"""
Exercise 5.18 from " A primer on..."
Read data from file and plot it. The exercise does
not ask us to write a function, but we do it anyway, 
since this makes it easier to reuse the code
in exercise 5.18. The call to the function is placed
in a so-called "test block" at the bottom, which ensures that
this code is executed when the file is run as a stand-alone program,
but not if the file is imported as a module into another program. 
"""

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

def read_density_data(filename):
    temp = []
    density = []

    with open(filename) 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)

if __name__ == "__main__":
    filename = "density_air.txt"  #sys.argv[1]
    temp, dens = read_density_data(filename)
    plt.plot(temp, dens, 'ro')
    plt.show()