Python error : Extra plot while plotting multiple data files using glob

Question

I have the following code where I'm plotting two data files "data_1.txt" (quadratic function) and "data_2.txt" (cubic function) using the glob with wildcard * .

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

#fig,ax = plt.subplots()

path = "data_*.txt"
for datafile in glob.glob(path):

    #data = np.loadtxt(datafile)
    data = np.genfromtxt(datafile)
    #print(data)
    #ax.plot(data)
    plt.plot(data)
    #plt.plot(data[:,0], data[:,1])  # This line works

plt.show()

However, this is generating an extra plot (red colored line)!

If I use plt.plot(data[:,0], data[:,1]) instead of plt.plot(data) , the red line doesn't appear any more.

Any explanation? Also is there any difference between genfromtxt and loadtxt in this context?

Answer 1

The red line you see is really a blue line and a red line on top of each other. Those are the first columns of your data files plotted against their index.

In order to produce a line plot with matplotlib's plot function, one should specify the x and y coordinate array. One option is indeed to use

data = np.genfromtxt(datafile)
plt.plot(data[:,0],data[:,1])

Another option would be to unpack the data. So for a two column datafile,

x,y = np.genfromtxt(datafile, unpack=True)
plt.plot(x,y)

Both options are equally available with np.loadtxt . In the respective documentations you will find that genfromtxt has a lot more options than loadtxt . Those extra options may be usefull for excluding header or footer data or replacing missing values. The return type of both commands is the same.