Calculating Standard Deviation & Variance in C++

Question

So, I've posted a few times and previously my problems were pretty vague. I started C++ this week and have been doing a little project.

I'm trying to calculate standard deviation & variance. My code loads a file of 100 integers and puts them into an array, counts them, calculates the mean, sum, variance and SD. But I'm having a little trouble with the variance.

I keep getting a huge number - I have a feeling it's to do with its calculation.

My mean and sum are ok.

NB:

 using namespace std;
    int main()

{

int n = 0;
int Array[100];
float mean;
float var;
float sd;
string line;
float numPoints;

ifstream myfile("numbers.txt");

if (myfile.is_open())

{
    while (!myfile.eof())
      
    {
        getline(myfile, line);
            
        stringstream convert(line);
        
        if (!(convert >> Array[n]))
        
        {
            Array[n] = 0;
        }
        cout << Array[n] << endl;
        
        n++;
        
    }
    
    myfile.close();

    numPoints = n;

}
else cout<< "Error loading file" <<endl;

int sum = accumulate(begin(Array), end(Array), 0, plus<int>());

cout << "The sum of all integers: " << sum << endl;

mean = sum/numPoints;

cout << "The mean of all integers: " << mean <<endl;

var = ((Array[n] - mean) * (Array[n] - mean)) / numPoints;

sd = sqrt(var);

cout << "The standard deviation is: " << sd <<endl;

return 0;

}

Answer 1

As the other answer by horseshoe correctly suggests, you will have to use a loop to calculate variance otherwise the statement

var = ((Array[n] - mean) * (Array[n] - mean)) / numPoints;

will just consider a single element from the array.

Just improved horseshoe's suggested code:

var = 0;
for( n = 0; n < numPoints; n++ )
{
  var += (Array[n] - mean) * (Array[n] - mean);
}
var /= numPoints;
sd = sqrt(var);

Your sum works fine even without using loop because you are using accumulate function which already has a loop inside it, but which is not evident in the code, take a look at the equivalent behavior of accumulate for a clear understanding of what it is doing.

Note: X ?= Y is short for X = X ? Y where ? can be any operator. Also you can use pow(Array[n] - mean, 2) to take the square instead of multiplying it by itself making it more tidy.