Hello, I started c++, and I recently got an assignment, which is supposed to calculate the sum, variance, mean deviation, and mean in a set. The user can input any number positive or negative for the set. Because it is a set, the person may enter as many numbers as they want. I have succeeded in making the program do addition and mean/average. I need help in coming up with a function which will make a variance, and then square root of that will give mean deviation. Use the following website to get the formula (online algorithm). The problem is that there are some things that i don't know what it stands for or what it does: http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#On-line_algorithm
any help is appreciated.

``````#include <iostream>
#include <cmath>
#include <iomanip>

using namespace std;

void main()
{
double value = 0;
double sum = 0;
int count = 0;
double cnt = 0;
double mean = 0;

while(value != -1)
{
cout << "Enter a value: ";
cin >> value;
if (value != -1)
{
cnt++;
sum += value;
}

}

cout << "Number:  " << cnt << endl;
cout << "Sum:  " << sum << endl;
mean = sum/cnt;
cout << fixed << showpoint << setprecision(2) << "Average: " << mean;
fflush(stdin);
cout << "\n press enter to end" ;
cin.get();

}

double deviation(double mean, int cnt, double sum)
{

}``````

>> The problem is that there are some things that i don't know what it stands for or what it does

Like what? If you don't understand the formulas, then a math/statistics forum might be a better place to ask. Google "standard deviation example" and you'll get thousands …

## All 3 Replies

>> The problem is that there are some things that i don't know what it stands for or what it does

Like what? If you don't understand the formulas, then a math/statistics forum might be a better place to ask. Google "standard deviation example" and you'll get thousands of hits. I generally DO NOT go with wikipedia. This one isn't bad, and there are others. Get out a calculator and pencil and peper and follow the example. Put down the C++ part until you get the math part under control.

I had to do this very exercise last month. However, the solution looked nothing like what you have.

The values of the set were input into a vector using the push_back() function. Then the stated statistics were found using the sum() function and loops.

However, Vernon is absolutely correct. If you don't understand statistics (mean, variance, etc.), then you can't write a program to compute them. The author of the book I am using has that as the very first rule: You can't program what you don't understand. First, understand the problem.

From the link you gave, first, these are recursion formulas, meaning that they are calculated as the data comes in, so all those calculations should be within that value-entering loop you have (or you can loop through the data in another function and apply the formulas). Since there aren't that many symbols to understand I can give you a quick definition of them:
x_n : is the new element that was just given by the user.
x(bar)_n-1 : is the average or mean computed up to and including the previous element (x_n-1)
x(bar)_n : is the average or mean that includes the new element x_n
n : is the number of elements inputted so far (cnt), including the new element x_n
s2_n : is the sample variance (s_n being the mean deviation) that includes the new element x_n

That's about it. The sigma one is for a population instead of sample, that's just a weird statistical technicality. You will notice that the formula for the sample variance will be undefined for the first element (n-1 == 0), but that's alright because the variance of a set containing only one element has to be zero (the element does not deviate from itself!). So both the variance and mean deviation start with a value of zero, and you start counting them with those formulas from the second element and on.

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