"""Ex. 5.17 in "A primer on... "
Read data from file and plot it. The exercise does not
require writing a function, but this is convenient 
since we can then reuse the function in ex 5.19.
"""

import matplotlib.pyplot as plt
import sys
import numpy as np

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

"""This if-test ensures that the following lines are only run
if the code is run as a stand-alone program, and not if it is imported
as a module into another program."""
if __name__ == "__main__":
    filename = sys.argv[1]
    temp, dens = read_density_data(filename)
    plt.plot(temp,dens,'o')
    plt.show()

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