Problems in generating random numbers

Question

I'm using the following code for generate the random numbers in C programming, but for every compilation a particular set of random numbers are generating repeatedly. Can anybody mention the correction here???

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
main()
{   
  int i,a[40];

  for(i=0;i<40;i++)
  {    
    a[i] = rand() % 6;
    printf("%d\n", a[i]);
  } 
  getch();
}

You aren't initializing the random seed correctly. There are multiple correct answers below which address this. — Cloud, Apr 28 '14 at 18:00
This question is asked repeatedly; please show some effort and do a search before asking the question. You will get your answer faster that way and not waste anyone's time. It's a win for everyone. — Eric Lippert, Apr 28 '14 at 18:06

score 4 · Answer 1 · answered Apr 28 '14 at 17:44

4

You need to initialize random seed to a different value, please see http://www.cplusplus.com/reference/cstdlib/rand/

answered Apr 28 '14 at 17:44

Bogdan

484
2
7

Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackoverflow.com/q/8259) to include the essential parts of the answer here, and provide the link for reference. – Andrew Barber Apr 28 '14 at 18:31
@AndrewBarber Yeah, but I am kinda lazy, please downvote so that the other answer gets on top :) – Bogdan Apr 28 '14 at 18:35

Nicholas Carey · Answer 2 · 2014-04-28T18:17:49.980

Per the documentation,

If rand() is called before any calls to srand() are made, the same sequence shall be generated as when srand() is first called with a seed value of 1. [emph. mine]

You need to mix it up, something like this:

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#include <time.h> 

main()
{   
  int i,a[40];

  // seed the PRNG with some random value
  // the current time is often used. You might want to incorporate
  // other sources of entropy as well.
  srand(time(NULL)); 

  for( i = 0 ; i < 40 ; i++ )
  {    
    a[i] = rand() % 6;
    printf("%d\n", a[i]);
  } 
  getch();
}

One should note that

seed the PRNG just once at beginning of execution
The reason for incorporating more sources of entity, rather than just relying on the current system clock is that the system clock itself is not very random -- to invocations quite close to each other my result in similar sequences. Other good sources of entropy include:
- hard disk statistics
- network I/O
- process activity
- keystroke latency
- mouse movements
- etc.

Although looking at user interface components such as keystrokes and mouse movements won't buy you much in the way of entropy if your software is running, say, on a server, where the keyboard and mouse are likely to be idle for long periods of time.

Your's should be the accepted answer :) – Bogdan Apr 28 '14 at 18:02 — Bogdan, Apr 28 '14 at 18:02

score 1 · Answer 3 · answered Apr 28 '14 at 17:48

Do something like this (inserted line) to seed the random number generator:

  //...
  int i,a[40];
  srand(clock());  //inserted line
  for(i=0;i<40;i++)
  //...

The clock() function will provide a different seed value each clock tick, resulting in a new and different initialization to the rand() function.

Problems in generating random numbers

3 Answers3