If your familiar with probabilities and the birhday paradox (help me ): )

Hello,i tried to solve the birthday problem with probabilities
and i have a problem,there is an error and i cant find it....
If you are patient plz look my source code and give me your ideas.

#1.This program returns the maximum number of people in a group
when at least 2 of them have a 50% probability to selebrate their birthday the same day.

#3.probality=success trials/number of trials

#4.the array gets random numbers in the interval [1,365] from rand()
I start with a group of 2 people ,the number of the group isnt bigger from 100 people.
#4a. number of people [2,100]

#5.I already solve this problem in a mathimatical form and i know for sure that the probability that at least 2 of them have a 50% to selebrate their birthday the same day when the group is composed by 23
people.

#6.i already isolated the problem but i dont know why this bring me an error )~~:

#7.The error is in the first for loop.If you munually put the number of people i.e people=23 you find the correct probab.(approximately 0.5).With the first loop my program is a total disaster.

Code:

---

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

int main()
{
int people,array[100],number,counter0,counter1,counter2;
float probability,success_trials,limit=0.50,number_of_trials;
         
printf("Enter the number of trials\n");
scanf("%f",&number_of_trials);
  for(probability=0.0,people=2;probability<limit;people++)
      {
        for (counter0=0;counter0<number_of_trials;counter0++)
            {
        for (counter1=0;counter1<people;counter1++)
          {
            number=array[counter1]=rand()%365+1;
                  /*random numbers in the interval[1,365]*/
                                                            for (counter2=0;counter2<counter1;counter2++)
                {
                  if (number==array[counter2])
/*we compare the first array element with the others
 and if we find a duplicated number then there
 are at least two people with the same birthday*/
                {
                          success_trials++;                         
                          counter1=counter2=people;               
                        } 
                    }
                }
                                               
            }
       
  probability=success_trials/number_of_trials;
  } 
}

---

#7.The error is in the first for loop.If you munually put the number of people i.e people=23 you find the correct probab.(approximately 0.5).With the first loop my program is a total disaster.put i need it to show the probability for 2,3,4...23 people :)

Code:

---

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

int main()
{
int people=23,array[100],number,counter0,counter1,counter2;
float probability,success_trials,limit=0.50,number_of_trials;
         
printf("Enter the number of trials\n");
scanf("%f",&number_of_trials);
     
      for (counter0=0;counter0<number_of_trials;counter0++)
            {
        for (counter1=0;counter1<people;counter1++)
          {
            number=array[counter1]=rand()%365+1;
                  /*random numbers in the interval[1,365]*/
                                                            for (counter2=0;counter2<counter1;counter2++)
                {
                  if (number==array[counter2])
/*we compare the first array element with the others
 and if we find a duplicated number then there
 are at least two people with the same birthday*/
                {
                          success_trials++;                         
                          counter1=counter2=people;               
                        } 
                    }
                }
                                               
            }
       
  probability=success_trials/number_of_trials;
printff("%f\n",probability);
return 0;
}

---

Would you mind posting code that isn't 2 miles wide? Use spaces, not tabs. And give us better details on what is wrong, where it's wrong, and why it's wrong.

Use the PREVIEW button when posting

i already told you ...:) if i knew exactly i wouldnt sent a thread
you are very demanding sorry...

Quote:

---

Originally Posted by dionys

With the first loop my program is a total disaster.

---

Expand

Perhaps if you didn't try and write it all in one go (a whole bunch of loops inside main), then perhaps the problem / solution would be clearer.

In other words, use some functions.

Oh yeah, WaltP is correct, its up to you to make sure your post is presentable. If it looks a mess, people will ignore it.

Your problem was that you weren't resetting (or indeed setting)
the variable 'success_trials'. Here is the code re-formatted and
corrected. I also re-named 'number_of_trials' as 'n_trials' and
made it an int which just made more sense to me...

Code:

---

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

int main(void)
{
  int people, array[100], counter0, counter1, counter2, n_trials;
  float probability, success_trials = 0, limit = 0.50;
         
  printf("Enter the number of trials\n");
  scanf("%d", &n_trials);
  for(probability = 0.0, people = 2; probability<limit; people++) {
    success_trials = 0;
    for (counter0 = 0; counter0 < n_trials; counter0++) {
      for (counter1 = 0; counter1 < people; counter1++) {
        array[counter1] = rand()%365+1;
        for (counter2 = 0; counter2 < counter1; counter2++) {
          if (array[counter2] == array[counter1]) {
            success_trials++;                         
            counter1 = counter2 = people;               
          } 
        }
      }                                       
    }
    probability = success_trials / n_trials;
  }
       
  printf("probability = %f when people = %d\n", probability, people-1);

  return 0;
}

---

thanks a lot :)
thanks
thanks
:)