Just what!? Standart rand() in C++ or undeniable proof of the existence of magic!

Revision en3, by seo, 2019-02-15 14:29:11

Until recently, I knew that rand() in C++ works awfully with random_shuffle and it was not credible, but continued to use it for most tasks.. But recently I wrote some code and I cannot explain the result. Maybe you can?

I advise you to read to the end, it is really very interesting!

At each step, this code generates a random value from 0 to 1. As soon as 11 (for example) identical values equal 1 were received in a row, index number of current value is added to the vector. This operation is repeated a number of times and each time it starts from the beginning.

Let's output vector values and look at the last 15 of them for convenience.

Code here:

CODE
OUTPUT1
OUTPUT2
OUTPUT3
OUTPUT4

At first glance it seems that the values are random. But let's try to sort the vector.

Code here:

CODE
OUTPUT1
OUTPUT2
OUTPUT3
OUTPUT4

It should be noted that the output is different each time, but the individual values are the same. Moreover, these individual values are repeated three to four times.

• Using of mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count()); instead of rand() gives a random output array.

• If we add a line that will not affect the answer in any way, but will affect the time difference between the rand() calls we also get random values in the output array. In doing so, we have to use random delay. If the delay is fixed, we will not get random values.

CODE1, rnd(), normal random values
CODE2, random delay between calls rand(), normal random values

With a fixed delay, we also get magic values, even if the delay is large.

CODE3, fixed delay between calls rand(), magic values

Now let's instead of an empty loop in the third code call rand().

CODE
OUTPUT1
OUTPUT2
OUTPUT3

Values become repeated much more often..

CODE
OUTPUT

If we call the method twice, the values are repeated more often.

I believe that this is not quite a trivial algorithm and the result is rather strange. If you have any idea how to explain this, please state your thoughts in the comments. Thanks for reading!

#### History

Revisions

Rev. Lang. By When Δ Comment
en3 seo 2019-02-15 14:29:11 2257
en2 seo 2019-02-15 01:00:46 2 (published)
en1 seo 2019-02-15 01:00:06 6059 Initial revision (saved to drafts)