# Calculate the average of the nth column (with 0 as the first column) of a csv file
# This is somewhat more general than the task we did in class

def IsFloat(sf):
    # return True if sf is a valid string representation of a floating-point number
    try:
        f = float(sf)
        return True
    except:
        return False

def average_csv(filename, nth_column):
    # return the average (mean) of the numbers in the nth column of the requested csv file
    # return 0 if file is not readable or there's no data in the nth column
    
    # create a list of lines from reading the file
    try:
        f = open(filename,'r')
    except:
        return 0
    lines = f.read().split('\n')
    f.close()
    
    # set up for calculating
    subtotal = 0
    count = 0
    
    # do it
    for aline in lines:
        words = aline.split(',')
        # check if the nth column exists and is a number
        if len(words) > nth_column:
            if IsFloat(words[nth_column]):
                subtotal += float(words[nth_column])
                count += 1
    
    if count > 0:
        return subtotal/count
    return 0