021
 
MGP
answered 5 Months ago
MGP
45

You need to seed the psuedo-random number generator first using srand.

#include <algorithm>
#include <cstdlib>

...

std::srand(std::time(0));

vector <Card> deck;
//some code to add cards to the deck here
random_shuffle ( deck.begin(), deck.end() );

Note from link above:

Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to rand(), and the start of the program. It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers.

Friday, August 6, 2021
 
Amalina Aziz
answered 4 Months ago
39

I found quite a simple way to do it:

A.lua:

local A = {}
local B

function A.foo()
    B = B or require 'B'
    return B.bar()
end

function A.baz()
    return 42
end

return A

B.lua:

local B = {}
local A

function B.bar()
    A = A or require 'A'
    return A.baz()
end

return B
Wednesday, September 22, 2021
 
zi3guw
answered 3 Months ago
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