Srand and Rand not working correctly?

**lebronlin** · 04-05-2011

Hi!

For some reason my srand and rand functions in my program are returning different numbers, but only thousandths to ten thousandths apart, so not very random. A few days before however, the program was running fine. I even reinstalled my compiler, CodeBlocks.
Here is an example of a program that has this issue.

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

Code:

int main(void)
{
srand(time(NULL));

double z;

  z = (2.0 * rand()/RAND_MAX - 1.0) *;

  printf("%f", z);

  return 0;


  }

this returns me values such as -0.79283 and then -0.79285

Please help!

**quzah** · 04-05-2011

So what's the problem?

Quzah.

**lebronlin** · 04-05-2011

rand() is basically giving me values that are all extremely close to eachother

Ah nevermind i think ive fixed the problem, sorry

nevermind again..... for some reason srand and rand are just not working for me!!! rand() is returning values that are only a thousandth or so in difference each time

**quzah** · 04-05-2011

Code:

  z = (2.0 * rand()/RAND_MAX - 1.0) *;

Why is that there?

Anyway, there's nothing that says if I flip a coin that it has to alternate between values. I can flip heads 100 times in a row if I get lucky enough.

Quzah.

**lebronlin** · 04-05-2011

sorry that wasnt supposed to be there

but shouldn't the usage of srand(time(NULL)) cause the rand() function to deviate a bit more each time the program is used?

**~~CommonTater~~** · 04-05-2011

Originally Posted by lebronlin

Hi!

For some reason my srand and rand functions in my program are returning different numbers, but only thousandths to ten thousandths apart, so not very random. A few days before however, the program was running fine. I even reinstalled my compiler, CodeBlocks.
Here is an example of a program that has this issue.

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

Code:

int main(void)
{
srand(time(NULL));

double z;

  z = (2.0 * rand()/RAND_MAX - 1.0) *;

  printf("%f", z);

  return 0;


  }

this returns me values such as -0.79283 and then -0.79285

Please help!

what is rand()/RAND_MAX ... think about this, RAND_MAX is always higher than the return from rand()... that's what it describes the maximum return. Thus the returned value is always going to be less than 1 and greater than 0...

If you're looking for a little more range try

Code:

z = (double) rand() % MyMax;

Where MyMax = the highest number you want.

**lebronlin** · 04-05-2011

Thanks for the recommendation, but that's not the issue that I'm trying to solve (sorry if I sound a bit mean)

Here's a simpler piece of code

Code:

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

int main()
{
    double x;
    srand(time(NULL));
    x = rand();
    printf("%f", x);
    return 0;
}

this piece of code again returns me values such as 14267.00000, 14270.00000, 14273.00000 and just went on as I ran the program more.

**brewbuck** · 04-05-2011

Originally Posted by lebronlin

Thanks for the recommendation, but that's not the issue that I'm trying to solve (sorry if I sound a bit mean)

Here's a simpler piece of code

Code:

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

int main()
{
    double x;
    srand(time(NULL));
    x = rand();
    printf("%f", x);
    return 0;
}

This seems to be due to a really stupid algorithm used for srand(). Probably it just sets the initial random state to the value you pass to srand() -- since that value increases slowly, the initial random value also increases slowly along with it.

Try looping multiple times and print out the values you get:

Code:

int main()
{
    double x;
    int i;
    srand(time(NULL));

    for (i = 0; i < 50; ++i)
    {
        x = rand();
        printf("%f", x);
    }
    return 0;
}

I think you'll see that it's just the first value which is bogus. Either way, it seems the rand() implementation that came with your compiler is pretty crappy.

**quzah** · 04-05-2011

So instead of running the same thing over and over, call rand in a loop.

Code:

for( x = 0; x < 1000; x++ )
    printf( "%08d  %c", rand(), x % 9 == 0 ? '\n' : ' ' );

edit - brew beat me to it

Quzah.

**lebronlin** · 04-05-2011

Yeah, I was just wondering if there was a reason why the first value was always bogus

, I guess ill be getting rid of CodeBlocks

**brewbuck** · 04-05-2011

Originally Posted by lebronlin

Yeah, I was just wondering if there was a reason why the first value was always bogus

, I guess ill be getting rid of CodeBlocks

You might be able to work around it by calling rand() once to "prime the pump" and skip past that bogus first value. You could also look at a different random number generator like Mersenne Twister...

**nonoob** · 04-05-2011

Apart from the fact that you shouldn't print an int using %f.
[cue sound of Doh!!]

**quzah** · 04-05-2011

Originally Posted by nonoob

Apart from the fact that you shouldn't print an int using %f.

x is a double, assuming you are talking to brewbuck.

Originally Posted by nonoob

[cue sound of Doh!!]

Quzah.

**iMalc** · 04-06-2011

Originally Posted by lebronlin

Yeah, I was just wondering if there was a reason why the first value was always bogus

, I guess ill be getting rid of CodeBlocks

Why would you trash a perfectly good IDE when the issue you having is related to the libraries for the compiler?
What compiler btw?

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