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#include <bits/stdc++.h>
using namespace std;
int main() {
srand(time(0));
vector<int> v;
for(int i = 0; i < 100; ++ i) {
int it = 0, tec = 0;
while(true) {
++ it;
if(tec == 11) {
v.push_back(it);
break;
}
if(rand() % 2) ++ tec;
else tec = 0;
}
}
for(int i = v.size() - 15; i < v.size(); ++ i)
cout << v[i] << endl;
}
OUTPUT16906 4109 125 9564 245 7841 10130 3685 265 272 9041 6916 4918 9028 13739
OUTPUT23685 265 272 9041 6916 4918 9028 13739 1796 133 7302 645 201 1207 192
OUTPUT316608 6906 4109 125 9564 245 7841 10130 3685 265 272 9041 6916 4918 9028
OUTPUT4245 7841 10130 3685 265 272 9041 6916 4918 9028 13739 1796 133 7302 645
At first glance it seems that the values are random. But let's try to sort the vector.
Code here:
CODE#include <bits/stdc++.h>
using namespace std;
int main() {
srand(time(0));
vector<int> v;
for(int i = 0; i < 100; ++ i) {
int it = 0, tec = 0;
while(true) {
++ it;
if(tec == 11) {
v.push_back(it);
break;
}
if(rand() % 2) ++ tec;
else tec = 0;
}
}
sort(v.begin(), v.end());
for(int i = v.size() - 15; i < v.size(); ++ i)
cout << v[i] << endl;
}
OUTPUT19041 9564 9564 9564 10130 10130 10130 10130 13739 13739 13739 13739 16608 16608 16608
OUTPUT29041 9041 9564 9564 9564 10130 10130 10130 13739 13739 13739 13739 16608 16608 16608
OUTPUT39564 9564 9564 9564 10130 10130 10130 10130 13739 13739 13739 16608 16608 16608 16608
OUTPUT49564 9564 9564 9564 10130 10130 10130 10130 13739 13739 13739 13739 16608 16608 16608
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#include <bits/stdc++.h>
using namespace std;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
int main() {
srand(time(0));
vector<int> v;
for(int i = 0; i < 100; ++ i) {
int it = 0, tec = 0;
while(true) {
++ it;
if(tec == 11) {
v.push_back(it);
break;
}
if(rnd() % 2) ++ tec;
else tec = 0;
}
}
sort(v.begin(), v.end());
for(int i = v.size() - 15; i < v.size(); ++ i)
cout << v[i] << endl;
}
CODE2, random delay between calls rand(), normal random values#include <bits/stdc++.h>
using namespace std;
int main() {
srand(time(0));
vector<int> v;
for(int i = 0; i < 100; ++ i) {
int it = 0, tec = 0;
while(true) {
//
int u = 1;
for(int j = 0; j < rand() % 10; ++ j) u *= 3;
//
++ it;
if(tec == 11) {
v.push_back(it);
break;
}
if(rand() % 2) ++ tec;
else tec = 0;
}
}
sort(v.begin(), v.end());
for(int i = v.size() - 15; i < v.size(); ++ i)
cout << v[i] << endl;
}
With a fixed delay, we also get magic values, even if the delay is large.
CODE3, fixed delay between calls rand(), magic values#include <bits/stdc++.h>
using namespace std;
int main() {
srand(time(0));
vector<int> v;
for(int i = 0; i < 100; ++ i) {
int it = 0, tec = 0;
while(true) {
//
int u = 1;
for(int j = 0; j < 10; ++ j) u *= 3;
//
++ it;
if(tec == 11) {
v.push_back(it);
break;
}
if(rand() % 2) ++ tec;
else tec = 0;
}
}
sort(v.begin(), v.end());
for(int i = v.size() - 15; i < v.size(); ++ i)
cout << v[i] << endl;
}
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!