""" 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 = []
    density = []
    with open(filename) as infile:
        for line in infile:
            if line[0] == '#' or line.isspace():
                continue #skip the rest and move to next loop iteration
            words = line.split()
            temp.append(float(words[0]))
            density.append(float(words[1]))
    return temp, density

"""
The lines below will be run if we run the code as
a standalone program, i.e. python read_density_data.py
but not if we import it as a module into another
program:
"""
if __name__ == '__main__':
    file = sys.argv[1]
    temp, dens = read_data(file)

    plt.plot(temp,dens,'ro')
    plt.show()

"""
Terimal> python read_density_data.py density_water.dat
Output is a plot
"""