# comp.py - gets data from file named by user
#    and calculates mean and standard deviation
# cmc, 11/1/09

# getData - asks user for filename, then
#    reads data values from the file
# returns list of values
def getData():
    filename = input("enter input file name --> ")
    dataFile = open(filename, "r")
    data = []
    for line in dataFile:
        xString = line.split()
        x = float(xString[0])
        data.append(x)
    dataFile.close()
    return data

# meanAndStdev - calculates both mean and sample
#    standard deviation using computation formula
# assumes data is a list of float values
# returns mean and std. dev. as a tuple
def meanAndStdev(data):
    n = len(data)
    
    # first find sum and sum of squares
    sumX = sumX2 = 0.0
    for x in data:
        sumX = sumX + x
        sumX2 = sumX2 + x*x

    # apply formulas for mean and std. dev.
    mean = sumX / n
    numerator = n * sumX2 - sumX**2
    variance = numerator / (n * (n-1))
    import math
    stdev = math.sqrt(variance)

    return (mean, stdev)

# main execution
mean, stdev = meanAndStdev( getData() )
print("mean =", mean)
print("standard deviation =", stdev)