Mean and standard deviation.

Write a program that reads a set of floating-point data values from the input. When the user indicates the end of input, print out the count of the values, the average, and the standard deviation. The average of a data set x₁, . . . , x_n is 
a = (Σx_i)/n
where Σx_i = x₁ + . . . + x_n is the sum of the input values. The standard deviation is
s = √(Σ(x_i - a)2 / (n - 1)) However, that formula is not suitable for our task. By the time you have computed the mean, the individual x_i are long gone. Until you know how to save these values, use the numerically less stable formula
s = √(((Σx_i²) - (Σx_i) / n)/(n - 1)) You can compute this quantity by keeping track of the count, the sum, and the sum of squares in the DataSet class as you process the input values.