"""
Exercise 5.16 from "A primer on...". The exercise
does not ask for a function, but it is convenient
to write one that can be reused in Ex. 5.18. 
"""


import sys
import matplotlib.pyplot as plt

def read_density_file(filename):
    temp = []
    dens = []
    with open(filename) as infile:
        for line in infile:
            if line[0] == '#' or line.isspace():
                continue  #continue to next loop iteration (do nothing)
            else:
                words = line.split()
                temp.append(float(words[0]))
                dens.append(float(words[1]))
    
    return temp, dens


if __name__ == '__main__': 
    """
    Because of the if-test above, the following code is only run
    if this file is the main program (i.e. run on its own), not if it
    is imported as a module into another program.
    """
    density_file = sys.argv[1]
    temp, dens = read_density_file(density_file) 
    plt.plot(temp, dens,'o')
    plt.show()


    """
    Terminal> python read_density_data.py density_air.txt
    (output is a plot)
    Terminal> python read_density_data.py density_water.txt
    (output is a plot)
    """