Help writing a code to find the median

[jeffsak]

I am trying to write a program that finds the mean and median for a set of numbers, based on how many movies a number a students see a month. I have no included the mean function, as it is working great. I notice with the median function though, it is giving the median based on the order of input the user gives and isn't sorting from smallest to biggest. I am not sure how to do that, can someone please help me?

  #include <iostream>
#include <iomanip>
using namespace std;

// Function prototypes
double calculateMean(int *, int);
double calculateMedian(int *, int);


int main()
{

	int *nums;			// To dynamically allocate an array
	int num_students;	// To hold the number of students
	char repeat = ' ';	// Does the user want to go again?

	do
	{

		// Prompt the user to enter in how many students were surveyed.
		cout << "Enter in how many students were surveyed: ";
		cin >> num_students;

		// Determine input validation.
		while (num_students < 0)
		{
			cout << "Invalid number of students!\n";
			cout << "Enter in how many students were surveyed: ";
			cin >> num_students;
		}

		// Dynamically allocate an array large enough to hold
		// that many numbers of students.
		nums = new int[num_students];

		// Get the number of movies for each student;
		for (int count = 0; count < num_students; count++)
		{
			cout << "Number of movies say by Person #" << (count + 1) << ": ";
			cin >> nums[count];

			// Determine input validation.
			while (nums[count] < 0)
			{
				cout << "Invalid number. Please enter in a positive number: ";
				cout << "\nNumber of movies say by Person #" << (count + 1) << ": ";
				cin >> nums[count];
			}

		}

		// Set output formatting.
		cout << fixed << showpoint << setprecision(1);

		// Display the mean.
		cout << "\nThe mean is: ";
		cout << calculateMean(nums, num_students) << endl;

		// Display the median.
		cout << "\nThe median is: ";
		cout << calculateMedian(nums, num_students) << endl;

		// Free dynamically allocated memory.
		delete[] nums;
		nums = 0;		// Make nums point to null.

		// Ask the user if he/she wants to go again.
		cout << "Do you want to go again? (Enter Y if yes and N if no) ";
		cin >> repeat;


	} while (repeat == 'Y' || repeat == 'y');
	cout << "Program ending.\n";

	return 0;
}


// Definition of calculateMedian. The nums parameter is a pointer.	   
// The num_students parameter holds the number of students. The        
// function calculates and returns the median back to the main function

double calculateMedian(int *nums, int num_students)
{
	
	double median = 0.0;
	cout << fixed << showpoint << setprecision(1);

	// int answer = num_students % 2;

	if (num_students % 2 == 0)
	{
		median = (nums[num_students / 2] + nums[(num_students / 2) + 1]) / 2.0;
		// median = even_median;
	}
	else
		median = nums[num_students / 2]; 

	return median; 
}

[integralfx]

I notice with the median function though, it is giving the median based on the order of input the user gives and isn't sorting from smallest to biggest

http://www.cplusplus.com/reference/algorithm/sort/

http://www.cplusplus.com/reference/iterator/begin/
http://www.cplusplus.com/reference/iterator/end/

[pnoid]

You have to sort the array before you calculate median.
http://www.cplusplus.com/articles/NhA0RXSz/

[jeffsak]

Okay, so I have sorted the number and now my median works great for when there is an odd number. It's a lot easier to find i guess since it's the actual number from the array. When there is an even number it is looking through I get 1 or .5 more than it should be. When I had it looking for the median of 1,2,3,4 it gave me 3 instead of 2.5. How can I fix this problem?

double calculateMedian(int *nums, int num_students)
{
	int middle;
	double median;
	middle = num_students / 2.0;

	if (num_students % 2 == 0 )
	{
		median = nums[middle];
	}
	else
	{
		
		median = (nums[middle] + nums[middle]) / 2.0;
	}
	return median;  
}

[integralfx]

Let's say, you have four numbers: 1, 2, 3, 4.

middle = num_students / 2.0; // pointless 2.0, as middle is an int 

This will make middle = 2, due to integer truncation.

median = nums[middle];
This will access the element at index 2, which is 3.

median = (nums[middle] + nums[middle]) / 2.0;

Redundant?

median = (nums[middle] + nums[middle]) / 2.0;

// which is
median = 2 * nums[middle] / 2.0;

// which is
median = nums[middle];

which is the same as your even elements case.

1,2,3,4 it gave me 3 instead of 2.5

Why did you expect the answer to be 2.5 instead of 3?
How did you go about working this out?

[Moschops]

In some schools, given an even number of values, the median is defined to be the mean of the two central values. Which in many ways is a bit rubbish, as one of the key features of a median that makes it actually useful is that it's definitely represented in the set. This is statisticians missing the point, in my opinion, but it is what it is.