How to use function srand() with time.h?

Question

My program contains code that should generate a random positive integer number every time I execute it. It generates random numbers but only once. After that, when I execute same code, it gives me same values, and it is making my code useless.

I started with the rand function, and then I used the srand() function with the time.h header file, but still it is not working properly.

#define size 10
for(i=0;i<size;i++)
    Arr[i] = rand()%size;

First call (random):

6 0 2 0 6 7 5 5 8 6

Second call (random but same as previous):

6 0 2 0 6 7 5 5 8 6

Later I visited Stack Overflow questions and I read about the srand() function, and I used it as:

#include<time.h>
for(i=0;i<size;i++)
    Arr[i] = srand(time(NULL));

First call:

-10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327 -10327

Second call:

-10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326 -10326

It is giving me different (but not random values). I've defined Arr[i] as unsigned int, and still I am getting negative values.

did you set your seed? Set your seed with the time function. — KrisSodroski, May 15 '13 at 15:33
You only need to call `srand` once to seed the `rand` generator. Don't call `srand` each time to get your numbers — TJD, May 15 '13 at 15:36
You probably ought to read the [manpage](http://linux.die.net/man/3/rand), which explains how these functions work, and provides a working example. — Aya, May 15 '13 at 15:38
You set your seed with the current time: srand(time(null)). Then you call rand. That way you get a unique seed every single time. The seed changes the random algorithm. Do you understand now? — KrisSodroski, May 15 '13 at 15:44
is there any specific reason for using srand(), if not may be you can follow [**How to use /dev/random or urandom in C? stackoverflow** ](http://stackoverflow.com/questions/2572366/how-to-use-dev-random-or-urandom-in-c) — asio_guy, May 15 '13 at 16:08

score 46 · Accepted Answer · answered May 15 '13 at 15:35

46

You need to call srand() once, to randomize the seed, and then call rand() in your loop:

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

#define size 10

srand(time(NULL)); // randomize seed

for(i=0;i<size;i++)
    Arr[i] = rand()%size;

answered May 15 '13 at 15:35

Paul R

208,748
37
389
560

srand() takes an unsigned int as an input, so time(NULL) should be recast. – Jiminion Apr 26 '17 at 13:20

score 4 · Answer 2 · edited May 23 '17 at 12:02

4

Try to call randomize() before rand() to initialize random generator.

(look at: srand() — why call it only once?)

edited May 23 '17 at 12:02

Community

1
1

answered May 15 '13 at 15:35

Ze..

163
1
10

Please write me the code, it would be really helpful if you put it – May 15 '13 at 15:39
1

`randomize` is not a standard function - perhaps you mean `srand` ? – Paul R May 15 '13 at 15:39
Paul R, yes, i mean srand() from the "stdlib.h". Thank you for adding comment. My mistake. – Ze.. May 15 '13 at 15:45
http://www.cplusplus.com/reference/cstdlib/srand/ has an example where srand is called more than once. – Ze.. May 15 '13 at 15:53
2

That example is to illustrate that not calling `srand()` has the same effect as calling `srand(1)`. It should be made clearer, though. – Daniel Fischer May 15 '13 at 16:05
@Ðeepak, may this information will help you: http://stackoverflow.com/questions/7343833/srand-why-call-only-once – Ze.. May 15 '13 at 16:09
@Daniel Fischer, thank you for the hint. – Ze.. May 15 '13 at 16:10

score 3 · Answer 3 · edited May 23 '17 at 11:54

3

If you chose to srand, it is a good idea to then call rand() at least once before you use it, because it is a kind of horrible primitive psuedo-random generator. See Stack Overflow question Why does rand() % 7 always return 0?.

srand(time(NULL));
rand();
//Now use rand()

If available, either random or arc4rand would be better.

edited May 23 '17 at 11:54

Community

1
1

answered May 15 '13 at 16:05

Grady Player

14,399
2
48
76

score -2 · Answer 4 · edited Sep 19 '14 at 08:07

-2

#include"stdio.h"
#include"conio.h"
#include"time.h"

void main()
{
  time_t t;
  int i;
  srand(time(&t));

  for(i=1;i<=10;i++)
    printf("%c\t",rand()%10);
  getch();
}

edited Sep 19 '14 at 08:07

mikdiet

9,859
8
59
68

answered Sep 19 '14 at 07:58

R.M.VIVEK Arni

1
3

score -2 · Answer 5 · edited Mar 06 '16 at 05:42

-2

#include"stdio.h"//rmv coding for randam number access

#include"conio.h"

#include"time.h"

void main()
{
    time_t t;
    int rmvivek;

    srand(time(&t));
    rmvivek=1;

    while(rmvivek<=5)
    {
        printf("%c\t",rand()%10);
        rmvivek++;
    }
    getch();
}

edited Mar 06 '16 at 05:42

aghast

14,785
3
24
56

answered Sep 19 '14 at 08:01

R.M.VIVEK Arni

1
3

this method encryption and decryption progamming used subpart – R.M.VIVEK Arni Sep 19 '14 at 08:12

How to use function srand() with time.h?

5 Answers5

Linked

Related