Birthday Paradox loop question

[jly]

Hello everyone, I'm trying to understand this loop that simulates the number of trials in the birthday paradox and compares the birthdays.

What I don't understand is why the first loop - Number of people in the room - exists. For example,

If I pass
nPpl = 2 //2 people in the room
nTrials = 1 // just 1 trial

this function will perform the second loop - trials loop - nPpl times
which in this case is 2 times. However, I only want it to perform the trails
once with 2 people.

  int compare(int bdays[], int nPpl, int &count, int nTrials) {

	// Number of People in the room
	for (int ppl = 0; people < nPpl; ppl++)
	{
		count = 0;

		// do trials_ number of trials
		for (int trial = 0; trial < nTrials; trial++)
		{       
                        // function that sets the birthdays
			setBdays(bdays, nPp_);

			//compare birthdays that are set in i and j
			for (int i = 0; i < nPpl; i++) {
				for (int j = i + 1; j < nPpl_; j++) {
					if (bdays[i] == bdays[j]) {
						count++;
					}
				}
			}

		}
	}
	return count;
}

If someone can explain to me if my thought process is correct or incorrect that would be great.
Also, If i switch the trials loop and the number of people loop my output is completely wrong.

Thanks in advance!

[jonnin]

correct me if I missed something...

for (int trial = 0; trial < nTrials; trial++) //Do this one time
{
..
for (int i = 0; i < nPpl; i++) //do this 2 times...

so these 2 loops cause you to do the deep stuff twice. Is this what you are unhappy about? Does this imply that some of your loops were not supposed to be nested?

[jly]

Hello, thanks for the reply.

I'm not understanding this first loop

// Number of People in the room - if nPpl =  2 this means that the next loop will be executed twice
for (int ppl = 0; people < nPpl; ppl++)

          // do trials number of trials - here if nTrials = 1, this loop will still be executed twice since 
          // the first loop has nPpl = 2
          for (int trial = 0; trial < nTrials; trial++)

So, I guess I'm confused because if i switch nPpl and nTrials so that the whole function will loop nTrial times, the result is completely wrong. However, using nPpl, the result is closer to the right result.

// Number of Trials
	for (int trials = 0; people < nTrials; trials++)
	{
		count = 0;

		// Number of people
		for (int people = 0; people < nPpl; peoplel++)

                       // function that sets the birthdays
			setBdays(bdays, nPp_);

			//compare birthdays that are set in i and j
			for (int i = 0; i < nPpl; i++) {
				for (int j = i + 1; j < nPpl_; j++) {
					if (bdays[i] == bdays[j]) {
						count++;
					}
				}
			}

		}
	}
	return count;
}

This gives a wrong result but I guess my thinking is.
Should I not loop it nTrialls times and put the number of people loop nested within so that it will execute the amount of trials I want?

[jonnin]

I honestly have not studied the problem and its solutions. I simply showed you why you got the result you complained about in hopes it would help.