"""
Exercise 5.16 from "A primer on..."
Read files with a given srtucture and plot
the results. The exercise does not require
writing a function, but it is convenient
to do this in order to reuse the solution for
exercise 5.18.
"""


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

def read_density_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 np.array(temp), np.array(dens)

if __name__ == "__main__":
    try:
        filename = sys.argv[1]
    except IndexError:
        print('No file given, using "density_air.txt" as default')
        filename = "density_air.txt"
    
    temp, dens = read_density_data(filename)
    plt.plot(temp, dens, 'ro')
    plt.show()

"""
Terminal> python read_density_data.py 
No file given, using "density_air.txt" as default
(output is a plot)
"""