""" Ex. 5.16 from A primer on...
Two main tasks:
1. Read data from file, store in lists or arrays
2. Plot data
We create a function and a module to facilitate
code reuse in Ex 5.18.
"""

import sys
import matplotlib.pyplot as plt

def read_data(filename):
    temp = []
    dens = []
    with open(filename) as infile:
        for line in infile:
            if line[0] == '#' or line.isspace():
                continue
            words = line.split()
            temp.append(float(words[0]))
            dens.append(float(words[1]))
    return temp, dens


if __name__ == "__main__":
    """The if-test above ensures that this code is run
    if the file is used as a stand-alone program, but
    not if it is imported into another program as a module.
    """
    filename = sys.argv[1]
    t, d = read_data(filename)
    plt.plot(t,d,'ro')
    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)
"""