read three columns from a text file using matplotlib - matplotlib

Question

My input file (text.txt) includes three columns. First one is belongs to x-axis, second column represents y-axis and third column represents y-axis again. When i run my code, i get "x.append(float(line.split()[0])) IndexError: list index out of range". How can I fix that error?
my code:
#!/usr/bin/python
import numpy as np
import matplotlib.pyplot as plt
with open("text.txt", "r") as data_file:
lines=data_file.readlines()
x=[]
y1=[]
y2=[]
counter=0
for line in lines:
if((line[0]!='#') and (line[0]!='#')):
x.append(float(line.split()[0]))
y1.append(float(line.split()[0]))
y2.append(float(line.split()[1]))
counter+=1
plt.plot(x, y1, y2)
plt.savefig("text.png", dpi=300)
my text.txt:input
# Carbon
# Gallium
#
# title
# xaxis
1.00 2.12 14.51
2.00 4.54 18.14
3.00 6.12 45.11
4.00 9.02 89.15
5.00 6.48 49.99
6.00 8.01 92.33
7.00 7.56 95.14
8.00 5.89 96.01

Answer 1

You are getting the error
IndexError: list index out of range
because your data file contains empty lines. They may be at the end of the file.
You could fix it by including
for line in lines:
if not line.strip(): continue
but instead of the code you posted, I would use NumPy's genfromtxt to parse the file this way:
import numpy as np
import matplotlib.pyplot as plt
with open("text.txt", "r") as f:
lines = (line for line in f if not any(line.startswith(c) for c in '##'))
x, y1, y2 = np.genfromtxt(lines, dtype=None, unpack=True)
plt.plot(x, y1, x, y2)
plt.savefig("text2.png", dpi=300)
If you want to fix your original code with minimal changes, it might look something like this:
with open("text.txt", "r") as f:
x = []
y1 = []
y2 = []
for line in f:
if not line.strip() or line.startswith('#') or line.startswith('#'):
continue
row = line.split()
x.append(float(row[0]))
y1.append(float(row[1]))
y2.append(float(row[2]))
plt.plot(x, y1, x, y2)
plt.savefig("text.png", dpi=300)
Tip: readlines()
reads the entire file and returns a list of strings. This can require a lot of
memory if the file is large. Therefore, never use lines=data_file.readlines() unless you really need the entire file converted into a list of strings. Otherwise, it requires less memory if you can process each line one-at-a-time using
for line in f: