Python - plot from txt file, skipping lines

Question

I have a problem with my script. I have (blabla.)txt file, for example:

blablaba bla
dsadsadsa
dsadsadsa
50 2323
60 2839
70 9832
80 0000
.....
....
...

and script I wrote:

import numpy as np
import matplotlib.pyplot as plt

with open("blabla.txt") as f:
    for line in xrange(3):
        next(f)
    for line in f:

      data = f.read()

      data = data.split('\n')

      x = [row.split()[0] for row in data]
      y = [row.split()[1] for row in data]

      index = [i for i,val in enumerate(x)]

      fig = plt.figure()
      ax1 = fig.add_subplot(111)
      ax1.set_title("graph")    
      ax1.set_xlabel('time')
      ax1.set_ylabel('distance')
      ax1.set_xticklabels(x)
      ax1.plot(index ,y, c='r', label='distance1')
      leg = ax1.legend()
      plt.locator_params(nbins=len(index)-1)
      plt.show()

The first question is: Is it true (syntax in the script) when I want to skip (because of the graph) the first three lines in the txt file?

The second: After runscript it says:

data = f.read()
ValueError: Mixing iteration and read methods would lose data.

What is the problem, its because of size? (txt file has about 600 000 lines)

Thanks for all the help....Funstorm60

Answer 1

You can use:

with open("<fname>.txt", 'r') as datafile:
    __data = datafile.readlines()[3:]

data = [[float(value) for value in line.split()] for line in __data]

This will give you a 2D array of your data - although it doesn't include the index information that you've included in your question.

EDIT: Sorry I missed the call to .split()

EDIT: As an example

1 2 3 4
2 3 4 5
6 5 4 3

will give the output:

[[1.0, 2.0, 3.0, 4.0], [2.0, 3.0, 4.0, 5.0], [6.0, 5.0, 4.0, 3.0]]