problem with my srand programm

Question.Write a program that simulates coin tossing.For each toss of the coin the program should print heads or tails.Let the program toss the coin 100 times, and count the number of time each side of the coin appears.Print the result.The program should call a seperate function cointoss that takes no argument and returns 0 for tails and return 1 for head.



Well my code it aint workin' it keeps on printing heads or talles hundredc time.that when it is executed each time.

Code:

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



int cointoss ( void ); //fucntion prototype

int main()
{
 int sum, cnt;
      srand (time(0));
      sum = cointoss ();


 for ( cnt =1; cnt <= 100; cnt ++){
    if ( sum == 0 )
    printf ("Heads\t");
    else
        if ( sum == 1 )
        printf ("Tails\t");      }


      getch();
      return 0;
}

int cointoss ( void )

{
    int toss;
    toss = 1+rand() % 2;

    if ( toss == 1 )
    return (0);
    else
        if ( toss == 2 )
        return (1);

}

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Quote:

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Originally posted by datainjector
Well my code it aint workin' it keeps on printing heads or talles hundredc time.that when it is executed each time.

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This is your main loop. Tell me, how many times are you calling the cointoss() function within this loop?

Code:

---

for (cnt = 1; cnt <= 100; cnt++)
{
    if (sum == 0)
        printf("Heads\t");
    else if (sum == 1)
        printf("Tails\t");
}

---

Just in case you don't get the answer, I'll tell you. It's none, you're not calling cointoss(), you're printing the value from the first call of cointoss() 100 times over.

1) You're not calling your cointoss function anywhere in the loop, so the same result is printed 100 times.

2) You don't keep track of the totals for both heads and tails.

3) You're overcomplicating the cointoss function and using a less than optimal call to rand. Remember that using rand() % N causes not very random sequences.

Here's another way to do it:

Code:

---

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

int cointoss ( void );

int main(void)
{
  int heads = 0,
      tails = 0,
      cnt;
  srand ((unsigned)time(NULL));

  for ( cnt = 0; cnt < 100; cnt++){
    if ( cointoss() != 0 ) {
      printf ("Heads\t");
      heads++;
    }
    else {
      printf ("Tails\t");
      tails++;
    }
  }
  printf ( "Heads: %d\nTails: %d\n", heads, tails );
  getchar();
  return 0;
}

int cointoss ( void )
{
  return rand() < RAND_MAX / 2;
}

---

-Prelude

Code:

---

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



int cointoss ( void ); //fucntion prototype

int main()
{
 int sum,cnt, head=0, tails =0;
  srand (time(0));


  for ( cnt =1; cnt <= 100; cnt ++ ){
    if ( cointoss() == 0 ) {
        printf ("Heads\t");
        head++;              }
    else
        {
                printf ("Tails\t");
                tails++;              } }

                puts (" ");
                printf ("Heads is %d, tails is %d", head, tails );


      getch();
      return 0;
}

int cointoss ( void )

{
    int toss;
      toss = 1+rand() % 2;

        if ( toss == 1 )
    return (0);
    else
        if ( toss == 2 )
        return (1);

}

---

Now i know what was wrong with the program.Thanks to u guys..The first problem was i said sum = cointoss(); and used it in the loop..which was wrong coz it will be counting the same thing 100 times..and ya i said

if ( cointoss()= 0 )
printf ("Heads");
else
IF ( cointoss() == 1 )
printf ("Tails");

well it should print the stuff 100 times right..but the if else stament was wrong because it would count say tails 20 and heads 50 but thats
70 ..so i removed the else if statment and replacet it with else which finally.worked

Well u guys dint c that mistake;)

>Well u guys dint c that mistake
My code corrected it, I just didn't feel it needed mentioning. And please read my post again on using rand() % N. I go over this every time someone mentions random numbers, but it's like nobody listens.

-Prelude

Quote:

---

Originally posted by datainjector
and ya i said

if ( cointoss()= 0 )
printf ("Heads");
else
IF ( cointoss() == 1 )
printf ("Tails");

well it should print the stuff 100 times right..but the if else stament was wrong because it would count say tails 20 and heads 50 but thats
70 ..so i removed the else if statment and replacet it with else which finally.worked

Well u guys dint c that mistake;) [/B]

---

We "didn't see that mistake" because it is not in your original code!

The real reason that code would be wrong is because you're actually calling the function twice, getting two potentially different results from the call.

This is why that'd be wrong. Again, the reason "we didn't see it", is because that code snipped is not in your orignial code.

And you are wrong. They were both right. Your code example didn't call the coin toss function inside the loop.

Quzah.

>>Remember that using rand() % N causes not very
>>random sequences

Excuse me but how do you measure randomness ????

Especialy in this case ... where N is 2 ???

>> but it's like nobody listens.
Awww I listen to ya :)

Quote:

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Originally posted by borko_b
>>Remember that using rand() % N causes not very
>>random sequences

Excuse me but how do you measure randomness ????

Especialy in this case ... where N is 2 ???

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One way would be to see the distribution of the sequence
Assuming , rand() is implemented really well (highly unlikely) well and generates uniformly on the set of integers {0,1,2,..........,RAND_MAX-1}
then
rand()%N, will generate a random variable with the following distributuion ,
it will take values 0,1,2,...,N-1 with the probability
([(RAND_MAX-1)/N]+1)/RAND_MAX,([(RAND_MAX-2)/N]+1)/RAND_MAX,............., [(RAND_MAX-N)/N]+1)/RAND_MAX

If N is very small compared to RAND_MAX all are close to 1/N
o.w there can be a problem
if your N is say (RAND_MAX-1)/2 then
the probability of
rand()%N being 0 is 3/RAND_MAX rather than 1/N (=2/(RAND_MAX-1) ) which means it will appear approx 1.5 times more than it should .

There might be other problems depending on how rand() is actually implemented .

Quote:

---

One way would be to see the distribution of the sequence
Assuming , rand() is implemented really well (highly unlikely) well and generates uniformly on the set of integers {0,1,2,..........,RAND_MAX-1}
then
rand()%N, will generate a random variable with the following distributuion ,
it will take values 0,1,2,...,N-1 with the probability
([(RAND_MAX-1)/N]+1)/RAND_MAX,([(RAND_MAX-2)/N]+1)/RAND_MAX,............., [(RAND_MAX-N)/N]+1)/RAND_MAX

If N is very small compared to RAND_MAX all are close to 1/N
o.w there can be a problem
if your N is say (RAND_MAX-1)/2 then
the probability of
rand()%N being 0 is 3/RAND_MAX rather than 1/N (=2/(RAND_MAX-1) ) which means it will appear approx 1.5 times more than it should .

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A well thought out and detailed explanation. I usually don't bother and just say Check Eskimo. :)

-Prelude

>>...One way would be to see the distribution of the sequence ...

Well...

Nice explanation !