CODEWITHSUNDEEP
c++random
Smatik
Smatik

Reputation: 407

Random Number Generator mechanism

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..

Upvotes: 0

Views: 530

Answers (4)

Jona
Jona

Reputation: 1825

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:

1
2
8
5
4

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

1
2
8
5
4

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.

Upvotes: 1

sonicwave
sonicwave

Reputation: 6092

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));

Upvotes: 0

Ron
Ron

Reputation: 918

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

Upvotes: 1

Stefan Birladeanu
Stefan Birladeanu

Reputation: 294

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

Upvotes: 1

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