Random Number Generator mechanism

Question

i had the following problem in my book:

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
void main()
{
    clrscr();
    randomize();
    int Game[]={10,16},P;
    int Turn=random(2)+5;
    for(int T=0;T<20;T++)
       {
           P=random(2);
           cout<<Game[P]+Turn<<"#";
       }
getch();
}

The output comes like 16#22#16#16#16#22#....20 times... Why the output of this always comes either 16# or 22#?? why not 15# or 21#?? i would like to the mechanism of this program. Thanks. turn=random(2)+5; if random(2) gives 0 then turn becomes turn=0+5=5 which implies that i should get 10+5=15 and 16+5=21 along with 16 and 22 but i m not getting them.

We got the above question in our computer science theory exam and we were to chose the correct answer(i.e it generates 16 and 22) but how will i am going to know that it will generate only 16 and 22. As i explained above 15 and 21 are also possible..

This code won’t compile on a modern C++ compiler (and was never valid C++ code!) which leads me to believe that you seriously need to upgrade your working environment. — Konrad Rudolph, Mar 21 '13 at 14:08

score 1 · Answer 1 · edited May 23 '17 at 11:57

1

maybe this helps:

The seed for the random number generator is not set.

If you call srand(time(NULL)) then you will get more random results

C++ rand() gives same number when running process

edited May 23 '17 at 11:57

Community

1
1

answered Mar 21 '13 at 14:05

Stefan Birladeanu

294
2
10

score 1 · Answer 2 · answered Mar 21 '13 at 14:06

1

You need to give a seed value that would help get "really" random. mumbers

answered Mar 21 '13 at 14:06

Ron

886
13
39

score 1 · Answer 3 · edited Dec 13 '13 at 11:06

A computer cannot randomize numbers by itself, it uses a seed for that.

But seed's aren't completely random they just have a specific order, like:

These numbers look pretty random but when you run the program the next time you will get:

The exact same.

To prevent this we use the time as a seed, time always changes so it will always generate new numbers.

#include <time.h>
srand(time(NULL));  // srand is the function to randomize numbers from a seed, we use 'time' as seed here

this video explains it.

score 0 · Accepted Answer · answered Mar 21 '13 at 14:11

0

Because Turn is only randomized once - at the beginning of the loop. If you move the assignment of Turn into your loop, you should get 15 and 21 also:

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
void main()
{
    clrscr();
    randomize();
    int Game[]={10,16},P;
    int Turn;
    for(int T=0;T<20;T++)
       {
           P=random(2);
           Turn=random(2)+5;
           cout<<Game[P]+Turn<<"#";
       }
    getch();
}

Also, as said by others, if you want the output to differ between runs, you will need to seed your random number generator, for instance by calling srand() with a seed. For instance:

#include <time.h>
(...)
srand(time(NULL));

answered Mar 21 '13 at 14:11

sonicwave

5,952
2
33
49

int Turn=random(2)+5; if random(2) gives out zero then value of turn becomes(turn=0+5) 5..then i should get 15 and 21 also..but i m not getting it! – Smatik Mar 21 '13 at 14:15
Yes, but as your random generator is not seeded, you'll always get the same value from run to run. Seed it with a different value (for instance by using `time(NULL)`), and you should see a difference. However, without moving the assignment of `Turn`, you'll always get runs that contain either 16 and 22 or 15 and 21. – sonicwave Mar 21 '13 at 14:22
That's what I said. Seed your generator. – sonicwave Mar 21 '13 at 14:25
you said "you'll always get runs that contain either 16 and 22 or 15 and 21" i m only getting 16 and 22(i ran the program again and again and i m still running it ) combination of 15 or 21 never comes can you tell me why. i dont thing seeding is a problem – Smatik Mar 21 '13 at 14:28

Random Number Generator mechanism

4 Answers4