Problem With Calculations

[Stripeb49]

I'm trying to write a program that calculates the mean and standard deviation for a set of numbers. I broke the calculation down into a couple parts, and for some reason when I try to sum the differences squared I keep getting the wrong answer. Not sure why though. For example if I enter 1,2,3,4,5 the sum of the differences squared should be 10 but I get 80.

// INCLUDES
#include<iostream>
#include<iomanip>
#include<cmath>
	using namespace std;

// PROTOTYPES
double meanStandDev(double, int); // sum & userArraySize
double standardDeviation(double sumTwo, int userArraySize); // difference squared, userArray, radicand

// MAIN
int main(void) {

	// declare variables
	double mean = 0.0; 
	double standDev = 0.0;
	int i;
	double sum = 0.0;
	double sumTwo = 0.0;
	double radicand = 0.0;
	double differenceSquared = 0.0;

	// ARRAY, create variable for the array
	int arraySize = 50; 
	
	// declare an array and fill it with 0.0
	double zeroArray[50] = {0.0}; 

	// PROMPT user to fill array
	int userArraySize = 0;
	double userArray[50]; // maximum size of 50

	do {
		cout << "Enter a value for the array (negative value to quit): ";
		cin >> userArray[userArraySize];

		// increment our counter
		userArraySize++;
	} while (userArray[userArraySize -1] >= 0.0);

	// fix to make to eliminate the negative value
	userArraySize--;

	// print new array
	for (int i=0; i < userArraySize; i++) {

		cout << userArray[i] << endl;
	}

	// sum of all elements in the array
	for (i=0; i<userArraySize; i++)
	{
		sum = sum + userArray[i];
	}

	// sum of differences squared
	for (i=0; i<sum; i++) {

		differenceSquared = pow(userArray[i] - mean, 2);
		sumTwo = sumTwo + differenceSquared;
	}
	

	// call function to calculate mean
	mean = meanStandDev(sum, userArraySize);
	// call function to calculate standard deviation
	standDev = standardDeviation(sumTwo, userArraySize);

	// output
	cout << "The mean is " << mean << "." << endl;
	cout << "Array total is " << sum << endl;
	cout << "Array size is " << userArraySize << endl;

	cout << "The sum of the differences squared is " << sumTwo << endl;
	cout << "The radicand is " << radicand << endl;
	cout << "The standard deviation is " << standDev << endl;

	system("pause");
	return 0;
} // end of main

// function definitions for the mean
double meanStandDev(double sum, int userArraySize) {

	// declare a return value for the mean
	double mean = 0.0;
	//calculate the mean
	mean = (sum / userArraySize);
	// delcare a return value for the mean
	return mean;
}

// function definitions for standard deviation
double standardDeviation(double sumTwo, int userArraySize) {
	
	double radicand = 0.0;
	double standDev = 0.0;
	
	// divide by n-1
	radicand = sumTwo / (userArraySize - 1);
cout << radicand << endl;
	// square root
	standDev = sqrt(radicand);

	//declare a return value
	return standDev;
}

[buffbill]

Compare your working with this:

double stdev(double a[], int n)
{ assert(n > 1);
  double sum=0;//initialize sum to zero
  for (int i=0; i<n; i++)
    sum += a[i];
  double mean = sum/n;  
  sum=0;
  double deviation;
  for (int i=0; i<n; i++)
  { deviation = a[i] - mean;
    sum += deviation*deviation;
  }
  return sqrt(sum/(n-1));  
}

[Pravesh Koirala]

Its because you are haven't calculated your mean. You are just using 0.0 as the mean.
calculate the mean just before calculating the square of differences.
Enjoy!